0
AS 90944 Chemistry Notes Chemistry 1.3 External assessment
Demonstrate understanding of acids and bases 4 Credits
This achievement standard involves demonstrating understanding of atomic structure, particle theory and rates of reaction relating to acids and base properties, uses and reactions. A c h i e v e m e n t C r i t e r ia ia
Achievement
Demonstrate understanding of aspects of acids and bases.
Achievement with Merit
Demonstrate in-depth understanding of aspects of acids and bases.
Achievement with Excellence
Demonstrate comprehensive comprehensive understanding of aspects of acids and bases.
Aspects of acids and bases will be selected from: Atomic structure electron arrangement of atoms and monatomic ions of the first 20 elements (a periodic table will be provided) isotopes ionic bonding names and formulae of ionic compounds using a given table of ions.
Properties acids release hydrogen ions in water reactions (of acids with bases) to form salts. pH and effects on indicators. i ndicators.
Rates of reaction and particle theory. Uses neutralisation carbon dioxide formation salt formation.
Acids and bases are restricted to HCl, H2SO4, HNO3, metal oxides, hydroxides, carbonates and hydrogen hydrogen carbonates. Other acids may be included included in examination questions. The names and formulae of any such such acids will be given in the question question
1
Basic Atomic Structure Atoms are extremely small particles. Each atom is made up of a number of sub-atomic particles protons, neutrons and electrons. protons - positively charged particles found in the nucleus at the centre of the atom. neutrons - these particles, also found in the nucleus, have no charge so they are described as neutral. electrons - negatively charged particles that rapidly move around the nucleus.
The atomic number is number is the number of protons in the nucleus. This is also equal equal to the number of electrons around the nucleus of a neutral atom i.e. an atom is neutral overall as the number of protons and electrons are equal. Most of the mass mass of the atom is in the nucleus. nucleus. The mass number is number is the number of protons plus neutrons in an atom. e.g. sodium atoms have 11 protons and 12 neutrons in the nucleus. The atomic number of sodium sodium is 11 and the mass number number is 23 (i.e. 11 + 12). Electrons are found in SHELLS The first 2 electrons are always placed in the lowest energy level, the next 8 electrons go into the second energy level, the third row has 8 electrons (although the third energy level can in fact hold a maximum of 18 electrons) and the last level holds the last 2 (as we only have to know up to 20). The electron arrangement of an atom is a series of numbers stating how many electrons there are in each level e.g. fluorine, atomic number 9, has electron arrangement 2.7 while calcium, atomic number 20 has electron arrangement 2.8.8.2. The outermost shell shell (level) of electrons electrons is called the valence level, and it is these t hese electrons which are involved in chemical reactions e.g. there are 2 electrons in the valence shell of a calcium atom. Elements that are in the same Group of the periodic table (same column) have the same number of electrons in the outer shell and this is why they react in much the same way (and also why they form ions of the same charge. Ions An ion is formed when an atom atom gains or loses electrons. electrons. This happens because because the atom becomes more stable ifif it has a full outer shell. Atoms that are metals form ions by losing the valence shell electrons. electrons. Because the ions formed formed now have more protons (+) than electrons (-) these ions are positively charged (called cations) e.g. Sodium atom, with 11 protons and electron arrangement 2.8.1, loses one valence electron and forms Na+, the sodium ion which has 11 protons and 10 electrons (2.8).
