Chemistry · Class 12 Science
Ch 7Chemical Kinetics — Class 12 Chemistry, concept-first.
You already know, from Class XI thermodynamics, how to predict whether a reaction CAN happen under a given set of conditions -- but thermodynamics is completely silent on how FAST it happens.
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Integrated Rate Law for First Order Reactions
A first order reaction has . Integrating gives , or in base-10 form, -- the single most-used formula in this chapter.
Most relevant Q&A
- For a first order reaction $A \rightarrow B$, the rate constant is $x\ \text{min}^{-1}$. If the initial concentration of A is 0.01 M, the co…Free
- Describe the graphical representation of first order reaction.Preview
- Cyclopropane $\rightarrow$ Propene; this reaction follows first order kinetics. The rate constant at a particular temperature is $2.303\time…Preview
- For a first order reaction, the rate constant is $6.909\ \text{min}^{-1}$. The time taken for 75% conversion in minutes is (a) $\left(\dfrac…Preview
- The decomposition of $Cl_2O_7$ at 500 K in the gas phase to $Cl_2$ and $O_2$ is a first order reaction. After 1 minute at 500 K, the pressur…Preview
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
You already know, from Class XI thermodynamics, how to predict whether a reaction CAN happen under a given set of conditions -- but thermodynamics is completely silent on how FAST it happens.
Rate of a Chemical Reaction
Why kinetics, when thermodynamics already exists? Thermodynamics (Class XI, Unit 7) tells you whether a reaction CAN happen at all under given conditions -- but it is completely silent on how long it…
Stoichiometry and Rate of a Reaction
Why stoichiometry complicates a single 'the' rate. For (matching coefficients on both sides), the rate of A's disappearance and B's appearance are numerically identical, so there is no ambiguity in ca…
Average and Instantaneous Rate
Two different meanings of 'the rate'. Take the isomerisation of cyclopropane to propene at 780 K, followed by measuring at 5-minute intervals: 2.00, 1.67, 1.40, 1.17, 0.98, 0.82, 0.69 mol L at t = 0,…
Rate Law and Rate Constant
The rate law is an empirical fact, not a stoichiometric guess. For the general reaction , the observed relationship between rate and concentration is written as Here k, the rate constant, is a fixed p…
Molecularity
From rate laws to mechanism. Kinetics does not stop at measuring an overall rate law -- it also aims to propose a believable step-by-step REACTION MECHANISM that could produce that rate law.
The Integrated Rate Equation
The problem with the differential rate law. The rate law , written as , is a differential equation -- it tells you the INSTANTANEOUS rate at whatever concentration currently prevails, but it cannot di…
Integrated Rate Law for a First Order Reaction
Setting up the integral. A first order reaction has for , so Integrating both sides between (concentration ) and (concentration ): Converting the natural log to a base-10 log (multiply by 2.303, since…
Pseudo First Order Reaction
The problem pseudo first order kinetics solves. A reaction that is genuinely second (or higher) order, involving two DIFFERENT reactants, is awkward to follow kinetically -- you would need to measure…
Integrated Rate Law for a Zero Order Reaction
Setting up the integral. A zero order reaction has a rate that does NOT depend on reactant concentration at all -- the book notes such reactions are genuinely rare, but the derivation is instructive.
General Rate Equation for an nth Order Reaction
One formula that covers every order except exactly one. For a single reactant obeying with any order (the case needs the separate logarithmic derivation of Section 7.5.1, because integrating produces…
Half Life Period of a Reaction
Definition. The half life of a reaction, , is the time it takes for the reactant's concentration to fall to exactly HALF its initial value.
