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Chemistry Unit 5
Rates
Collision theory In order to react in the first place, minus all the temperature and surfac area malarkey, malarkey, to particles must must collide with each other and and they “may react”. May react? Isn’t it enough to have two particles colliding with ea other? Well no they have to collide the correct way round AND with the right energy for the bonds to break. Orientation o Consider a simple reaction between ethene CH 2=CH2 and HC they react to form chloroethane. o As a result of the collision a double bond is broken and a sing bond if formed o The reaction can only happen happen if the end end of the hydrogen approaches the carbon-carbon double bond, any other orientation and the reaction cannot happen o In unsymmetrical compounds the orientation is very importa as to whether the reaction will take place Activation energy o Even if the particles are the right way round they still won’t react unless they have a minimum amount of energy called t activation energy. Surface Area The more finely divided the the solid is, the the faster the the reaction happens. happens. A powdered solid will normally produce normally produce a faster reaction than if the same mass is present as a single lump. The powdered solid has a greater surface area than the single lump. The explanation explanation o As we know from previous section particles has to collide Master your semester withthe Scribd Read Free Foron 30this Days Sign up to vote title there is order to react so if a particle has more surface area & The New York Useful Not useful moreTimes places for the particles to collide and there fore react Cancel anytime. Special offer for students: Only $4.99/month. o Consider the reaction between hydrochloric acid and magnesium •
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Concentration Simply if you increase i ncrease the concentration of the reactants you increase t rate of reaction. Pressure Increasing the pressure on a reaction involving reacting gases increase the rate of reaction. Changing the pressure on a reaction, which involve only solids or liquids, has no effect on the rate. Temperature Temperature Increasing temperature temperature increases the rate of reaction, because when yo increase the temperature of a substance this increases the kinetic ener of the particles within it. (They move faster) this increases the amount o collisions and more collisions have the correct activation energy, so the fore an increase in the rate of reaction. Catalysts Catalysts speed up rate of reaction by creating an alternate reaction pathway with lower activation energy. Usually by creating intermediate compounds, the most important thing about catalysts is that they aren’ used up in the reaction or if they are they are recreated. Catalysts can b homogeneous(same state as reactants) or heterogeneous ( different state)
Rate equations
Measuring rates COLORIMETRY: COLORIMETRY: Measures change in intensity of colour CLOCK REACTIONS: Time taken for a particular amount of reactan react//product to be formed Master your semester with Scribd Read Free Foron 30this Days MASS CHANGE/VOLUME CHANGE: Used for reactants/products Signgas up to vote title & The New York Times Useful Not useful inCancel of TITRIMETRIC TITRIMETRIC ANALYSIS: Measures Measures changes changes conc anytime. Special offer forreactant/product students: Only $4.99/month. reactant/pro duct when one is acid, alkali or iodine by reacting with known volume of standard solution to neutralise • •
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Concentration Simply if you increase i ncrease the concentration of the reactants you increase t rate of reaction. Pressure Increasing the pressure on a reaction involving reacting gases increase the rate of reaction. Changing the pressure on a reaction, which involve only solids or liquids, has no effect on the rate. Temperature Temperature Increasing temperature temperature increases the rate of reaction, because when yo increase the temperature of a substance this increases the kinetic ener of the particles within it. (They move faster) this increases the amount o collisions and more collisions have the correct activation energy, so the fore an increase in the rate of reaction. Catalysts Catalysts speed up rate of reaction by creating an alternate reaction pathway with lower activation energy. Usually by creating intermediate compounds, the most important thing about catalysts is that they aren’ used up in the reaction or if they are they are recreated. Catalysts can b homogeneous(same state as reactants) or heterogeneous ( different state)
Rate equations
Measuring rates COLORIMETRY: COLORIMETRY: Measures change in intensity of colour CLOCK REACTIONS: Time taken for a particular amount of reactan react//product to be formed Master your semester with Scribd Read Free Foron 30this Days MASS CHANGE/VOLUME CHANGE: Used for reactants/products Signgas up to vote title & The New York Times Useful Not useful inCancel of TITRIMETRIC TITRIMETRIC ANALYSIS: Measures Measures changes changes conc anytime. Special offer forreactant/product students: Only $4.99/month. reactant/pro duct when one is acid, alkali or iodine by reacting with known volume of standard solution to neutralise • •
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[A] and [B] = concentrations of substances A and B x and y = partial orders Overall order = x + y The Arrhenius Arrhenius equation • • •
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luckily we don’t need to memorise this but its nice to understand it s you don’t panic in the exam so here are what the various symbols mean Starting with the easy ones . . . Temperature, Temperature, o T to fit into into the equation, equation, this has to be measured measured in kelvi The gas constant, R • •
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This is a constant which which comes from an equation, equation, pV=nRT which relates the pressure, volume and temperature of a particular number of moles of gas. It turns up in all sorts o unlikely places!
