Chemistry · Class 12 Science
Ch 8Chemical Kinetics — Class 12 Chemistry, concept-first.
Chemical kinetics is concerned with the speed of a chemical reaction -- how quickly reactants disappear and products appear -- and, later in this chapter, with the molecular-level reasons a reaction proceeds at the particular speed it does.
Key concepts
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Average Rate Of Reaction
Imagine you are watching a simple reaction:
Most relevant Q&A
- For the reaction $R \rightarrow P$, the concentration of $R$ falls from $0.500\ \text{mol L}^{-1}$ to $0.400\ \text{mol L}^{-1}$ in $50\ \te…Free
- In the gas-phase decomposition $2\text{N}_2\text{O}_5(g) \rightarrow 4\text{NO}_2(g) + \text{O}_2(g)$, the concentration of $\text{N}_2\text…Free
- For the reaction $2A \rightarrow 3B$, the rate of disappearance of $A$ at a certain instant is $6.0\times 10^{-3}\ \text{mol L}^{-1}\text{s}…Free
- In the reaction 3A -> 2B, the rate of production of B is +d[B]/dt when the rate of reaction of A is (a) -(1/2) d[A]/dt (b) -(2/3) d[A]/dt (c…Preview
In previous exams
How often this chapter’s concepts have been examined — real appearance data, never estimated.
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Rate of a Chemical Reaction: Average and Instantaneous Rate
Chemical kinetics is concerned with the speed of a chemical reaction -- how quickly reactants disappear and products appear -- and, later in this chapter, with the molecular-level reasons a reaction p…
Factors Affecting the Rate of a Reaction
Three conditions, above all others, determine how fast a given reaction proceeds: the concentration of the reactants, the temperature at which the reaction is carried out, and whether a catalyst is pr…
Rate Law and the Order of a Reaction
The rate law. For a reaction, the rate law is the experimentally determined equation expressing the instantaneous rate of the reaction as a function of the molar concentrations of the reactants, each…
Molecularity of a Reaction
While order is found purely from experimental rate data, molecularity is a theoretical concept that comes from the reaction mechanism -- the sequence of individual molecular-level steps, called elemen…
Integrated Rate Equation for Zero Order Reactions
A reaction is zero order in a reactant if its rate does not depend on at all:
Integrated Rate Equation for First Order Reactions
A reaction is first order in a reactant if its rate is directly proportional to raised to the first power:
Half-Life of a Reaction
The half-life of a reaction, written , is the time required for the concentration of a reactant to fall to exactly half of its value at the start of that interval.
Collision Theory of Chemical Reactions
Collision theory, developed principally by Max Trautz and William Lewis, gives a molecular-level picture of why a reaction proceeds at the particular rate it does.
Activation Energy
Activation energy, , is the minimum extra energy, over and above the average energy the reactant molecules already possess, that colliding molecules must have in order to react -- it is the energy bar…
Temperature Dependence of the Rate of a Reaction: The Arrhenius Equation
The Arrhenius equation. Svante Arrhenius proposed, on the basis of extensive experimental measurements of how rate constants vary with temperature, that the rate constant of almost every reaction obey…
Calculating Activation Energy from the Arrhenius Equation
When rate-constant data are available at only two temperatures, rather than a full series suitable for a graph, the Arrhenius equation can still be used to extract the activation energy directly, with…
Catalysis: Homogeneous and Heterogeneous
A catalyst is a substance that changes the rate of a chemical reaction -- in almost every practically useful case, increases it -- without itself undergoing any net, permanent chemical change over the…
Enzyme Catalysis
Enzymes are biological catalysts -- large protein molecules, produced by living cells, that catalyze the countless biochemical reactions of metabolism with a speed, and under such remarkably mild cond…
Summary
This chapter developed chemical kinetics from first principles. It began by defining average and instantaneous rate, and the single unambiguous "rate of reaction" obtained by dividing each species' ra…
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
+−Show 11 questionsHide questions11 questions
- Q1What is meant by a zero order reaction? Give an example of such a reaction. Establish the integrated rate equation for a zero order reaction…Preview
- Q2(i) Draw the graph of half-life period (t1/2) versus initial concentration of reactant ([A]0) for a zero order reaction. Give reasons in fav…Preview
- Q3What is the Order of chemical reaction? Deduce the mathematical expression for half-life period of a First Order Decomposition Reaction in w…Preview
- Q4What is the molecularity of the reaction? Establish the integrated rate equation of the First Order Reaction. (2+3) **OR** State with indivi…Preview
- Q5In the reaction 3A -> 2B, the rate of production of B is +d[B]/dt when the rate of reaction of A is (a) -(1/2) d[A]/dt (b) -(2/3) d[A]/dt (c…Preview
- Q6State law of Mass action. **OR** What is active mass?Preview
- Q7(i) For a chemical reaction R -> P, the plot of concentration (R) versus time (t) is given as shown (see figure). Predict the order of the r…Preview
- Q8(i) Establish the integrated form of rate equation of first order reaction. (ii) Show that in a first order reaction, time required for comp…Preview
- Q9(i) Write two differences between order and molecularity of a chemical reaction. [2] (ii) After 20 years of radioactive decay 0.0625 g remai…Preview