Collision Theory
The core idea. Proposed independently by Max Trautz (1916) and William Lewis (1918), and grounded in the kinetic theory of gases: a chemical reaction happens because reacting molecules physically COLL…
Arrhenius Equation -- The Effect of Temperature on Reaction Rate
The empirical starting point. Reaction rates generally rise as temperature rises (with only rare exceptions), though by how much varies reaction to reaction; a widely quoted rough rule near room tempe…
Factors Affecting the Reaction Rate
Rounding up the whole chapter. Every rate-affecting influence discussed across the chapter -- and one new one, surface area -- is drawn together here into a single list of five factors that experiment…
Nature and State of the Reactant
Chemical identity changes the energy cost of reacting. Every reaction involves breaking some of the reactant's existing bonds and forming new ones in the product; the net energy this costs is fixed by…
Concentration of the Reactants
Why more concentration means more rate. Collision theory (Section 7.7) already supplies the explanation: packing more reactant particles into the same volume raises the CHANCE that any two of them col…
Effect of Surface Area of the Reactant
Surface area only matters for heterogeneous (solid-involving) reactions. When a solid reacts with a liquid or a gas, only the molecules sitting right at the solid's exposed SURFACE can actually take p…
Effect of Presence of a Catalyst
Catalysts act differently from the other four factors. Concentration, temperature and surface area all speed a reaction up only to a limited extent, and only by changing conditions the chemist directl…
EVALUATION
55 Q+−Choose the Best Answer25 questions
- Q1For a first order reaction $A \rightarrow B$, the rate constant is $x\ \text{min}^{-1}$. If the initial concentration of A is 0.01 M, the co…Free
- Q2A zero order reaction $X \rightarrow \text{Product}$, with an initial concentration 0.02 M has a half life of 10 min. If one starts with con…Free
- Q3Among the following graphs showing variation of rate constant with temperature (T) for a reaction, the one that exhibits Arrhenius behavior…Free
- Q4For a first order reaction $A \rightarrow \text{product}$ with initial concentration $x\ \text{mol L}^{-1}$, has a half life period of 2.5 h…Preview
- Q5For the reaction $2NH_3 \rightarrow N_2 + 3H_2$, if $-\dfrac{d[NH_3]}{dt}=k_1[NH_3]$, $\dfrac{d[N_2]}{dt}=k_2[NH_3]$, $\dfrac{d[H_2]}{dt}=k_…Preview
- Q6The decomposition of phosphine ($PH_3$) on tungsten at low pressure is a first order reaction. It is because the (NEET) (a) rate is proporti…Preview
- Q7For a reaction $\text{Rate}=k[\text{acetone}]^{3/2}$ then unit of rate constant and rate of reaction respectively is (a) $(\text{mol L}^{-1}…Preview
- Q8The addition of a catalyst during a chemical reaction alters which of the following quantities? (NEET) (a) Enthalpy (b) Activation energy (c…Preview
- Q9Consider the following statements: (i) increase in concentration of the reactant increases the rate of a zero order reaction. (ii) rate cons…Preview
- Q10In a reversible reaction, the enthalpy change and the activation energy in the forward direction are respectively $-x\ \text{kJ mol}^{-1}$ a…Preview
- Q11What is the activation energy for a reaction if its rate doubles when the temperature is raised from 200 K to 400 K? ($R=8.314\ \text{JK}^{-…Preview
- Q12Cyclopropane $\rightarrow$ Propene; this reaction follows first order kinetics. The rate constant at a particular temperature is $2.303\time…Preview
- Q13For a first order reaction, the rate constant is $6.909\ \text{min}^{-1}$. The time taken for 75% conversion in minutes is (a) $\left(\dfrac…Preview
- Q14In a first order reaction $x \rightarrow y$; if k is the rate constant and the initial concentration of the reactant x is 0.1 M, then the ha…Preview
- Q15Predict the rate law of the following reaction based on the data given below: $2A+B \rightarrow C+3D$ | Reaction number | [A] (M) | [B] (M)…Preview