Activation energy, E A
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mole - not in kJ mol -1 You may also come across it in a different form form created by by a
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Entropy • • •
A measure of disorder Zero entropy = perfectly crystalline solid at 0k 2nd law of thermodynamics = entropy always increases ΔStotal = ΔSsystem + ΔSsurroundings
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Entropy of System
• •
ΔSsystem = ΔSproducts - ΔSreactants Use of a data booklet Entropy of surroundings
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ΔSsurroundings = -ΔH/T Units =JK -1mol-1
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get further apart, so there are more possibilities for Special offer for students:Particles Only $4.99/month. their position in space o
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Change of state: o Rapid change in entropy because particles move further apa closer together Feasibility Total entropy: o Must increase and must be positive for a reaction to be possible Increasing Temperature: o increases entropy, so can be used to make a reaction feasibl Dissolving Enthalpy of solution: o Enthalpy change when 1 mole of solute is dissolved in suffici solvent to produce an infinitely dilute solution Lattice enthalpy: o Enthalpy change when 1 mol of a solid ionic compound is produced from gaseous ions that start infinitely far apart •
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Enthalpy of hydration: o Enthalpy change when 1 mol of aqueous ions is formed from gaseous ions You're Reading a Preview Enthalpy of solution: Unlock full access with a free trial. o Sum of enthalpy of solutions Factors affecting lattice enthalpy Download With Free Trial Charge of ions: o Larger charge on ions increases lattice enthalpy Ionic radii: o Larger ionic radii decreases lattice enthalpy •
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Equilibrium
Explaining the basics a reversible reaction is a chemical change in which the products c be converted back to the original reactants under suitable conditions. Shown by the sign You're Reading When a reversible reaction occurs in aaPreview closed system, a chemical equilibrium is formed Unlock full access with a free trial. o A closed system means nothing can enter or leave o Eventually the ‘system settles and a state of Download Withdown’ Free Trial concentration balance exsists o BUT! The reactions don’t stop! Reactants are continually forming products, and products are re-forming reactants, hence dynamic equilibrium. Le chatelier’s Principle If changing the conditions disturbs a dynamic equilibrium, the position o equilibrium moves to counteract the change. Master your semester with Scribd Read Free Foron 30this Days Sign up to vote title Rule 1 – raising the temperature favours the endothermic directio & The New York Times Useful Not useful Cancel anytime. and lowering favours the exothermic Special offer for students: Only $4.99/month. Rule 2 – increasing the pressure favours the side of the equation •
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equilibrium concentrations of everything, you can combine these concentrations into an expression known as equilibrium constant. The equilibrium constant always has the same value (provided yo don't change the temperature), irrespective of the amounts of A, C and D you started with. It is also unaffected by a change in pressure whether not you using a catalyst.
Compare this with the chemical equation for the equilibrium. The convention is that the substances on the right-hand side of the equation are written at the top of the K c expression, and those on the left-hand side at the bottom. The indices (the powers that you have to raise the concentrations - for example, squared or You're cubed or whatever) Reading a Preview are just the numbers that appear in the equation Unlock full access with a free trial. K c is governed by temperature, this is only factor that can change o If the forward reaction is exothermic, K will decrease in valu Download With Free Trial c with temperature increase o If the forward reaction is endothermic K c will increase in valu with increase in temperature If K c is > 105 the posision of equilibrium is on the right hand side, almost 100% completion If K c is aproximetly 1 the equilibrium is dead centre (dynamic Master your semester with Scribd equilibrium) Read Free Foron 30this Days Sign up to vote title -5 If K c is < 10 (Very small) the equilibrium lies on the left hand side & The New York Times Useful Not useful Cancel anytime. very low RHS yield, backwards reaction is favoured •
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the total pressure by the mole fraction.
ACID-BASE Equilibrium
The Arrhenius theory You're Reading a Preview The theory Unlock full access with a free trial. • Acids are substances which produce hydrogen ions in solution. • Bases are substances which produce hydroxide ions in solution. Download With Free Trial Neutralisation happens because hydrogen ions and hydroxide ions reac to produce water
Limitations of the theory Hydrochloric acid is neutralised by both sodium hydroxide solution and ammonia solution. In both cases, you get a colourless solution which yo can crystallise to get a white - either sodium chloride or ammonium Master your semester withsalt Scribd Read Free Foron 30this Days Sign up to vote title chloride. & The New York Times Useful Not useful equations These are clearly very similar reactions. The full are: Cancel anytime. Special offer for students: Only $4.99/month.
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there are hydroxide ions there, and we can squeeze this into the Arrhen theory. However, this same reaction also happens between ammonia gas and hydrogen chloride gas.
In this case, there aren't any hydrogen ions or hydroxide ions in solution - because there isn't any solution. The Arrhenius theory wouldn't count this as an acid-base reaction, despite the fact that it is producing the same product as when the two substances were in solution. That's silly! The bronsted-lowry acid theory The theory • An acid is a proton (hydrogen ion) donor. • A base is a proton (hydrogen ion) acceptor. The relationship between the Bronsted-Lowry theory and the Arrhenius theory The Bronsted-Lowry theory doesn't go against the Arrhenius theory in way - it just adds to it. Hydroxide ions are still bases because they accept hydrogen ions from acids and form water. An acid produces hydrogen ions in solution because it reacts with the water molecules by giving a proton them. You're to Reading a Preview When hydrogen chloride gas dissolves in water to produce hydrochloric Unlock full access with a free trial. acid, the hydrogen chloride molecule gives a proton (a hydrogen ion) to water molecule. A co-ordinate (dative covalent) bond is formed betwee Download With Free Trial one of the lone pairs on the oxygen and the hydrogen from the HCl. + Hydroxonium ions, H 3O , are produced.
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reacts with a base, what is actually functioning as the acid is the hydroxonium ion. For example, a proton is transferred
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solution, H+(aq), what you are actually talking about are hydroxonium ion Conjugate pairs When hydrogen chloride dissolves in water, almost 100% of it reacts wi the water to produce hydroxonium ions and chloride ions. Hydrogen chloride is a strong acid, and we tend to write this as a one-way reactio
n fact, the reaction between HCl and water is reversible, but only to a minor extent. In order to generalise, consider an acid HA, and think of th reaction as being reversible.
