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An aqueous acid solution of hydrogen peroxide, reacts with the iodide ions to form iodine. H2O2 (aq) + 2I (aq) + 2H+ (aq)→ I2(aq) + 2H2O(l). We can detect iodine though its colour. Iodine detection can be enhanced if we add the starch solution to it. For each of the reactants, the concentrations can vary and the time that it takes to produce some fixed amount of iodine is to be measured.
A way of measuring this rate is to time how long it takes to produce a certain amount of iodine. This can also be done using a fixed amount of sodium thiosulphate to the reaction mixture, which then reacts with a fixed amount of iodine.
I2(aq) + 2S2O3(aq) \(\rightarrow\) 2I (aq) + S4O6 (aq)
When all the sodium thiosulphate gets used up the iodine/starch colour suddenly appears in the solution. By the amount of thiosulphate that is added the amount of iodine produced can be easily calculated.
| Table of Contents |
Aim
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To study the effect of variation in concentration of iodide ions in the reaction rate between iodide ions and hydrogen peroxide at room temperature.
Theory
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Chemical kinetics is that branch of chemistry that deals with the study of reaction rates as well as their mechanisms. The study of kinetics also helps one describe the conditions by which we can alter the rates of reaction. Temperature, concentration, pressure and catalyst are the factors that affect the rate of a chemical reaction.
Ionic reactions are the type of reactions that occur very quickly. An example is, the precipitation of silver chloride which occurs very quickly when the aqueous solutions of silver nitrate and sodium chloride get mixed. Then there are some reactions that occur very slowly, as the rusting of iron in the presence of air and moisture.
Change in concentration of a reactant:
Change in concentration of a reactant or product is defined as the speed of a reaction in unit time. It is known as the rate of decrease of concentration of any of the reactants or the rate of increase of concentration of any product.
Let’s take a reaction,
R \( \rightarrow\) P
We assume the volume of the system to remain constant, 1 mole of the reactant is producing 1 mole of the product. [R1] and [P1] are concentrations of R and P respectively at a time t1 and then [R2] and [P2] are the concentrations of R and P respectively at the time t2, then,
How the reactant concentration depends on rate of chemical reaction?
Hydrogen peroxide acts as an oxidising agent which oxidises the iodide ions to iodine in an acidic medium.
This reaction gets monitored when a known volume of sodium thiosulphate solution and starch solution are added to the reaction mixture. The iodine salt which is liberated reacts with sodium thiosulphate and gets reduced to iodide ions.
When the thiosulphate ions get completely consumed, then the liberated iodine react with the starch solution and give a blue colour.
Material Required
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The material required for the experiment includes:
- 5 conical flasks of 250 mL volume
- Stop-watch
- 1 conical flask of 500 mL volume
- Trough
- Measuring cylinder; volume 100 mL
- Starch solution
- 0.1 M Potassium Iodide solution
- 2.5 M Sulphuric acid solution
- 0.04M Sodium thiosulphate solution
- 3% Hydrogen peroxide solution
Procedure
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The experiment follows the following procedure:
- Take a 500 mL conical flask.
- Add:
- 25 mL starch solution which is freshly prepared
- 25 mL 3% hydrogen peroxide
- 195 mL of distilled water
- 25 mL of 2.5 M H2SO4 solution
Then stir all this well and mark the flask as X.
- Then place this flask X in a water bath being maintained at room temperature.
- Label 4 conical flasks of volume 250 mL as Z, Q, R, S.
- Take 10 mL of 0.04 Na2S2O3 solution in P, and 10 mL of 0.1 M KI solution in Q, and 80 mL distilled water in flask R.
- Keep S flask aside to carry out the reaction.
- Now in flask S add the above solutions, shake the contents well and put it in a water bath.
- Take 10 mL of 0.04 M Na2S2O3 solution, 20 mL of 0.1M KI solution and 70 mL of distilled water in the conical flask marked Q.
- Shake the resulting solution well and place it in the same water bath.
- Next, take 10 mL of 0.04 M Na2S2O3 solution, 30 mL of 0.1 M KI solution and then 60 mL of distilled water in the conical flask labelled R.
- Shake flask R well and put it in water bath.
- In flask S, using a measuring cylinder pour 25 mL solution from flask X into it.
- Then add 25 mL solution P into it with constant stirring. Stop-watch should start when half the solution from flask P is transferred.
- Then keep flask S in water bath to maintain a constant temperature and write the time required for the blue colour to appear.
- Repeat the same steps for the solutions of flasks Q and R separately.
- Then repeat the experiment with solutions of flasks P, Q and R twice to calculate the average time.
- Record observations in the below table.
Observation Table
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| S. No. | System Composition | Time taken for appearance of the blue colour | Average time | |
| First try | Second try | |||
| 1. | 25 mL solution from flask X + 25 mL solution from flask P | |||
| 2. | 25 mL solution from flask X + 25 mL solution from flask Q | |||
| 3. | 25 mL solution from flask X + 25 mL solution from flask R | |||
Conclusion
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As the concentration of iodide ions increases the rate of reaction increases.
