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Chemistry · Class 12 Science

Ch 3Chemical Kinetics — Class 12 Chemistry, concept-first.

Chemistry, at its heart, is the study of change: one substance with a definite set of properties turns into another substance with different properties. For any reaction a chemist wants to know three separate things about that change — whether it can happen at all, how far it will go, and how fast it happens.

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Key concepts

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Average Rate Of Reaction

Imagine you are watching a simple reaction:

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Chapter contents

The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.

Introduction

Chemistry, at its heart, is the study of change: one substance with a definite set of properties turns into another substance with different properties.

3.1

Rate of a Chemical Reaction

4 Q

Different reactions proceed at very different speeds. Ionic reactions, such as the precipitation of silver chloride when solutions of silver nitrate and sodium chloride are mixed, are essentially inst…

+Worked Examplesi2 questions
  1. Example 3.1From the concentrations of $C_4H_9Cl$ (butyl chloride) at different times given below, calculate the average rate of the reaction: $C_4H_9Cl…Free
  2. Example 3.2The decomposition of $N_2O_5$ in $CCl_4$ at 318 K has been studied by monitoring the concentration of $N_2O_5$ in the solution. Initially th…Preview
+Intext Questions 3.1-3.2i2 questions
  1. 3.1For the reaction $R \rightarrow P$, the concentration of a reactant changes from 0.03 M to 0.02 M in 25 minutes. Calculate the average rate…Free
  2. 3.2In a reaction, $2A \rightarrow$ Products, the concentration of A decreases from $0.5\ \text{mol L}^{-1}$ to $0.4\ \text{mol L}^{-1}$ in 10 m…Preview
3.2

Factors Influencing Rate of a Reaction

The rate measured for a reaction is not a fixed number attached to that reaction alone — it depends on the experimental conditions under which the reaction is carried out.

3.2.1

Dependence of Rate on Concentration

At a fixed temperature, the rate of a chemical reaction can depend on the concentration of one or more of the reactants — and sometimes even on the concentration of a product.

3.2.2

Rate Expression and Rate Constant

Take a general reaction

3.2.3

Order of a Reaction

In the general rate law

+Worked Examplesi2 questions
  1. Example 3.3Calculate the overall order of a reaction which has the rate expression (a) Rate $= k[A]^{1/2}[B]^{3/2}$ (b) Rate $= k[A]^{3/2}[B]^{-1}$Free
  2. Example 3.4Identify the reaction order from each of the following rate constants. (i) $k = 2.3\times10^{-5}\ \text{L mol}^{-1}\text{s}^{-1}$ (ii) $k =…Preview
3.2.4

Molecularity of a Reaction

The number of reacting species (atoms, ions, or molecules) that must collide simultaneously in an elementary reaction, in order to bring about that chemical change, is called the molecularity of the r…

+Intext Questions 3.3-3.4i2 questions
  1. 3.3For a reaction, $A + B \rightarrow$ Product; the rate law is given by, $r = k[A]^{1/2}[B]^2$. What is the order of the reaction?Free
  2. 3.4The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the r…Preview
3.3

Integrated Rate Equations

The differential rate law tells us how the rate of a reaction depends on the concentration of the reactant(s) at any instant.

3.3.1

Zero Order Reactions

A reaction is zero order when its rate does not change at all as the reactant is used up — the rate is proportional to the reactant concentration raised to the power zero. For a generic reaction

3.3.2

First Order Reactions

A reaction is first order when its rate is proportional to the first power of the concentration of one reactant. For a generic reaction

+Worked Examplesi2 questions
  1. Example 3.5The initial concentration of $N_2O_5$ in the following first order reaction $N_2O_5(g) \rightarrow 2NO_2(g) + \frac{1}{2}O_2(g)$ was $1.24\t…Free
  2. Example 3.6The following data were obtained during the first order thermal decomposition of $N_2O_5(g)$ at constant volume: $2N_2O_5(g) \rightarrow 2N_…Preview
3.3.3

Half-life of a Reaction

4 Q

The half-life of a reaction, written , is the time taken for the concentration of a reactant to fall to exactly one-half of its initial value.