Thinking about the forward reaction: • The HA is an acid because it is donating a proton (hydrogen ion) to the water. • The water is a base because it is accepting a proton from the HA. But there is also a back reaction between the hydroxonium ion and the ion: + • The H3O is an acid because it is donating a proton (hydrogen ion) to t A- ion. + • The A ion is a base because it is accepting a proton from the H 3O . The reversible reaction contains two acids and two bases. We think of You're Reading a Preview them in pairs, called conjugate pairs. Unlock full access with a free trial.
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When the acid, HA, loses a proton it forms a base, A -. When the base, A accepts a proton back again, it obviously refoms the acid, HA. These tw are a conjugate pair. Members of a conjugate pair differ from each other by the presence or absence of the transferable hydrogen ion. If youyour are thinking aboutwith HA asScribd the acid, then A - is its conjugate base. Master semester Read Free For 30 Days Sign up to vote on this title If you are thinking about A as the base, then HA is its conjugate acid. & The New York Times Useful Not useful pair. Thinking The water and the hydroxonium ion are also a conjugate Cancel anytime. Specialthe offer water for students: $4.99/month. asOnly a base, the hydroxonium ion is its conjugate acid because has the extra hydrogen ion which it can give away again.
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Think first about the forward reaction. Ammonia is a base because it is accepting hydrogen ions from the water. The ammonium ion is its conjugate acid - it can release that hydrogen ion again to reform the ammonia. The water is acting as an acid, and its conjugate base is the hydroxide The hydroxide ion can accept a hydrogen ion to reform the water. Looking at it from the other side, the ammonium ion is an acid, and ammonia is its conjugate base. The hydroxide ion is a base and water is its conjugate acid. Amphoteric substances You may possibly have noticed (although probably not!) that in one of t last two examples, water was acting as a base, whereas in the other on was acting as an acid. A substance which can act as either an acid or a base is described as being amphoteric. You're Reading a Preview Unlock full access with a free trial.
Strong Acids Download With Free Trial Explaining the term “strong acid” When an acid dissolves in water, a proton (hydrogen ion) is transferred a water molecule to produce a hydroxonium ion and a negative ion depending on what acid you are starting from. In the general case . . .
Theseyour reactions are all reversible, but in some cases, the acid is so Master semester with Scribd goo Read Free For 30 Days Sign up to vote on this title at giving away hydrogen ions that we can think of the reaction as being & The New York Times Useful Not useful one-way. The acid is virtually 100% ionised. SpecialFor offer example, for students: Only $4.99/month. when hydrogen
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chloride dissolves in water to make hydrochloric acid, so little of the reverse reaction happens that we can
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This shows the hydrogen chloride dissolved in the water splitting to give hydrogen ions in solution and chloride ions in solution. This version is often used in this work just to make things look easier. I you use it, remember that the water is actually involved, and that when you write H +(aq) what you really mean is a hydroxonium ion, H 3O+. Strong acids and pH pH is a measure of the concentration of hydrogen ions in a solution. Strong acids like hydrochloric acid at the sort of concentrations you normally use in the lab have a pH around 0 to 1. The lower the pH, the higher the concentration of hydrogen ions in the solution. Defining pH
Working out the pH of a strong acid Suppose you had to work out the pH of 0.1 mol dm -3 hydrochloric acid. You're Reading a Preview you have to do is work out the concentration of the hydrogen ions in the solution, and then use your calculator to convert Unlock full access with a free trial.it to a pH. With strong acids this is easy. Download With Free Trial ionised. Each mole of Hydrochloric acid is a strong acid - virtually 100% HCl reacts with the water to give 1 mole of hydrogen ions and 1 mole of chloride ions That means that if the concentration of the acid is 0.1 mol dm -3, then th concentration of hydrogen ions is also 0.1 mol dm -3. Use your calculator to convert this into pH. My calculator wants me to enter 0.1, and then press the "log" button. Yours might want you to do i Master your semester Scribd in a different order. Youwith need to find out! Read Free Foron 30this Days Sign up to vote title log10 [0.1] = -1 & The New York Times Useful Not useful Cancel anytime. But pH = - log 10 [0.1] Special offer for students: Only $4.99/month. - (-1) = 1
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converted into ions. The rest remain as simple ethanoic acid molecules. Most organic acids are weak. Hydrogen fluoride (dissolving in water to produce hydrofluoric acid) is a weak inorganic acid that you may come across elsewhere . Comparing the strengths of weak acids The position of equilibrium of the reaction between the acid and water varies from one weak acid to another. The further to the left it lies, the weaker the acid is.
The acid dissociation constant, K a You can get a measure of the position of an equilibrium by writing an equilibrium constant for the reaction. The lower the value for the consta the more the equilibrium lies to the left. The dissociation (ionisation) of an acid is an example of a homogeneo reaction. Everything is present in the same phase - in this case, in solut in water. You can therefore write a simple expression for the equilibrium constant, K c. Here is the equilibrium again: You might expect the equilibrium constant to be written as: You're Reading a Preview
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However, if you think about thisDownload carefully, is something odd about With there Free Trial At the bottom of the expression, you have a term for the concentration the water in the solution. That's not a problem - except that the number going to be very large compared with all the other numbers. In 1 dm3 of solution, there are going to be about 55 moles of water. If you had a weak acid with a concentration of about 1 mol dm -3, and about 1% of it reacted with the water, the number of moles of water is only going fall by about 0.01. In other words, if the acid is weakthe Master yourtosemester with Scribd Read Free Foron 30this Days Sign up to vote title concentration of the water is virtually constant. & The New York Times Useful Not useful it Cancel expression as In that case, there isn't a lot of point in including in the anytime. Specialitoffer for students: Only $4.99/month. were a variable. Instead, a new equilibrium constant is defined which leaves it out. This new equilibrium constant is called K
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An introduction to pK a pK a bears exactly the same relationship to K a as pH does to the hydroge ion concentration: Remember this: • The lower the value for pK a, the stronger the acid. The higher the value for pK a, the weaker the acid.
