Chemical kinetics Lab Report
Contents Objective of the Lab ........................................................................................................................................................... 3 Information about the Dye and Bleach Structure ............................................................................................. 3 Reaction Equation.......................................................................................................................................................... 4 Rate Expression.............................................................................................................................................................. 5 Order and Rate Constant............................................................................................................................................. 5 Factors affecting the rate of reaction ..................................................................................................................... 6 The Ionic or Molecular Nature of the Reactants ............................................................................................... 6 The Concentration of the Reactants ....................................................................................................................... 7 The Temperature of the Reactants ......................................................................................................................... 7 Presence of a catalyst ................................................................................................................................................... 7 Initial rate method ......................................................................................................................................................... 8 Arrhenius Equation....................................................................................................................................................... 8 Procedure............................................................................................................................................................................... 8 For dermination of the order of the reaction ..................................................................................................... 8 For the determination of activation energy of oxidation .............................................................................. 9 Tables....................................................................................................................................................................................... 9 Table 1: Experimental runs for Kinetics Order of Reaction ......................................................................... 9 Table 2: Experimental runs for Kinetic Energy of Activation................................................................. 10 Graphs ...................................................................................................................................................................................10 For order reaction w.r.t blue dye ......................................................................................................................... 10 For order of reaction w.r.t bleach .........................................................................................................................10 For activation energy ................................................................................................................................................. 11 Discussion ............................................................................................................................................................................11 Observation: ................................................................................................................................................................... 11 Results: .............................................................................................................................................................................12 Conclusion: ..........................................................................................................................................................................12 References ...........................................................................................................................................................................12
What is Chemical kinetics Chemical Kinetics is the branch of chemistry which is concerned with the study of the rate of chemical reactions. The rate of a reaction is a measure of how quickly reactants are turned into products. This area of study directly complements the study of thermodynamics which focuses exclusively upon the energetic favorability of reactions. For example,
A + 2B → 2C + D Reaction rate Rate: Change in some variable per unit time. Reaction rate: Change in concentration per unit time. Rates are determined by monitoring concentration as a function of time. Rates are negative for reactants and positive quantities for products.
Objective of the Lab The objective of this lab is to determine the order of the reaction, and thereby to gain familiarity with rate laws . Here are some following objectives which we have to do in this lab.
To calculate the order of reaction with respect to dye. To calculate the order of reaction with respect to bleach. To calculate the average rate constant of the reaction. To calculate the activation energy for the oxidation of blue dye by sodium hypochlorite.
Information about the Dye and Bleach Structure
Bleach is a 5%(m/v) solution of sodium hypochlorite [naocl]. Blea ch neutralizers refer to chemicals that neutralize the harmful effects of sodium hypochlorite. Despite the benefits of bleach, bleach gives off toxic fumes, and it can be quite damaging to plumbing or fabrics if used in excess or if disposed inappropriately. Bleach needs to be neutralized after it has accomplished its intended purpose and prior to sink disposal. How to Make Bleach Neutralizer?
Sodium Metabisulfite Sodium thiosulfate, or Sodium sulfite, or 3 percent hydrogen peroxide Ascorbic Acid Water
Protective clothing, gloves and goggles Caution: Acids besides those mentioned above should not be used in an effort to
neutralize bleach. Structure of a Bleach
Blue dye is an organic compound with a distinctive blue color. Historically, blue dye was a natural dye extracted from the leaves of certain plants, and this process was important economically. A large percentage of blue dye produced today – several thousand tons each year – is synthetic. The structure of dye is given below
Reaction Equation
Rate Expression
The rate of a reaction can be represented either by the disappearance of reactants or the appearance of products. Since Blue dye is the only coloured species in the reaction, we can monitor the rate of the reaction shown above by recording the decrease in the colour of solution with time. Where the exponents a and b indicate the order of the
Reaction with respect to each reagent, and k is the overall rate constant for the reaction at room temperature. The overall rate of reaction is the sum of a and b. The square brackets,[], represent the concentration of the given reagent. Order and Rate Constant 6
Order with respect to dye is ‘first order’. Looking at the rate constants and the volume of bleach one can work out the order the reaction with respect to the bleach. If the reaction is first order with respect to the bleach, then doubling the volume of the bleach will double the rate constant. Therefore, the reaction is first order with respect to the bleach.
“The overall order of the reaction is second order.” Rate Constant 6:In chemical kinetics, a reaction rate constant or reaction rate coefficient, k,
quantifies the rate of a chemical reaction. For a reaction between reactants A and B to form product aA+bB→cC The reaction rate is often found to have the form
Here k(T) is the reaction rate constant that depends on temperature. [A] and [B] are the molar concentrations of substances A and B in moles per unit volume of solution, assuming the reaction is taking place throughout the volume of the solution. Overall Rate Law 5
The rate law or rate equation for a chemical reaction is an equation that links the reaction rate with the concentrations or pressures of the reactants and constant parameters. For many reactions the rate is given by a power law such as
Where [A] and [B] express the concentration of the species A and B, respectively. The exponents x and y are the partial reaction orders and must be determined experimentally.