2
Non-metal atoms form negatively charged ions (called anions) by gaining electrons t o form a complete outer shell e.g. oxygen atom (2.6.) gains 2 electrons to form the oxide ion, O2-, with electron arrangement 2.8. Note: The names of these monatomic non-metal ions all end in -ide, -ide, whereas the names of the metal ions are unchanged. Note: You are provided with a table of ions in the exam but you must know the names of them. Io n i c Co Co m p o u n d s
Ionic compounds are substances that are made up of positive and negative ions. The ions are held together by ionic bonds – bonds – the attraction between the oppositely charged ions. In any sample of the compound the total positive and total negative charge must be equal so that overall the sample is neutral. neutral. The formula of any ionic compound compound is written so that the ratio of positive to negative ions ensures that the sample is in fact neutral. When asked to write a formula for an ionic compound you must select the correct positive and negative ions by considering the relative size of the positive and negative charges on the ions decide the ratio that you will need in the formula when writing the formula for the compound leave out the charge on the ions Examples: 1. Magnesium oxide - contains magnesium ions, Mg2+ and oxide ions, O2-. Since there is the same number of positive charges as negative charges, the formula will only need one of each. ANSWER = MgO + 2. Sodium sulfate - contains sodium ions, Na , and sulfate ions, SO42-. Since the positive charge is 1+ but the negative charge is 2-, the charges are not equal. We will will need two Na+ ions to balance the SO42- ion. i.e. Na+ Na+ + SO42ANSWER = Na2SO4
3.
Calcium nitrate - contains calcium ions, Ca2+ and nitrate ions, NO3-. Again in this example size size of the charges is different - +2 and -1. Clearly we will need two of the nitrate ions to balance a single positive ion, but since this involves two of a group we will need to place brackets around the nitrate ion when writing this formula. ANSWER = Ca(NO3)2
Chemical Equations A chemical reaction can be written as an equation – equation – either using using words words or formulae. The substances that react (the reactants) are shown on the left hand side of the equation and those that are formed (products) are written on the right side of the equation. Example: Reaction of sodium hydroxide with hydrochloric acid to form sodium chloride and water. Word equation sodium hydroxide and hydrochloric acid Symbol equation NaOH + HCl
→ →
sodium chloride and water NaCl + H2O
3
When writing symbol equations it is important to write the correct formula of all substances before “balancing the equation”. To be balanced the equation must have the same elements present on both side side of the equations. In addition the number of atoms of of each element must be the same on both sides of the equation. When balancing an equation you must not change any of the individual formula! Acids Some important common acids are
Sulfuric acid, H2SO4, (also used in car batteries).
Hydrochloric acid, HCl, (also found in the stomach to aid digestion).
Nitric acid, HNO3, (used in the manufacture of fertilisers and explosives).
Acids are characterised characterised by their sour taste, their pH, by the colour change change when tested with acid-base indicators, and by the way they react with (1) metal carbonates and hydrogen carbonates (2) metal oxides and hydroxides. The behaviour of acids is due to the fact they produce H+ ions in water. It is these H+ ions that make the solution acidic. Bases Bases are substances substances that neutralise acids. This means that when a base is added added to an + acid it reacts with the H ions and removes them by converting them into neutral water. (H+ + OH- → H2O) The types of compounds that typically act as bases are metal oxides and hydroxides, metal carbonates and hydrogen carbonates (also called bicarbonates). Some common bases are (note they all end in “carbonate” or “oxide”)
sodium hydroxide, NaOH, also called caustic soda, used in oven cleaners.
calcium hydroxide, Ca(OH)2, a dilute aqueous solution is called limewater.
magnesium hydroxide, Mg(OH)2, found in indigestion tablets
sodium hydrogen carbonate (also called sodium bicarbonate), NaHCO3, sold as baking soda and used in antacid tablets, toothpaste and in baking powder sodium carbonate, Na2CO3, found in washing soda and also in antacid tablets. calcium carbonate, CaCO3, (marble, limestone, chalk) used in making lime and cement Aluminium oxide, Al2O3, (found in the ore bauxite) used to make aluminium metal for building materials and making drink cans. Iron(III) oxide Fe2O3 used to make iron metal.