- Q16Assertion: rate of reaction doubles when the concentration of the reactant is doubled if it is a first order reaction. Reason: rate constant…Preview
- Q17The rate constant of a reaction is $5.8\times10^{-2}\ \text{s}^{-1}$. The order of the reaction is (a) First order (b) zero order (c) Second…Preview
- Q18For the reaction $N_2O_5(g) \rightarrow 2NO_2(g) + \dfrac{1}{2}O_2(g)$, the value of rate of disappearance of $N_2O_5$ is given as $6.5\time…Preview
- Q19During the decomposition of $H_2O_2$ to give dioxygen, 48 g $O_2$ is formed per minute at a certain point of time. The rate of formation of…Preview
- Q20If the initial concentration of the reactant is doubled, the time for half reaction is also doubled. Then the order of the reaction is (a) Z…Preview
- Q21In a homogeneous reaction $A \rightarrow B+C+D$, the initial pressure was $P_0$ and after time t it was P. The expression for rate constant…Preview
- Q22If 75% of a first order reaction was completed in 60 minutes, 50% of the same reaction under the same conditions would be completed in (a) 2…Preview
- Q23The half life period of a radioactive element is 140 days. After 560 days, 1 g of element will be reduced to (a) $\dfrac{1}{2}$ g (b) $\dfra…Preview
- Q24The correct difference between first and second order reactions is that (NEET) (a) A first order reaction can be catalysed; a second order r…Preview
- Q25After 2 hours, a radioactive substance becomes $\left(\dfrac{1}{16}\right)^{\text{th}}$ of the original amount. Then the half life (in min)…Preview
+−Write Brief Answer30 questions
- Q1Define average rate and instantaneous rate.Free
- Q2Define rate law and rate constant.Free
- Q3Derive integrated rate law for a zero order reaction $A \rightarrow \text{product}$.Free
- Q4Define half life of a reaction. Show that for a first order reaction half life is independent of initial concentration.Preview
- Q5What is an elementary reaction? Give the differences between order and molecularity of a reaction.Preview
- Q6Explain the rate determining step with an example.Preview
- Q7Describe the graphical representation of first order reaction.Preview
- Q8Write the rate law for the following reactions. (a) A reaction that is 3/2 order in x and zero order in y. (b) A reaction that is second ord…Preview
- Q9Explain the effect of catalyst on reaction rate with an example.Preview
- Q10The rate law for a reaction of A, B and L has been found to be $\text{rate}=k[A]^2[B][L]^{3/2}$. How would the rate of reaction change when…Preview
- Q11The rate of formation of a dimer in a second order reaction is $7.5\times10^{-3}\ \text{mol L}^{-1}\text{s}^{-1}$ at 0.05 mol $L^{-1}$ monom…Preview
- Q12For a reaction $x+y+z \rightarrow \text{products}$ the rate law is given by $\text{rate}=k[x]^{3/2}[y]^{1/2}$. What is the overall order of…Preview
- Q13Explain briefly the collision theory of bimolecular reactions.Preview
- Q14Write Arrhenius equation and explain the terms involved.Preview
- Q15The decomposition of $Cl_2O_7$ at 500 K in the gas phase to $Cl_2$ and $O_2$ is a first order reaction. After 1 minute at 500 K, the pressur…Preview
- Q16Give two examples for zero order reaction.Preview
- Q17Explain pseudo first order reaction with an example.Preview
- Q18Identify the order for the following reactions (i) Rusting of Iron (ii) Radioactive disintegration of $_{92}U^{238}$ (iii) $2A+3B \rightarro…Preview
- Q19A gas phase reaction has energy of activation $200\ \text{kJ mol}^{-1}$. If the frequency factor of the reaction is $1.6\times10^{13}\ \text…Preview
- Q20For the reaction $2x+y \rightarrow L$ find the rate law from the following data. | [x] (M) | [y] (M) | rate ($\text{M s}^{-1}$) | |---|---|-…Preview
- Q21How do concentrations of the reactant influence the rate of reaction?Preview
- Q22How do nature of the reactant influence rate of reaction.Preview
- Q23The rate constant for a first order reaction is $1.54\times10^{-3}\ s^{-1}$. Calculate its half life time.Preview
- Q24The half life of the homogeneous gaseous reaction $SO_2Cl_2 \rightarrow SO_2+Cl_2$ which obeys first order kinetics is 8.0 minutes. How long…Preview