The ionic product of water, K W and PK w The important equilibrium in water Water molecules can function as both acids and bases. One water molecule (acting as a base) can accept a hydrogen ion from a second o (acting as an acid). This will be happening anywhere there is even a tra of water - it doesn't have to be pure. A hydroxonium ion and a hydroxide ion are formed.
You're Reading a Preview accessstrong with a free trial. However, the hydroxonium ion Unlock is a full very acid, and the hydroxide is a very strong base. As fast as they are formed, they react to produce Download With Free Trial water again. The net effect is that an equilibrium is set up.
At any one time, there are incredibly small numbers of hydroxonium ion and hydroxide ions present. Further down this page, we shall calculate concentration of hydroxonium ions present in pure water. It turns out be 1.00 x 10 -7 mol dm-3 at room temperature. Master your semester with Scribd You may well find this equilibrium written in a Read simplified form: Free Foron 30 Days Sign up to vote this title
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You may wonder why the water isn't written on the bottom of these equilibrium constant expressions. So little of the water is ionised at any one time, that its concentration remains virtually unchanged - a constan K w is defined to avoid making the expression unnecessarily complicated by including another constant in it The value of K w Like any other equilibrium constant, the value of K w varies with temperature. Its value is usually taken to be 1.00 x 10 -14 mol2 dm-6 at ro temperature. In fact, this is its value at a bit less than 25°C. pK w The relationship between K w and pK w is exactly the same as that betwee K a and pK a, or [H+] and pH. The K w value of 1.00 x 10 -14 mol2 dm-6 at room temperature gives you a pK w value of 14. Try it on your calculator! Notice that pK w doesn't have units
Strong Bases Explaining the term "strong base" A strong base is something like sodium hydroxide or potassium hydroxi which is fully ionic. You can think of Reading the compound You're a Preview as being 100% split into metal ions and hydroxide ions in solution. full access with a free trial. Each mole of sodium hydroxideUnlock dissolves to give a mole of hydroxide io in solution. Download With Free Trial
Some strong bases like calcium hydroxide aren't very soluble in water. That doesn't matter - what does dissolve is still 100% ionised into calciu Master your semester with Scribd base ions and hydroxide ions. Calcium hydroxide still counts aDays strong Read Free Foras 30this Sign up to vote on title because that 100% & The NewofYork Timesionisation. Useful Not useful Cancel anytime. Working out the pH of a strong base Special offer for students: Only $4.99/month. Remember that:
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Explaining the term "weak base" Ammonia is a typical weak base. Ammonia itself obviously doesn't cont hydroxide ions, but it reacts with water to produce ammonium ions and hydroxide ions.
However, the reaction is reversible, and at any one time about 99% of t ammonia is still present as ammonia molecules. Only about 1% has actually produced hydroxide ions. A weak base is one which doesn't convert fully into hydroxide ions in solution. Comparing the strengths of weak bases in solution: K b When a weak base reacts with water, the position of equilibrium varies from base to base. The further to the left it is, the weaker the base.
You can get a measure of the position of equilibrium by writing an equilibrium constant for the reaction. The lower the value for the consta the more the equilibrium lies to the left. You're Reading a Preview In this case the equilibrium constant is called K b. This is defined as: Unlock full access with a free trial.
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again just like with weak acids you can use this to calculate pH, again th top is equal so you can rearrange to calculate OH- concentration then using Kw and the log equation you can calculate pH pK b The relationship between K b and pK b is exactly the same as all the othe "p" terms in this topic:
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the corresponding term for acids, pK a - the smaller the value, the stronger the acid.
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pH Curves
Equivalence point Sorting out some confusing terms When you carry out a simple acid-base titration, you use an indicator to tell you when you have the acid and alkali mixed in exactly the right proportions to "neutralise" each other. When the indicator changes colo this is often described as the end point of the titration. In an ideal world, the colour change would happen when you mix the tw solutions together in exactly equation proportions. That particular mixtu is known as the equivalence point . For example, if you were titrating sodium hydroxide solution with hydrochloric acid, both with a concentration of 1 mol dm -3, 25 cm3 of You're Reading a Preview sodium hydroxide solution would need exactly the same volume of the full access with free trial. acid - because they react 1 : 1 Unlock according to athe equation.
Download Withbe Freethe Trialneutral point of the In this particular instance, this would also titration, because sodium chloride solution has a pH of 7. But that isn't necessarily true of all the salts you might get formed. For example, if you titrate ammonia solution with hydrochloric acid, you would get ammonium chloride formed. The ammonium ion is slightly acidic, and so pure ammonium chloride has a slightly acidic pH. That means that at the equivalence point (where you had mixed the Master yourin semester with Scribd solutions the correct proportions according to Read Free Foron 30this Days Signthe up toequation), vote title the solution actually be neutral. To use the term "neutral point" in & The Newwouldn't York Times Useful Not useful Cancel anytime. context would be misleading. Specialthis offer for students: Only $4.99/month. Similarly, if you titrate sodium hydroxide solution with ethanoic acid, at
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All the following titration curves are based on both acid and alkali havin concentration of 1 mol dm -3. In each case, you start with 25 cm 3 of one the solutions in the flask, and the other one in a burette. Although you normally run the acid from a burette into the alkali in a fla you may need to know about the titration curve for adding it the other way around as well. Alternative versions of the curves have been described in most cases.