Bases that dissolve in water are called alkalis. When they dissolve they produce solutions that contain hydroxide ions, OH -. pH scale The pH scale is a measure measure of how acidic or basic a solution is. The pH scale has a range from 0 to 14. Substances classed classed as acidic will form solutions in water water that have a pH less than 7, the lower the value the higher higher the acidity. Those solutions that are alkaline alkaline will form aqueous solutions solutions having a pH greater than 7 (less than 14). The closer the pH is to 14 the more alkaline the solution.
4
Indicators Indicators are substances that change colour depending on how acidic or alkaline a solution is. The most commonly commonly used used indicators are litmus and Universal Indicators. Indicators. Acidic substances dissolve in water to form solutions that turn litmus red. Note that blue litmus turns pink (or red) while red litmus stays red. red. Universal indicator (UI) is a solution that is a mixture of indicators and has a range of colours, from red through to blue (like the colours of the rainbow). Strongly acidic solutions turn universal indicator red (pH 1 - 2), while more weakly acidic solutions turn UI orange to yellow (pH 3 -6). Basic solutions turn red litmus blue and blue litmus stays blue. Universal indicator would change colours from blue to purple or violet (pH 9 -14) depending on whether the solution is weakly or strongly basic. Neutral solutions (e.g. water or solutions of salt, NaCl, or sugar) have a pH of 7 (universal indicator goes green) and if blue or red litmus is dipped in a neutral solution, no colour change occurs.
The pH of some common substances can be tested t ested using Universal Indicator. Reactions of metal oxides or metal hydroxides with acids (neutralisation reactions) Metal oxides and hydroxides are basic and will react with, and neutralise, acids. acids. When an acid reacts with a metal oxide or metal hydroxide (base) the products are the metal salt and water. water. Neither of these products products are gases, so there is no fizzing observed. If the acid and base are reacted reacted in the correct ratio the resulting solution solution will be neutral, pH = 7.
Overall the word equation for these neutralisation reactions can be written: metal oxide (base) + acid
metal salt + water
5
or
metal hydroxide (base) + acid
metal salt + water
The water must be evaporated in order to isolate the metal salt For a given acid, the type of salt is always the same. Sulfuric acid reacts to produce metal sulfates Hydrochloric acid reacts to produce metal chlorides Remember when writing an equation using formulae there are 5 steps: 1. Identify the type type of reaction reaction (acid + base OR acid + carbonate) carbonate) 2. Write a WORD equation 3. Write the formula of common common substances substances underneath the correct word (eg H2O H2O under “water”) 4. Use the table of of ions to write the more difficult formula correctly correctly (using a piece of scrap paper) 5. Balance the atoms atoms on each side side Reactions of metal carbonates or metal hydrogen carbonates with acid (another neutralisation reaction) Metal carbonates are basic because they react with, and neutralise, acids. Overall the word equation for the reaction between metal carbonate and acid is metal carbonate + acid
metal salt + water + carbon dioxide
or metal hydrogen carbonate + acid
metal salt + water + carbon dioxide
The reaction of a metal carbonate or bicarbonate with an acid will result in the acid being neutralised, and the reaction therefore has many everyday applications. Examples include sodium hydrogen carbonate (baking soda) in toothpaste will neutralise the acid saliva in your mouth.
antacid tablets will neutralise the excess acidity in your stomach that can lead to indigestion (the CO2 produced may well make you ‘burp’).
baking powder is a mixture of baking soda and some acidic powders which react together when liquid is added to the mixture producing CO2 which helps the cake to “rise”.
some recipes only require baking soda (sodium hydrogen carbonate) as the acid will be present in other ingredients such as dried fruit.
Sherbert contains a mixture of citric acid, tartaric acid and baking soda which react together in your mouth producing CO2 and hence a fizzy sensation.
statues made from limestone or marble (CaCO3) are destroyed by acid rain as the acid reacts with the t he calcium carbonate producing CO2, H2O and a soluble salt which washes away. a solution of baking soda can be used to neutralise the acid in a bee sting.