- Q25The time for half change in a first order decomposition of a substance A is 60 seconds. Calculate the rate constant. How much of A will be l…Preview
- Q26A zero order reaction is 20% complete in 20 minutes. Calculate the value of the rate constant. In what time will the reaction be 80% complet…Preview
- Q27The activation energy of a reaction is $22.5\ \text{k Cal mol}^{-1}$ and the value of rate constant at $40^\circ C$ is $1.8\times10^{-5}\ s^…Preview
- Q28Benzene diazonium chloride in aqueous solution decomposes according to the equation $C_6H_5N_2Cl \rightarrow C_6H_5Cl+N_2$. Starting with an…Preview
- Q29From the following data, show that the decomposition of hydrogen peroxide is a reaction of the first order: | t (min) | 0 | 10 | 20 | |---|-…Preview
- Q30A first order reaction is 40% complete in 50 minutes. Calculate the value of the rate constant. In what time will the reaction be 80% comple…Preview
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
+−Show 25 questionsHide questions25 questions
- Q1The half life period of a first order reaction is 10 minutes. Then its rate constant is : (a) $6.93\times10^{2}$ min$^{-1}$ (b) $0.693\times…Preview
- Q2What is a pseudo first order reaction ? Give an example.Preview
- Q3Write the Arrhenius equation and explain the terms.Preview
- Q4What are the characteristics of order of a reaction ?Preview
- Q5The sum of the powers of the concentration terms that occur in the rate equation is called : (a) rate (b) molecularity (c) rate constant (d)…Preview
- Q6The rate constants of the forward reaction and reverse reaction are 0.05 $sec^{-1}$ and 2 $sec^{-1}$ respectively, then $K_c$ is : (a) 0.035…Preview
- Q7Identify to which type does the following complex reactions belongs to : (i) dehydration of 2-methyl-2-butanol (ii) isomerisation of cyclopr…Preview
- Q8Write any two characteristics of first order reaction.Preview
- Q9Explain the experimental determination of rate constant for the decomposition of $H_2O_2$ in aqueous solution.Preview
- Q10Decomposition of nitrogen pentoxide $N_2O_5$ in $CCl_4$ is a _________ reaction. (a) Sequential (b) First order (c) Pseudo first order (d) P…Preview
- Q11Consider the potential energy diagrams of reactions (I) and (II) given below, predict which reaction will go faster and why ? [figure: two p…Preview
- Q12The activation energy of a certain reaction is 100 kJ/mole. What is the change in the rate constant of the reaction if the temperature is ch…Preview
- Q13If 75% of a first order reaction was completed in 60 min, 50% of the same reaction under the same conditions would be completed in : (a) 35…Preview
- Q14Give examples for the first order reaction.Preview
- Q15Write Arrhenius equation and explain the terms involved.Preview
- Q16The addition of a catalyst during a chemical reaction alters, which of the following quantities ? (a) Entropy (b) Internal energy (c) Activa…Preview
- Q17Show that in case of first order reaction the time required for the completion of 99% is twice the time required for the completion of 90% o…Preview
- Q18The rate constant of a reaction is $5.8 \times 10^{-2}\,s^{-1}$. The order of the reaction is : (a) Second order (b) First order (c) Third o…Preview
- Q19What is an order of a reaction ?Preview
- Q20Write Arrhenius equation and explain the terms involved.Preview
- Q21During the decomposition of $H_2O_2$ to give dioxygen, 48 g $O_2$ is formed per minute at certain point of time. The rate of formation of wa…Preview
- Q22The rate constant for a first order reaction is $1.54 \times 10^{-3}\,s^{-1}$. Calculate its half life time.Preview
- Q23The half life period of a radioactive element is 140 days. After 560 days 1 g of element will be reduced to : (a) $\left(\dfrac{1}{8}\right)…Preview
- Q24Write the rate law for the reaction in which the order with respect to the reactants X, Y and Z are $1/2$, $3/2$ and zero respectively.Preview
- Q25Prove that in case of first order reaction $t_{99.9\%} = 10\,t_{1/2}$.Preview