+Worked Examplesi2 questions
  1. Example 3.7A first order reaction is found to have a rate constant, $k = 5.5\times10^{-14}\ \text{s}^{-1}$. Find the half-life of the reaction.Free
  2. Example 3.8Show that in a first order reaction, time required for completion of 99.9% is 10 times of half-life ($t_{1/2}$) of the reaction.Preview
+Intext Questions 3.5-3.6i2 questions
  1. 3.5A first order reaction has a rate constant $1.15\times10^{-3}\ \text{s}^{-1}$. How long will 5 g of this reactant take to reduce to 3 g?Free
  2. 3.6Time required to decompose $SO_2Cl_2$ to half of its initial amount is 60 minutes. If the decomposition is a first order reaction, calculate…Preview
3.4

Temperature Dependence of the Rate of a Reaction

Almost every chemical reaction speeds up when the temperature is raised. The decomposition of shows this sharply: the same fraction of material decomposes in about 12 minutes at , in 5 hours at , and…

+Worked Examplesi2 questions
  1. Example 3.9The rate constants of a reaction at 500 K and 700 K are $0.02\ \text{s}^{-1}$ and $0.07\ \text{s}^{-1}$ respectively. Calculate the values o…Free
  2. Example 3.10The first order rate constant for the decomposition of ethyl iodide by the reaction $C_2H_5I(g) \rightarrow C_2H_4(g) + HI(g)$ at 600 K is $…Preview
3.4.1

Effect of Catalyst

A catalyst is a substance that speeds up a reaction without itself undergoing any permanent chemical change in the process.

3.5

Collision Theory of Chemical Reactions

Collision theory, put forward by Max Trautz and William Lewis around 1916–18, digs deeper than the Arrhenius equation into why a reaction proceeds at the rate it does.

+Intext Questions 3.7-3.9i3 questions
  1. 3.7What will be the effect of temperature on rate constant?Free
  2. 3.8The rate of the chemical reaction doubles for an increase of 10 K in absolute temperature from 298 K. Calculate $E_a$.Preview
  3. 3.9The activation energy for the reaction $2HI(g) \rightarrow H_2 + I_2(g)$ is $209.5\ \text{kJ mol}^{-1}$ at 581 K. Calculate the fraction of…Preview

Summary

- Rate of Reaction: Defined as change in concentration per unit time: for . Average rate over an interval; instantaneous rate from slope of tangent.