Titration curves for strong acid v strong base We'll take hydrochloric acid and sodium hydroxide as typical of a strong acid and a strong base. Running acid into the alkali
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You can see that the pH only falls a very small amount until quite nea equivalence point. Then there is a really steep plunge. If you calculate t values, the pH falls all the way from 11.3 when you have added 24.9 cm to 2.7 when you have added 25.1 cm 3. Running alkali into the acid Master your with Scribd This is very semester similar to the previous curve except, of course, that the pH Read Free Foron 30this Days Sign up to vote title starts off low and increases as you add more sodium hydroxide solution
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Running acid into the alkali
Because you have got a weak base, the beginning of the curve is obviously going to be different. However, once you have got an excess acid, the curve is essentially the same as before. At the very beginning of the curve, the pH starts by falling quite quickly the acid is added, but the curve very soon gets less steep. This is becau a buffer solution is being set up - composed of the excess ammonia and the ammonium chloride being formed. Notice that the equivalence point is Reading now somewhat You're a Preview acidic ( a bit less th pH 5), because pure ammonium chloride isn't neutral. However, the Unlock full access with a free trial. equivalence point still falls on the steepest bit of the curve. That will tur out to be important in choosing a suitable indicator for the titration. Download With Free Trial Running alkali into the acid At the beginning of this titration, you have an excess of hydrochloric ac The shape of the curve will be the same as when you had an excess of acid at the start of a titration running sodium hydroxide solution into th acid. It is only after the equivalence point that things become different. A buffer solution is formed containing excess ammonia and ammonium Master your semester with Scribd expe chloride. This resists any large increase in pH -Read notupFree that would Foryou 30this Days Sign to vote on title a very large increase anyway, because ammonia is only a weak base. & The New York Times Useful Not useful Special offer for students: Only $4.99/month.
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and a strong base.
Running acid into the alkali For the first part of the graph, you have an excess of sodium hydroxide. The curve will be exactly the same as when you add hydrochloric acid t sodium hydroxide. Once the acid is in excess, there will be a difference
Past the equivalence point you have a buffer solution containing sodium ethanoate and ethanoic acid. This resists any large fall in pH. Running alkali into the acid You're Reading a Preview Unlock full access with a free trial.
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The start the graph shows relatively rapid rise in pH but this slows Master yourof semester withaScribd Read Free For 30this Days Sign up to vote on title ethanoa down as a buffer solution containing ethanoic acid and sodium & The New York Times Useful Not useful is produced. Beyond the equivalence point (when the sodium hydroxide Specialin offer for students: Onlycurve $4.99/month. excess) the is just
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the same as that end of the HCl - NaOH grap
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Notice that there isn't any steep bit on this graph. Instead, there is just what is known as a "point of inflexion". That lack of a steep bit means th it is difficult to do a titration of a weak acid against a weak base. Calculations based on these graphs To calculate the equivalence point you (using a ruler) measure t steep bit, divide this value by two, mark a dot at this number, follo across to find pH, down for the volume of acid added. At half equivalence point pH=pKa •
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METHYL ORANGE: Use when there is a strong acid PHENOLPHTHALEIN : Use when there is a strong alkali NO INDICATOR AVAILABLE: When both the acid and alkali are wea as there is no sharp change in pH USE EITHER : When strong acid and strong alkali, as there is a long sharp change in pH Indicators are weak acids, The un-ionised indicator is colour 1 , whereas the ion is colour 2 Using litmus as example of how this works Adding hydroxide ions: • • •
•
Adding hydrogen ions:
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Download With Free Trial If the concentrations of HLit and Lit - are equal: At some point during the movement of the position of equilibrium, the concentrations of the two colours will become equal. The colour you see will be a mixture of the two.
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Buffers
Definition A buffer solution is one, which resists changes in pH when small quantit of an acid or an alkali are added to it. Acidic buffer solutions An acidic buffer solution is simply one, which has a pH less than 7. Acid buffer solutions are commonly made from a weak acid and one of its sa - often a sodium salt. A common example would be a mixture of ethanoic acid and sodium ethanoate in solution. In this case, if the solution contained equal molar concentrations of both the acid and the salt, it would have a pH of 4.76. wouldn't matter what the concentrations were, as long as they were the same. You can change the pH of the buffer solution by changing the ratio of to salt, or by choosing a different acid and one of its salts. Alkaline buffer solutions An alkaline buffer solution has a pH greater than 7. Alkaline buffer solutions are commonly made from a weak base and one of its salts. A frequently used example is a mixture of ammonia solution and ammonium chloride solution. IfYou're these wereamixed Reading Preview in equal molar proportions, the solution would have a pH of 9.25. Again, it doesn't mat Unlock full access with a free trial. what concentrations you choose as long as they are the same How do buffers work? With Free Trial A buffer solution has to containDownload things which will remove any hydrogen ions or hydroxide ions that you might add to it - otherwise the pH will change. Acidic and alkaline buffer solutions achieve this in different way Acidic buffer solutions We'll take a mixture of ethanoic acid and sodium ethanoate as typical. Ethanoic acid is a weak acid, and the position of this equilibrium will be well to the left:
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Principle, that will tip the position of the equilibrium even further to the left.
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Although the reaction is reversible, since the ethanoic acid is a weak ac most of the new hydrogen ions are removed in this way.