They are not in general equal to the stoichiometric coefficients. The constant k is the reaction rate constant or rate coefficient of the reaction. The value of this coefficient k may depend on conditions such as temperature, ionic strength, surface area of an adsorbent, or light irradiation. For elementary reactions, which consist of a single step, the order equals the molecularity as predicted by collision theory. For multistep reactions, the order of each step equals the molecularity, but this is not generally true for the overall rate. The rate equation is a differential equation and can be integrated to obtain an integrated rate equation that links concentrations of reactants or products with time. For example, we took a chemical reaction of ions 6 I(aq) + bro3(aq) + 6 H+(aq) → 3 I2(aq) + Br(aq) + 3 H2O The reaction which we took as an example has the rate law Rate=k[I − ] x [bro3− ] y [H + ] z Factors affecting the rate of reaction 4
There are four main factors that can influence the rate of a chemical reaction. These factors are given below.
The Ionic or Molecular Nature of the Reactants The Concentration of the Reactants The Temperature of the Reactants Presence of a Catalyst
The Ionic or Molecular Nature of the Reactants
In general, chemical reactions that occur between ions in aqueous solution are extremely rapid: they happen almost instantaneously when the solutions containing the reacting ions are mixed. This is because the attractive forces between the ions are broken as the compound dissolves and the ions become hydrated by the water molecules. Additionally, most ions exert attractive forces equally in all directions. There is no preferred orientation for collision, and usually no covalent bonds need to be broken.
The Concentration of the Reactants
For many reactions, the rate will increase as the concentration of the reactants increases. If concentration is expressed as the number of moles of a reactant in a particular volume of solution, realize that increasing the reactant concentration means raising the number of reactant molecules that are contained in that same volume. This result is easily understood when the collision theory of reactions is considered. If there are twice as many Molecules contained in the same volume, there will be twice as many collisions occurring over the same time period. The Temperature of the Reactants
In nearly all cases, the rate of a reaction increases with increasing temperature. A rule of thumb for many reactions: Each time the temperature is increased by 10oc, the
rate of the reaction doubles. Presence of a catalyst
A substance that increases the rate of a reaction (without itself being used up by the reaction) is called a catalyst.Catalysts work by providing an alternate pathway for the reaction: one in which less energy is required in order to have an effective collision. Put another way, a catalyst lowers the Activation Energy that is required for reactants to be converted into products. Some catalysts can increase the rate of a great number of different reactions, while other catalysts, such as the enzymes in living systems, are specific to just one reaction, or even to just a single type of reactant molecule.
Initial rate method 3
Method of determining the rate-law expression by carrying out a reaction with different initial concentrations and analyzing the resultant changes in initial rates It makes the concentration of a reactant effectively constant. The important thing about initial Rates is that they correspond to the initial concentrations, which, unlike concentrations later on in the reaction, are under our control and can be kept constant. When more data points are available, the initial rate can be determined more accurately from a graph of concentration vs. Time The method of initial rates relies upon the assumption that the concentration of each reactant and the temperature do not change significantly over the time interval that the reaction rate is being measured Arrhenius Equation2
Arrhenius equation happens to be one of the most important equations in physical chemistry. This equation gives the temperature behavior of rate constants and thus tells us the reaction rate for varying temperatures. The Arrhenius Equation is given below: =
– /T
Where
A=frequency factor
E=Activation Energy J/mol or cal/mol
R=gas constant= 8.314 J/mol.K=1.987cal/mol.K
T=Absolute temperature
This equation is the combines the concept of activation energy and the Boltzman Distribution Law. The activation energy Ea has been with equated with a minimum energy that must be possessed by reacting molecules before the reaction will occur. From the kinetic theory the factor – /T gives the fraction of the collisions between the molecules. The magnitude of the rate constant is the function of the factor –E/RT thus the increased value of this ratio represents the lower rate constant and thus the reaction rate (hence there is a negative sign). If the rat constant is found at different temperatures, we can find the Ea from the graph between ln(k) and 1/T.
Procedure For dermination of the order of the reaction
Label three clean bruets as water ,blue dye and bleach
Add reagents to each tube from stock solutions
Record the exact amount of each solution used by recording in itial and final buret reading
Record the temperature of the solution
Add contents of bleach tube to blue dye solution tube and record the time uptill solution become colorless Repeat the same procedure three times and record time elapsed each time for same concentration Now,repeat the experiment for each concentration and record readings in each run
For the determination of activation energy of oxidation
Label three clean bruets as water ,blue dye and bleach
Add reagents to each tube from stock solutions
Record the exact amount of each solution used by recording initial and final buret reading
Note that sodium hypochlorite tends to decompose at temperatures above 313k
Use content temperature bath to ensure constant temperature of stock solutions
As solutions comes in equilibrium with the bath tube temperature , mix the contents of both tubes until reaction mixture become color less
Record elapsed time
Dispose off the reaction solution after it becomes colorless.