Exercises

+Show 30 questions30 questions
  1. 3.1From the rate expression for the following reactions, determine their order of reaction and the dimensions of the rate constants. (i) $3NO(g…Free
  2. 3.2For the reaction: $2A + B \rightarrow A_2B$ the rate $= k[A][B]^2$ with $k = 2.0\times10^{-6}\ \text{mol}^{-2}\,\text{L}^{2}\,\text{s}^{-1}$…Free
  3. 3.3The decomposition of $NH_3$ on platinum surface is zero order reaction. What are the rates of production of $N_2$ and $H_2$ if $k = 2.5\time…Free
  4. 3.4The decomposition of dimethyl ether leads to the formation of $CH_4$, $H_2$ and $CO$ and the reaction rate is given by Rate $= k[CH_3OCH_3]^…Preview
  5. 3.5Mention the factors that affect the rate of a chemical reaction.Preview
  6. 3.6A reaction is second order with respect to a reactant. How is the rate of reaction affected if the concentration of the reactant is (i) doub…Preview
  7. 3.7What is the effect of temperature on the rate constant of a reaction? How can this effect of temperature on rate constant be represented qua…Preview
  8. 3.8In a pseudo first order reaction in water, the following results were obtained: | $t$/s | 0 | 30 | 60 | 90 | |---|---|---|---|---| | $[A]$/$…Preview
  9. 3.9A reaction is first order in A and second order in B. (i) Write the differential rate equation. (ii) How is the rate affected on increasing…Preview
  10. 3.10In a reaction between A and B, the initial rate of reaction ($r_0$) was measured for different initial concentrations of A and B as given be…Preview
  11. 3.11The following results have been obtained during the kinetic studies of the reaction: $2A + B \rightarrow C + D$ | Experiment | [A]/$\text{mo…Preview
  12. 3.12The reaction between A and B is first order with respect to A and zero order with respect to B. Fill in the blanks in the following table: |…Preview
  13. 3.13Calculate the half-life of a first order reaction from their rate constants given below: (i) $200\ \text{s}^{-1}$ (ii) $2\ \text{min}^{-1}$…Preview
  14. 3.14The half-life for radioactive decay of $^{14}C$ is 5730 years. An archaeological artifact containing wood had only 80% of the $^{14}C$ found…Preview
  15. 3.15The experimental data for decomposition of $N_2O_5$ $[2N_2O_5 \rightarrow 4NO_2 + O_2]$ in gas phase at 318 K are given below: | $t$/s | 0 |…Preview
  16. 3.16The rate constant for a first order reaction is $60\ \text{s}^{-1}$. How much time will it take to reduce the initial concentration of the r…Preview
  17. 3.17During nuclear explosion, one of the products is $^{90}Sr$ with half-life of 28.1 years. If 1 μg of $^{90}Sr$ was absorbed in the bones of a…Preview
  18. 3.18For a first order reaction, show that time required for 99% completion is twice the time required for the completion of 90% of reaction.Preview
  19. 3.19A first order reaction takes 40 min for 30% decomposition. Calculate $t_{1/2}$.Preview
  20. 3.20For the decomposition of azoisopropane to hexane and nitrogen at 543 K, the following data are obtained. | $t$ (sec) | P (mm of Hg) | |---|-…Preview
  21. 3.21The following data were obtained during the first order thermal decomposition of $SO_2Cl_2$ at a constant volume. $SO_2Cl_2(g) \rightarrow S…Preview
  22. 3.22The rate constant for the decomposition of $N_2O_5$ at various temperatures is given below: | T/°C | 0 | 20 | 40 | 60 | 80 | |---|---|---|--…Preview
  23. 3.23The rate constant for the decomposition of hydrocarbons is $2.418\times10^{-5}\ \text{s}^{-1}$ at 546 K. If the energy of activation is $179…Preview
  24. 3.24Consider a certain reaction $A \rightarrow$ Products with $k = 2.0\times10^{-2}\ \text{s}^{-1}$. Calculate the concentration of A remaining…Preview
  25. 3.25Sucrose decomposes in acid solution into glucose and fructose according to the first order rate law, with $t_{1/2} = 3.00$ hours. What fract…Preview
  26. 3.26The decomposition of hydrocarbon follows the equation $k = (4.5\times10^{11}\ \text{s}^{-1})\,e^{-28000\ K/T}$ Calculate $E_a$.Preview
  27. 3.27The rate constant for the first order decomposition of $H_2O_2$ is given by the following equation: $\log k = 14.34 - 1.25\times10^4\ K/T$ C…Preview
  28. 3.28The decomposition of A into product has value of $k$ as $4.5\times10^{3}\ \text{s}^{-1}$ at 10°C and energy of activation $60\ \text{kJ mol}…Preview
  29. 3.29The time required for 10% completion of a first order reaction at 298 K is equal to that required for its 25% completion at 308 K. If the va…Preview
  30. 3.30The rate of a reaction quadruples when the temperature changes from 293 K to 313 K. Calculate the energy of activation of the reaction assum…Preview

Answers to Intext Questions

These are the final answers printed at the end of the NCERT chapter. The book gives answers for some intext questions only — every question below links to our complete worked solution.

NCERT Exemplar

Higher-order thinking problems from the NCERT Exemplar.