Since most of the new hydrogen ions are removed, the pH won't change very much - but because of the equilibria involved, it will fall a little bit. Adding an alkali to this buffer solution Alkaline solutions contain hydroxide ions and the buffer solution remove most of these. This time the situation is a bit more complicated because there are two processes which can remove hydroxide ions. Removal by reacting with ethanoic acid The most likely acidic substance which a hydroxide ion is going to collid with is an ethanoic acid molecule. They will react to form ethanoate ion and water.
Because most of the new hydroxide ions are removed, the pH doesn't increase very much. Removal of the hydroxide ions by reacting with hydrogen ions Remember that there are some hydrogen ions present from the ionisati of the ethanoic acid. You're Reading a Preview
Hydroxide ions can combine with these to make water. As soon as this Unlock full access with a free trial. happens, the equilibrium tips to replace them. This keeps on happening until most of the hydroxide ionsDownload are removed. With Free Trial
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small extent to give a few hydrogen ions and hydroxide ions .
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lots of unreacted ammonia; • lots of ammonium ions from the ammonium chloride; • enough hydroxide ions to make the solution alkaline. Other things (like water and chloride ions) which are present aren't important to the argument. Adding an acid to this buffer solution There are two processes which can remove the hydrogen ions that you are adding. Removal by reacting with ammonia The most likely basic substance which a hydrogen ion is going to collide with is an ammonia molecule. They will react to form ammonium ions. •
Most, but not all, of the hydrogen ions will be removed. The ammonium ion is weakly acidic, and so some of the hydrogen ions will be released again. Removal of the hydrogen ions by reacting with hydroxide ions Remember that there are some hydroxide ions present from the reactio between the ammonia and the water.
Hydrogen ions can combine with these hydroxide ions to make water. A You're Reading soon as this happens, the equilibrium tips atoPreview replace the hydroxide ions This keeps on happening until most of the hydrogen ions are removed. Unlock full access with a free trial. Download With Free Trial
Master your semester with Scribd Again, because you have equilibria involved, not the hydrogen ion Read Free For 30this Days Sign all up toof vote on title are New removed - just most of them. & The York Times Useful Not useful Cancel anytime. solution The hydroxide ions from the alkali are removed by a simple reaction w
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Aldehydes and Ketones
Aldehydes and ketones are simple compounds which contain a carbon group - a carbon-oxygen double bond. How do you name aldehydes? How do you name ketones? In aldehydes, the carbonyl group has a hydrogen atom attached to it together with a second hydrogen atom •
o
Shown by using ‘al’
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You'rethe Reading a Preview Based on alkane minus ‘ane’ e.g methane
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In ketones, the carbonyl group has two hydrocarbon groups attached o
Shown by using ‘one’ e.g propane
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propanone
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tendency to pull electrons in a carbon-oxygen bond towards itself. One the two pairs of electrons that make up a carbon-oxygen double bond is even more easily pulled towards the oxygen. That makes the carbonoxygen double bond very highly polar. Addition of hydrogen cyanide to aldehydes and ketones The reaction Hydrogen cyanide adds across the carbon-oxygen double bond in aldehydes and ketones to produce compounds known as hydroxynitriles These used to be known as cyanohydrins. For example, with ethanal (an aldehyde) you get 2-hydroxypropanenitri
With propanone (a ketone) you get 2-hydroxy-2-methylpropanenitrile:
The reaction isn't normally done using hydrogen cyanide itself, because this is an extremely poisonous gas. Instead, the aldehyde or ketone is mixed with a solution of sodium or potassium cyanide in water to which little sulphuric acid has been added. The pH of the solution is adjusted t Reading areaction. Preview about 4 - 5, because this gives You're the fastest Unlock full access with a free trial.
The solution will contain hydrogen cyanide (from the reaction between sodium or potassium cyanide and the sulphuric acid), but still contains Download With Free Trial some free cyanide ions. This is important for the mechanism. The mechanism for the addition of HCN to ethanal As before, the reaction starts with a nucleophilic attack by the cyanide i on the slightly positive carbon atom.
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of a hydrogen ion from, for example, a
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with both types of carbonyl compound o boiling points of these precipitates can be compared with a known table of values to determine its type warm a fews drops of the compound with Tollen’s reagent (ammoniacal silver nitrate) o only the aldehyde produces a silver mirror on the sides of th test tube o aldehydes are stronger reducing agents than ketones and reduce the metal ion Simmer with Fehling’s or Benedicts solution (a blue complex of Cu o The aldehyde will produce a brown/red precipitate o The ketone with stay blue Doing the triiodomethane (iodoform) reaction
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Carboxylic acids
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The background Carboxylic acids are compounds, which contain a -COOH group. For the purposes of this page we shall just look at compounds where the -COOH group is attached either to a hydrogen atom or to an alkyl group.
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Notice: • The bond between the sodium and the ethanoate is ionic. Don't draw line between the two (implying a covalent bond). That's absolutely wrong! Although the name is written with the sodium first, the formula is alway written in one of the ways shown. This is something you just have to ge used to. Physical properties of carboxylic acids The physical properties (for example, boiling point and solubility) of the carboxylic acids are governed by their ability to form hydrogen bonds. Boiling points Before we look at carboxylic acids, a reminder about alcohols: The boiling points of alcohols are higher than those of alkanes of similar size because the alcohols can form hydrogen bonds with each other as well as van der Waals dispersion forces and dipole-dipole interactions. The boiling points of carboxylic acids of similar size are higher still. The higher boiling points of the carboxylic acids are still caused by hydrogen bonding, but operating in a different way. In a pure carboxylic acid, hydrogen bonding can occur between two molecules of acid to produce a dimer . You're Reading a Preview Unlock full access with a free trial.