6
Rates of Reaction Some reactions are fast and and some are slow. When vinegar is added to baking soda, the reaction is fast and carbon dioxide dioxide is rapidly formed. Concrete setting is a slow reaction (taking many hours), while rusting is a very slow reaction taking several weeks or years. The rate of a chemical reaction describes describes how fast or slow a reaction is. Rate can be measured by finding out how fast a reactant is used up, or how fast a product is formed. This can be done in different ways eg by measuring: how fast a colour change occurs (if one of the reactants or products is coloured) the volume of gas produced after various times if one of the products is a gas the decrease in mass with time if one of the products is lost as a gas.
Depending on the speed of the reaction, r eaction, measurements can be made every few seconds, minutes, hours or even days. Consider the reaction of magnesium with hydrochloric acid to produce hydrogen gas and a solution of magnesium chloride. magnesium + hydrochloric acid
magnesium chloride + hydrogen
While the reaction is occurring we observe bubbles of gas being produced. As the reaction slows down down the rate at which bubbles appear appear decreases. When there are no more bubbles the reaction has stopped. The shorter the time taken for the magnesium to disappear (provided there is plenty of acid present) or for the bubbles to stop appearing then the faster the rate of reaction. Note: Note: When carrying out a reaction of a metal with with an acid, the metal must first of all be rubbed with sandpaper sandpaper to remove the metal oxide oxide coating from the surface. If this is not removed it may prevent the reaction occurring or simply slow down the rate of reaction.
Collision Theory A reaction occurs when when particles collide. Not all collisions between particles result in a chemical reaction. If a reaction is to occur, the particles must must collide with sufficient energy for a reaction to occur. This is the energy needed to break bonds in the reacting reacting particles so that the products can form. The reactants must also collide collide in the correct orientation (or position). So if a collision between between particles occurs occurs with sufficient energy and in the correct orientation a reaction will occur (it is an effective collision). collision). The more frequently effective collisions occur, the faster the rate of reaction. Factors affecting the rate of reaction It is possible to carry out investigations to find out how three differ ent factors can affect the rate of a reaction, by affecting the frequency (and possibly the energy) with which collisions occur. occur. The factors to be investigated are
concentration of reacting solutions
surface area of solids
temperature
the presence of a catalyst
1. Concentration As the concentration of of a solution increases, the number number of dissolved particles per volume increases. Increasing the concentration concentration therefore means there are more particles particles available in the same volume and therefore a greater chance that collisions will occur
7
between reacting reacting particles. particles. We can say that the frequency of collisions will increase which results in an increased rate of the chemical reaction. A sample of air contains about about 20% oxygen gas, most of the remaining 80% being nitrogen gas. This means that in a reaction involving oxygen, the reaction done in air will generally be slower than the same reaction done in PURE oxygen since the latter has a higher concentration and therefore there are more frequent collisions between reacting particles. Note that it is not correct to just say there are more collisions, collisions, as there would also be more conclusions if the quantities of reactants were increased without changing the concentration BUT this would not increase the rate, only make more product.
2. Surface area If a solid is chopped, crushed or ground into smaller pieces, the surface area of the sample is increased and more particles are exposed exposed at the surface. Thus powdered marble has a much greater surface area than marble chips. When the surface area is increased the rate of reaction is also increased since there are more particles exposed for collision with another reactant. reactant. Again it is the the frequency of collisions that increases leading to a faster rate of reaction. The total number number of collisions collisions does not increase assuming the amounts of reactants remains constant.
3. Temperature When temperature is increased the particles gain kinetic energy and this has two effects. Firstly they move faster and therefore therefore collide with other particles more more often. Secondly, and more importantly, since the particles have more energy, it is more likely that the collisions will have enough energy to result in a reaction. This means increasing the temperature results in both an increase in the frequency of collisions and, more importantly, in the effectiveness of the collisions so that there is a greater chance that the collision will result in a reaction (ie have enough energy to overcome the activation energy barrier.) This leads to a significant increase in the reaction rate.