Precautions
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The precautions to be followed during the experiment include:
- Use freshly produced sodium thiosulphate and starch solution always.
- Kl solution concentration should be higher than sodium thiosulphate solution concentration.
- Record observations when the solution color turns blue.
Things to remember
- Chemical Kinetics is a branch of Chemistry that deals with chemical reaction, its factors and mechanism. It is closely related to the chemical reaction and physical process.
- Rate of a chemical reaction is the change in the concentration of any one of the reactants or products per unit time.
- Hydrogen peroxide oxidizes iodide ions to iodine in acidic medium H2O2 + 2I– + 2H+ \(\rightarrow\) 2H2O + I2
- Solutions used for the experiment like hydrogen peroxide and starch solution should always be freshly prepared.
- Remember the solutions whose concentrations need to be kept higher than others.
Sample Questions
Ques. Write the concentration of Kl and Na2SO3 solution used for the experiment and give the color of the complex formed. (2 Marks)
Ans. Kl = 0.1 M
Na2SO3 = 0.04 M
Color of the complex formed = blue
Ques. What are the factors influencing the rate of reaction? (3 Marks)
Ans. The rate of reaction is influenced by the:
- A solid reactants surface area.
- The concentration and pressure of any reactant
- Temperature
- Nature of the reactants
- Presence or absence of any catalyst
Ques. Explain how an increase in the concentration of iodide ion affects the rate of reaction. (2 Marks)
Ans. The higher the iodide ion concentration, the higher the increase in the rate of the first reaction, this means that thiosulfate gets depleted more quickly. This brings the change in colour which signals the end of reaction to occur more quickly.
Ques. Define the order of reaction and activation energy of a reaction. (3 Marks)
Ans. Order of a reaction is known as the sum of powers of the molar concentrations of the reacting species in the rate equation of the reaction. This can be a whole number, zero, a fraction, positive or negative. This can be experimentally determined and is meant for reaction and not for individual steps.
Activation energy of a reaction is defined as the minimum extra amount of energy which is absorbed by the reactant molecules to form the activated complex is called activation energy.
Ques. Rate constant for reaction of zero order in A = 0.0030 mol L-1 s-1. Then, how long it will take for the initial concentration of A to drop from 0.10 M to 0.075 M? (2 Marks)
Ans. Zero- order reaction
Ques. The thermal decomposition of HCO2H is a first order reaction with rate constant= 2.4 × 10-3 s-1 at a certain temperature. Then calculate how long does it take for three-fourths of initial quantity of HCO2 H to decompose(Log 0.25 = -0.6021) (3 Marks)
Ques. Differentiate between ‘order of reaction’ and ‘molecularity of reaction’. (3 Marks)
Ans. The differences between the order of reaction and the molecularity of reaction are:
| Order of reaction | Molecularity of reaction |
|---|---|
| This is defined as the sum of tire concentration terms on which the die rate of reaction actually depends. | This is known as the number of atoms, ions or molecules which must collide with each other simultaneously to result into a chemical reaction. |
| This can be a fraction or even zero. | This will always be a whole number. |
Ques. Determine the rate of law of the reaction given below. (5 Marks)
2NO(g) + O2(g) \(\rightarrow\) 2NO2 (g) (3 Marks)
Ans. We can determine the rate law for this experimentally. First, we can take the concentration of one of the reactants suppose O2 be taken in large excess and then the reaction rate is determined with respect to NO (g). Then for the other reactant, NO is taken in large excess and the reaction rate is given with respect to O2. The rate law expression for the reaction may be expressed as
Rate = k [NO] x [O2] y
Here, x and y are the real coefficients of the reaction.
Ques. Can we determine the order of a reaction by taking into consideration the balanced chemical equation? (5 Marks)
Ans. No, the sum of the stoichiometric coefficients of the reactants in a perfectly balanced chemical equation might or might not represent its order. Determining the order of reaction is possible only for elementary reactions which happen in a single step. For multiple step reactions, the coefficients of the reactants which are involved in the rate law are responsible for determining the order of the reaction. For example,
KClO3 + 6FeSO4 + 3H2SO4 \(\rightarrow\) KCl + 3Fe2(SO4)3 + 3H2O
The sum of the stoichiometric coefficients in this reaction is ten. But this reaction is of the second order which means that this is a multistep reaction and in the slowest or the rate-determining step the sum of the coefficients of the reacting species involved is only two.
Ques. Give an explanation about how all energetically effective collisions do not result in a chemical change. (5 Marks)
Ans. The collision theory states that the reaction rate is dependent generally on two factors. That is, the energy factor and orientation factor.
- According to the Arrhenius Equation, all the molecules which have energy greater than or equal to the activation energy collide to form products. But this postulate is not true for every reaction. We can clearly observe a significant amount of deviation in reactions involving complex molecules.
- Some molecules have sufficient activation energy and therefore, did not collide to form the product. But, only a few of them have effective collisions, the collisions which leads to the formation of products. Many scientists have found that the kinetic energy of these molecules is not the only parameter which governs the reaction.
Scientists have therefore concluded that only the molecules which have the threshold activation energy and a proper orientation during their collision will form products.
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