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This immediately doubles the size of the molecule and so increases the van der Waals dispersion forces between one of these dimers and its neighbours - resulting in a high boiling point. Solubility in water In theyour presence of water, the carboxylic dimerise. Instead, Master semester with Scribd acids don't Read Free For 30this Days Sign up to vote on title hydrogen bonds are formed between water molecules and individual & The New York Times Useful Not useful molecules of acid. Cancel anytime. Special offer for students: Only $4.99/month. The carboxylic acids with up to four carbon atoms will mix with water in any proportion. When you mix the two together, the energy released
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using potassium dichromate(VI) solution in the presence of dilute sulphuric acid. During the reaction, the potassium dichromate(VI) soluti turns from orange to green. Primary alcohols are oxidised to carboxylic acids in two stages - first to aldehyde and then to the acid. We often use simplified versions of these equations using "[O]" to represent oxygen from the oxidising agent. The formation of the aldehyde is shown by the simplified equation:
"R" is a hydrogen atom or a hydrocarbon group such as an alkyl group. The aldehyde is then oxidised further to give the carboxylic acid:
If you start with an aldehyde, you are obviously just doing this second stage. Starting from the primary alcohol, you could combine these into one sin equation to give: You're Reading a Preview
For example, if you were converting ethanol into ethanoic acid, the Unlock full access with a free trial. simplified equation would be: Download With Free Trial
The alcohol is heated under reflux with an excess of a mixture of potassium dichromate(VI) solution and dilute sulphuric acid. Heating under reflux (heating in a flask with a condenser placed vertica in it) prevents any aldehyde formed escaping before it has time to be oxidised to the carboxylic acid. By hydrolysing nitriles Whatyour are nitriles? Master semester with Scribd Read Free For 30this Days Sign up to title Nitriles are compounds which contain -CN attached tovote a on hydrocarbon & The New York Times Useful Not useful group. Some common examples include: Cancel anytime. Special offer for students: Only $4.99/month.
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They are formed in the reaction between halogenoalkanes (haloalkanes alkyl halides) and cyanide ions. For example: . . . or during the reaction between aldehydes or ketones and hydrogen cyanide. For example, the reaction between ethanal and hydrogen cyanide to make 2-hydroxypropanenitrile is:
Converting the nitrile into a carboxylic acid There are two ways of doing this, both of which involve reacting the carbon-nitrogen triple bond with water. This is described as hydrolysis The two methods produce slightly different products - you just have to careful to get this right. Acid hydrolysis The nitrile is heated under reflux with a dilute acid such as dilute hydrochloric acid. A carboxylic acid is formed. For example, starting fro ethanenitrile you would get ethanoic acid. The ethanoic acid could be distilled off the mixture.
Alkaline hydrolysis The nitrile is heated under reflux with an alkali such as sodium hydroxid You're Reading a Preview solution. Unlock full access with a free trial. This time you wouldn't, of course, get a carboxylic acid produced - any acid formed would react with the sodium hydroxide present to give a sa Download With Free Trial You also wouldn't get ammonium ions because they would react with sodium hydroxide to produce ammonia. Starting from ethanenitrile, you would therefore get a solution containin ethanoate ions (for example, sodium ethanoate if you used sodium hydroxide solution) and ammonia.
You have remember to convert the ions into the free carboxylic acid, Master yourtosemester with Scribd Free For 30this Days Sign up to vote on title acid, because that's what we are trying to make. ToRead liberate the weak & The New York Times Useful Not useful ions from a strong acid ethanoic acid, you just have to supply hydrogen Specialsuch offer foras students: Only $4.99/month. hydrochloric acid.
mixture to make it acidic.
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You add enough hydrochloric acid to the
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You're Reading a Preview Unlock full access with a free trial.
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Esters Background Master your semester with Scribd the Esters are derived from carboxylic acids. A carboxylic acid contains Read Free For 30this Days Sign up to vote on title -COOH group, in an ester the hydrogen in this group is replaced by & The New Yorkand Times Useful Not useful group of some Specialhydrocarbon offer for students: Only $4.99/month.