Perform the reaction at 274K,278K,295K,303K and 316k and record respective readings
Tables Table 1: Experimental runs for Kinetics Order of Reaction For hypochlorite ion Average time (sec)
Vol. Bleach used (ml)
Hypochlorite ion concentration [clo-]
Blue dye concentration use for the trail [ERG] M
182.67 5.00
0.25
5.09533E-06
7.50
0.37
5.09533E-06
10.00
0.50
5.09533E-06
12.50
0.62
5.09533E-06
111 102.34 77.67
Rate of the reaction
2.78942E08 4.59039E08 4.97915E08 6.56052E08
Log([clo-])
Log(Rate)
-0.60
-7.55
-0.43
-7.34
-0.30
-7.30
-0.20
-7.18
For blue dye Average time (sec)
Vol. Blue dye used (ml)
78.34 89.34 102.34 96.34
5 7 10 12
Blue dye concentration use for the trail [ERG] M 2.54767E-06 3.8215E-06 5.09533E-06 6.36917E-06
Rate of the reaction
Log([ERG])
3.25234E-08 4.2778E-08 4.97915E-08 6.61159E-08
-5.59 -5.42 -5.29 -5.20
Table 2: Experimental runs for Kinetic Energy of Activation Temperature (k) 278.15 274.15 295.15 303.75 316.15
Average time (sec) 224.34 324.34 102.34 43.34 34
Rate constant k 22.04164688 2.52570991 0.000197553 0.000384611 0.000490191
1/T (K)
Ln(k)
0.003595182 0.003647638 0.003388108 0.003292181 0.003163056
3.092934 0.926522 -8.5295 -7.86328 -7.62072
Graphs For order reaction w.r.t blue dye -7.00 -7.05
-5.59
-5.42
-5.29
-7.10 -7.15 -7.20 ) e t -7.25 a r ( g -7.30 o l
-7.35 -7.40 -7.45 -7.50 -7.55
log(blue dye)
For order of reaction w.r.t bleach
-5.20
Log(Rate)
-7.49 -7.37 -7.30 -7.18
-6.90 -0.60
-0.43
-0.30
-0.20
-7.00
-7.10 ) e t a r (
-7.20
g o l
-7.30
-7.40
-7.50
-7.60
log(bleach)
For activation energy 4
2
0 0.003595182 0.003647638 0.003388108 0.003292181 0.003163056 ) k ( n l
-2
-4
-6
-8
-10
1/T
Discussion: The reaction between blue dye and bleach was studied to determine the effects of concentration on the rate of reaction. The experiments are conducted based on the rate equation, R=k[clo -]m[ERG]n, where k is the rate constant while m and n are the reaction orders of clo - and ERG respectively. Observation: The blueness of the stock solutions was dependent on the concentration; after the addition of the bleach, each solution immediately began to lose its color. Color loss became slower as the percentage of stock solution decreased.
Gradual color loss seemed to appear to increase in proportion to the concentration of the bleach (higher concentration of bleach, faster color loss proceeds).
Results: According to the rate law, blue dye concentration does affect the rate of the reactions. The k values (rate constant) and rate of reactions vary slightly depending o n the concentration.
Conclusion: Changing the concentration of the dye does c hange the rate of reaction. As concentration increases, rate should also increase.Changing the concentration of the bleach did change the rate of reaction. AS concentration increases, the rate increased. Experimental errors could have added to the varied k values theoretically1 the k values should be same. Changes in the k values could be due to the human error in recording time using stop watch. Rate laws can also be predicted from theory Concentration Dependent Term:If a reaction has a number of completing path it will proceed primarily by the one of the least resistance, this path usually dominates, only knowledge of energies of all possible intermediates will allow prediction of the dominant path and its corresponding rate expression.Temperature Dependent Term If we know the mechanism of the reaction whether or not it is elementary, we may predict the frequency factor and activation energy term of the rate constant. Activation energy can be estimated from the transition-state theory but the reliability is poor.By varying the different concentration of blue dye in the reaction and then varying the concentration of the bleach, we come to know about the chemical kinetics of the reaction and its dependence on the reactant concentrations. The knowledge of reaction order helps us to predict the speed of reaction over time for example we could know the decay life of the radioactive elements as it is first order reaction. It has vast applications in industrial processes e.g. When the reaction is first order, there is no need to use excess amount of reactants whenever it comes to second order reactions it would become advantageous to use excess of the reactant.
References 1. Chemical reaction engineering,3 rd ed. Octave Levenspiel 2. Elements of chemical reaction engineering, scott fogler
3. Https://faculty.uca.edu/kdooley/f2015_chem4450_lab_iodine_clock_experim ent.pdf 4. 10_rates_fall_2010.pdf 5. Rate Law.pdf 6. kinetics.pdf