- Q10Show that half-life time of a first order reaction is independent of initial concentration of the reactant.Preview
- Q11i) Mention two differences between order and molecularity of chemical reaction. ii) 2 N2O5(g) -> 4 NO2(g) + O2(g). For this reaction the con…Preview
More questions
34 Q+−Show 17 questionsHide questions17 questions
- Example 1For the reaction $R \rightarrow P$, the concentration of $R$ falls from $0.500\ \text{mol L}^{-1}$ to $0.400\ \text{mol L}^{-1}$ in $50\ \te…Free
- Example 2In the gas-phase decomposition $2\text{N}_2\text{O}_5(g) \rightarrow 4\text{NO}_2(g) + \text{O}_2(g)$, the concentration of $\text{N}_2\text…Free
- Example 3Explain, using the idea of molecular collisions, why increasing the concentration of a reactant generally increases the rate of a reaction.Free
- Example 4Explain why the rate of most chemical reactions increases sharply with even a modest rise in temperature, although the average kinetic energ…Preview
- Example 5The following data were obtained for the reaction $A + B \rightarrow \text{Product}$ at constant temperature: | Experiment | $[A]$ (mol L$^{…Preview
- Example 6The following data were obtained for a reaction of the type $A \rightarrow \text{Product}$: | Experiment | $[A]$ (mol L$^{-1}$) | Initial ra…Preview
- Example 7Distinguish between the order of a reaction and its molecularity. Why can the order of a reaction be zero or a fraction, while molecularity…Preview
- Example 8State the molecularity of each of the following elementary reactions: (a) $\text{NH}_4\text{NO}_2 \rightarrow \text{N}_2 + 2\text{H}_2\text{…Preview
- Example 9A reaction is found to be zero order in reactant $R$. Its concentration falls from $2.00\times 10^{-2}\ \text{mol L}^{-1}$ to $1.50\times 10…Preview
- Example 10For the zero order reaction of the previous example, with $k = 2.0\times 10^{-4}\ \text{mol L}^{-1}\text{min}^{-1}$ and initial concentratio…Preview
- Example 11A first order reaction has a rate constant of $1.15\times 10^{-3}\ \text{s}^{-1}$. How long will $5\ \text{g}$ of the reactant take to reduc…Preview
- Example 12A first order reaction has rate constant $k = 2.31\times 10^{-3}\ \text{s}^{-1}$. Calculate its half-life.Preview
- Example 13According to collision theory, why do not all molecular collisions between reactant molecules result in a chemical reaction? State the two c…Preview
- Example 14Describe, in words, the potential energy profile for an exothermic reaction $A \rightarrow B$, and use it to define activation energy and th…Preview
- Example 15For a certain first order reaction, the Arrhenius pre-exponential factor is $A = 5\times 10^{13}\ \text{s}^{-1}$ and the activation energy i…Preview
- Example 16A first order reaction is 50% complete in $30\ \text{minutes}$ at $300\ \text{K}$, and 50% complete in $10\ \text{minutes}$ at $320\ \text{K…Preview
- Example 17Classify each of the following catalyzed reactions as homogeneous or heterogeneous catalysis, and justify your answer: (a) hydrolysis of an…Preview
+−Show 17 questionsHide questions17 questions
- Q18Explain, in terms of activation energy, how a catalyst increases the rate of a reaction without itself being permanently consumed.Free
- Q19For the reaction $2A \rightarrow 3B$, the rate of disappearance of $A$ at a certain instant is $6.0\times 10^{-3}\ \text{mol L}^{-1}\text{s}…Free
- Q20Starting from the zero order integrated rate equation $[R] = [R]_0 - kt$, derive an expression for the half-life $t_{1/2}$ of a zero order r…Free
- Q21A first order reaction has a half-life of $20\ \text{minutes}$. Calculate its rate constant.Preview
- Q22A radioactive-decay-style first order process has a half-life of $30\ \text{years}$. What fraction of the original quantity remains after $9…Preview
- Q23A first order reaction has $k = 1.155\times 10^{-2}\ \text{min}^{-1}$. Calculate the time required for the reaction to reach 99% completion.Preview
- Q24A first order reaction has $k = 4.00\times 10^{-2}\ \text{min}^{-1}$. Calculate the time taken for the concentration to fall (a) from $0.80\…Preview
- Q25A first order reaction has rate constant $k = 3.46\times 10^{-5}\ \text{s}^{-1}$ at $298\ \text{K}$ and activation energy $E_a = 50\ \text{k…Preview
- Q26The rate constant of a reaction doubles when the temperature is raised from $300\ \text{K}$ to $310\ \text{K}$. Calculate the activation ene…Preview
- Q27A catalyst lowers the activation energy of a reaction from $100\ \text{kJ mol}^{-1}$ to $80\ \text{kJ mol}^{-1}$ at $300\ \text{K}$, without…Preview
- Q28Explain what is meant by the specificity of enzyme catalysis, and how this differs from the behaviour of most ordinary (inorganic) catalysts…Preview
- Q29Explain (a) why the rate of an enzyme-catalyzed reaction rises with increasing substrate concentration at low substrate concentration but be…Preview
- Q30The rate constant of a certain reaction has units $\text{mol L}^{-1}\text{s}^{-1}$. What is the order of the reaction? Justify using the gen…Preview
- Q31The rate constant of a certain reaction has units $\text{L mol}^{-1}\text{s}^{-1}$. What is the order of the reaction?Preview
- Q32The acid-catalyzed hydrolysis of ethyl acetate, $\text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O} \xrightarrow{\text{H}^+} \text{CH}_…Preview
- Q33The thermal decomposition of acetaldehyde, $\text{CH}_3\text{CHO} \rightarrow \text{CH}_4 + \text{CO}$, is found experimentally to obey the…Preview
- Q34State whether the acid-catalyzed inversion of cane sugar, $\text{C}_{12}\text{H}_{22}\text{O}_{11} + \text{H}_2\text{O} \xrightarrow{\text{H…Preview