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kind. Usually by that of an alcohol. They are named firstly after the alcohol then the carboxylic acid, e.g:
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Boiling points The small esters have boiling points, which are similar to those of aldehydes and ketones with the same number of carbon atoms. Like aldehydes and ketones, they are polar molecules and so have dipo dipole interactions as well as van der Waals dispersion forces. However they don't form hydrogen bonds, and so their boiling points aren't anything like as high as an acid with the same number of carbon atoms Hydrolysis of esters What is hydrolysis? You're Reading a Preview Technically, hydrolysis is a reaction with water. That is exactly what happens when esters are hydrolysed by water Unlock full access with a freeor trial.by dilute acids such as dilute hydrochloric acid. The alkaline hydrolysis of esters actually involves Download With Free Trial reaction with hydroxi ions, but the overall result is so similar that it is lumped together with th other two. Hydrolysis using water or dilute acid The reaction with pure water is so slow that it is never used. The reac is catalysed by dilute acid, and so the ester is heated under reflux with dilute acid like dilute hydrochloric acid or dilute sulphuric acid. Here your are two simple examples of hydrolysis using an acid catalyst. Master semester with Scribd Read Free For 30 Days Sign up to vote on this title First, hydrolysing ethyl ethanoate:
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excess of dilute acid. Hydrolysis using dilute alkali This is the usual way of hydrolysing esters. The ester is heated under reflux with a dilute alkali like sodium hydroxide solution. There are two big advantages of doing this rather than using a dilute a The reactions are one-way rather than reversible, and the products are easier to separate. Taking the same esters as above, but using sodium hydroxide solution rather than a dilute acid: First, hydrolysing ethyl ethanoate using sodium hydroxide solution:
. . . and then hydrolysing methyl propanoate in the same way:
Notice that you get the sodium salt formed rather than the carboxylic a itself. This mixture is relatively easy to separate. Provided you use an excess You're Reading Preview sodium hydroxide solution, there won't be aany ester left - so you don't have to worry about that. Unlock full access with a free trial. The alcohol formed can be distilled off. That's easy! If you want the acid rather thanDownload its salt,With all Free youTrial have to do is to add an excess of a strong acid like dilute hydrochloric acid or dilute sulphuric a to the solution left after the first distillation. If you do this, the mixture is flooded with hydrogen ions. These are pick up by the ethanoate ions (or propanoate ions or whatever) present in th salts to make ethanoic acid (or propanoic acid, etc). Because these are weak acids, once they combine with the hydrogen ions, they tend to sta combined. Master your semester with Scribd Free Foron 30this Days Sign up to vote title The carboxylic acid can now be distilled off. Read & The New York Times esters to make soap Useful Not useful Hydrolyzing complicated Cancel anytime. Specialifoffer forlarge students:esters Only $4.99/month. the present in animal or vegetable fats and oils are heate with concentrated sodium hydroxide solution exactly the same reaction
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Because of its relationship with soap making, the alkaline hydrolysis of esters is sometimes known as saponification. Polyesters What is a polyester? A polyester is a polymer (a chain of repeating units) where the individua units are held together by ester linkages.
The diagram shows a very small bit Reading of the apolymer You're Preview chain and looks pret complicated. But it isn't very difficult to work out - and that's the best full access with a free thing to do: work it out, not tryUnlock to remember it. trial. You will see how to do in a moment. Download With Free Trial The usual name of this common polyester is poly(ethylene terephthalat The everyday name depends on whether it is being used as a fibre or as material for making things like bottles for soft drinks. When it is being used as a fibre to make clothes, it is often just called polyester . It may sometimes be known by a brand name like Terylene When it is being used to make bottles, for example, it is usually called PET . Master your semester Scribd Making polyesters as anwith example of condensation polymerisation Read Free Foron 30this Days Sign up to vote title In condensation polymerisation, when the monomers join together a sm & The New York Times Useful Not useful Cancel anytime. gets That's different from addition polymerisation which Specialmolecule offer for students: Onlylost. $4.99/month. produces polymers like poly(ethene) - in that case, nothing is lost when
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Now imagine lining these up alternately and making esters with each ac group and each alcohol group, losing a molecule of water every time an ester linkage is made.
That would produce the chain shown above (although this time written without separating out the carbon-oxygen double bond - write it whichever way you like).
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Acyl (acid) chlorides A carboxylic acid such as ethanoic acid has the structure:
There are a number of related compounds in which the -OH group in the acid is replaced by something else. Compounds like this are described a acid derivatives. Acyl chlorides (also known as acid chlorides) are one example of an acid derivative. In this case, the -OH group has been replaced by a chlorine atom. You're Reading a Preview Unlock full access with a free trial.
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WITH WATER: Acyl chloride + water carboxylic acid + hydrochl acid WITH ALCOHOLS (REFLUX): Acyl chloride + alcohol ester + hydrochloric acid WITH AMMONIA: Acyl chloride + ammonia amide + hydrochlor acid Master your semester with Scribd + WITH AMINES: Acyl chloride + amine N-substituted amide Read Free Foron 30this Days Sign up to vote title hydrochloric acid & The New York Times Useful Not useful Cancel anytime. HCl: Released as gas so steamy fumes are observed Special offer for students: Only $4.99/month. •
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Optical isomers
What are isomers? Isomers are molecules that have the same molecular formula, but have different arrangement of the atoms in space. That excludes any differen arrangements which are simply due to the molecule rotating as a whole or rotating about particular bonds. Where the atoms making up the various isomers are joined up in a different order, this is known as structural isomerism. Structural isomerism is not a form of stereoisomerism, and is dealt with on a separate page You're Reading a Preview Why optical isomers? access with a free trial. Optical isomers are named likeUnlock thisfullbecause of their effect on plane polarised light. With Free Trialexist as two isomers simple substances which show Download optical isomerism known as enantiomers . A solution of one enantiomer rotates the plane of polarisation in a clockwise direction. This enantiomer is known as the (+) form. A solution of the other enantiomer rotates the plane of polarisation in an anti-clockwise direction. This enantiomer is known as the (-) form. So the otherwith enantiomer of alanine is known as or (-)alanine. Master your semester Scribd Free Foron 30this Days Sign up to vote title If the solutions are equally concentrated Read the amount of rotation & The New York Times Useful Not useful caused by the two isomers is exactly thesame - but in opposite Cancel anytime. Special offer fordirections. students: Only $4.99/month. When optically active substances are made in the lab, they often •
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If an achiral molecule (one with a plane of symmetry) looked in a mirro you would always find that by rotating the image in space, you could make the two look identical. It would be possible to superimpose the original molecule and its mirror image.
Some real examples of optical isomers Butan-2-ol The asymmetric carbon atom in a compound (the one with four differen groups attached) is often shown by a star.
It's extremely important to draw the isomers correctly. Draw one of the using standard bond notation to show the 3-dimensional arrangement around the asymmetric carbon You're atom. Thena draw Reading Previewthe mirror to show the examiner that you know what you are doing, and then the mirror image Unlock full access with a free trial.
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