+Show 66 questions66 questions
  1. Q1The role of a catalyst is to change ______________. (i) gibbs energy of reaction. (ii) enthalpy of reaction. (iii) activation energy of reac…Free
  2. Q2In the presence of a catalyst, the heat evolved or absorbed during the reaction ___________. (i) increases. (ii) decreases. (iii) remains un…Free
  3. Q3Activation energy of a chemical reaction can be determined by _____________. (i) determining the rate constant at standard temperature. (ii)…Free
  4. Q4A potential-energy profile (energy on the vertical axis, reaction coordinate on the horizontal axis) is drawn for a reaction. Reading from l…Preview
  5. Q5Consider a first order gas phase decomposition reaction given below: $A(g) \rightarrow B(g) + C(g)$ The initial pressure of the system befor…Preview
  6. Q6According to the Arrhenius equation the rate constant is $k = A\,e^{-E_a/RT}$. Four possible straight-line plots of $\ln k$ (vertical axis)…Preview
  7. Q7Consider the Arrhenius equation given below and mark the correct option. $k = A\, e^{-E_a/RT}$ (i) Rate constant increases exponentially wit…Preview
  8. Q8In the reaction of zinc with dilute hydrochloric acid, the volume of hydrogen gas collected is plotted against time. The curve starts at the…Preview
  9. Q9Which of the following statements is not correct about order of a reaction. (i) The order of a reaction can be a fractional number. (ii) Ord…Preview
  10. Q10For the same plot of the volume of hydrogen collected against time in the zinc + dilute HCl reaction (a curve through the origin, with volum…Preview
  11. Q11Which of the following statements is correct? (i) The rate of a reaction decreases with passage of time as the concentration of reactants de…Preview
  12. Q12Which of the following expressions is correct for the rate of reaction given below? $5Br^-(aq) + BrO_3^-(aq) + 6H^+(aq) \rightarrow 3Br_2(aq…Preview
  13. Q13Three reaction energy profiles (energy on the vertical axis versus reaction coordinate, each with a peak marked 'activated complex') are pro…Preview
  14. Q14Rate law for the reaction $A + 2B \rightarrow C$ is found to be Rate $= k[A][B]$ Concentration of reactant 'B' is doubled, keeping the conce…Preview
  15. Q15Which of the following statements is incorrect about the collision theory of chemical reaction? (i) It considers reacting molecules or atoms…Preview
  16. Q16A first order reaction is 50% completed in $1.26 \times 10^{14}$ s. How much time would it take for 100% completion? (i) $1.26 \times 10^{15…Preview
  17. Q17Compounds 'A' and 'B' react according to the following chemical equation. $A(g) + 2B(g) \rightarrow 2C(g)$ Concentration of either 'A' or 'B…Preview
  18. Q18Which of the following statement is not correct for the catalyst? (i) It catalyses the forward and backward reaction to the same extent. (ii…Preview
  19. Q19The value of rate constant of a pseudo first order reaction ____________. (i) depends on the concentration of reactants present in small amo…Preview
  20. Q20For the reversible reaction $A \rightleftharpoons B$, the concentrations of reactant and product change exponentially with time until equili…Preview
  21. Q21State a condition under which a bimolecular reaction is kinetically first order reaction.Preview
  22. Q22Write the rate equation for the reaction $2A + B \rightarrow C$ if the order of the reaction is zero.Preview
  23. Q23How can you determine the rate law of the following reaction? $2NO(g) + O_2(g) \rightarrow 2NO_2(g)$Preview
  24. Q24For which type of reactions, order and molecularity have the same value?Preview
  25. Q25In a reaction if the concentration of reactant A is tripled, the rate of reaction becomes twenty seven times. What is the order of the react…Preview
  26. Q26Derive an expression to calculate time required for completion of zero order reaction.Preview
  27. Q27For a reaction $A + B \rightarrow$ Products, the rate law is — Rate $= k[A][B]^{3/2}$ Can the reaction be an elementary reaction? Explain.Preview
  28. Q28For a certain reaction large fraction of molecules has energy more than the threshold energy, yet the rate of reaction is very slow. Why?Preview
  29. Q29For a zero order reaction will the molecularity be equal to zero? Explain.Preview
  30. Q30For a general reaction $A \rightarrow B$, the concentration of A is plotted against time and the plot is a straight line that falls steadily…Preview
  31. Q31The reaction between $H_2(g)$ and $O_2(g)$ is highly feasible yet allowing the gases to stand at room temperature in the same vessel does no…Preview
  32. Q32Why does the rate of a reaction increase with rise in temperature?Preview
  33. Q33Oxygen is available in plenty in air yet fuels do not burn by themselves at room temperature. Explain.Preview
  34. Q34Why is the probability of reaction with molecularity higher than three very rare?Preview
  35. Q35Why does the rate of any reaction generally decreases during the course of the reaction?Preview
  36. Q36Thermodynamic feasibility of the reaction alone cannot decide the rate of the reaction. Explain with the help of one example.Preview
  37. Q37Why in the redox titration of $KMnO_4$ vs oxalic acid, we heat oxalic acid solution before starting the titration?Preview
  38. Q38Why can't molecularity of any reaction be equal to zero?Preview
  39. Q39Why molecularity is applicable only for elementary reactions and order is applicable for elementary as well as complex reactions?Preview
  40. Q40Why can we not determine the order of a reaction by taking into consideration the balanced chemical equation?Preview
  41. Q41All energetically effective collisions do not result in a chemical change. Explain with the help of an example.Preview
  42. Q42What happens to most probable kinetic energy and the energy of activation with increase in temperature?Preview
  43. Q43Describe how does the enthalpy of reaction remain unchanged when a catalyst is used in the reaction.Preview
  44. Q44Explain the difference between instantaneous rate of a reaction and average rate of a reaction.Preview
  45. Q45With the help of an example explain what is meant by pseudo first order reaction.Preview
  46. Q46Rate law cannot be determined from balanced chemical equation if _______. (Two or more than two options may be correct.) (i) reverse reactio…Preview
  47. Q47Which of the following statements are applicable to a balanced chemical equation of an elementary reaction? (Two or more than two options ma…Preview
  48. Q48In any unimolecular reaction ______________. (Two or more than two options may be correct.) (i) only one reacting species is involved in the…Preview
  49. Q49For a complex reaction ______________. (Two or more than two options may be correct.) (i) order of overall reaction is same as molecularity…Preview
  50. Q50At high pressure the following reaction is zero order. $2NH_3(g) \xrightarrow[\text{Platinum catalyst}]{1130\ K} N_2(g) + 3H_2(g)$ Which of…Preview
  51. Q51During decomposition of an activated complex (Two or more than two options may be correct.) (i) energy is always released (ii) energy is alw…Preview
  52. Q52According to Maxwell Boltzmann distribution of energy, __________. (Two or more than two options may be correct.) (i) the fraction of molecu…Preview
  53. Q53In the graph showing Maxwell Boltzmann distribution of energy, ___________. (Two or more than two options may be correct.) (i) area under th…Preview
  54. Q54Which of the following statements are in accordance with the Arrhenius equation? (Two or more than two options may be correct.) (i) Rate of…Preview
  55. Q55Mark the incorrect statements. (Two or more than two options may be correct.) (i) Catalyst provides an alternative pathway to reaction mecha…Preview
  56. Q56Four plots are proposed for a zero order reaction: (i) reaction rate (vertical axis) versus time shown as a horizontal line (constant rate)…Preview
  57. Q57Four plots are proposed for a first order reaction: (i) half-life $t_{1/2}$ (vertical axis) versus initial concentration $[R]_0$ shown as a…Preview
  58. Q58Match each rate/concentration plot in Column I with the reaction order in Column II. More than one entry in Column I may correspond to the s…Preview
  59. Q59Match the statements given in Column I and Column II. Column I: (i) Catalyst alters the rate of reaction (ii) Molecularity (iii) Second half…Preview
  60. Q60Match the items of Column I and Column II. Column I: (i) Diamond (ii) Instantaneous rate (iii) Average rate Column II: (a) short interval of…Preview
  61. Q61Match the items of Column I and Column II. Column I: (i) Mathematical expression for rate of reaction (ii) Rate of reaction for zero order r…Preview
  62. Q62Assertion: Order of the reaction can be zero or fractional. Reason: We cannot determine order from balanced chemical equation. (i) Both asse…Preview
  63. Q63Assertion: Order and molecularity are same. Reason: Order is determined experimentally and molecularity is the sum of the stoichiometric coe…Preview
  64. Q64Assertion: The enthalpy of reaction remains constant in the presence of a catalyst. Reason: A catalyst participating in the reaction, forms…Preview
  65. Q65Assertion: All collision of reactant molecules lead to product formation. Reason: Only those collisions in which molecules have correct orie…Preview
  66. Q66Assertion: Rate constants determined from Arrhenius equation are fairly accurate for simple as well as complex molecules. Reason: Reactant m…Preview

CBSE Sample Papers

Questions from official CBSE sample papers.

+Show 2 questions2 questions
  1. Q1For the reaction A $\rightarrow$ B, the rate of reaction becomes three times when the concentration of A is increased by nine times. What is…Preview
  2. Q2A reaction is first order in A and second order in B (i) Write the differential rate equation. (ii) How is the rate affected on increasing t…Preview