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Chemical Kinetics — NEET & JEE MCQs with answers
Chemical kinetics studies the rates of chemical reactions and the factors that influence them - concentration, temperature, pressure and catalyst. Topics include rate laws, rate constants, order and molecularity of reactions, integrated rate equations for zero and first order reactions, half-life, the Arrhenius equation for temperature dependence (activation energy), and collision theory.
Topics in this chapter
- 3.1 Rate of a Chemical Reaction
- 3.2 Factors Influencing Rate of a Reaction
- 3.3 Integrated Rate Equations
- 3.4 Temperature Dependence of the Rate of a Reaction
- 3.5 Collision Theory of Chemical Reactions
Free practice questions
Which single statement correctly describes what 'zero order' means for a reaction?
- A. The concentration of the reactant stays constant throughout the reaction
- B. The rate constant of the reaction is zero
- C. The rate is proportional to the zero power of the concentration of the reactants, so the rate equals $k$ at all concentrations
- D. No reactant molecule takes part in the rate determining step
Show answer
Answer: C — NCERT defines a zero order reaction as one in which the rate of the reaction is proportional to zero power of the concentration of reactants; since any quantity raised to power zero is unity, the rate equals $k$ at every concentration. Being zero order says nothing about the rate constant being zero, nor about the number of reactant molecules.
NCERT Class 12 Chemistry Ch 3 ("Zero order reaction means that the rate of the reaction is proportional")
In the mechanism $\mathrm{H_2O_2 + I^- \rightarrow H_2O + IO^-}$ followed by $\mathrm{H_2O_2 + IO^- \rightarrow H_2O + I^- + O_2}$, the species $\mathrm{IO^-}$ is called
- A. A catalyst poison
- B. An inhibitor
- C. An intermediate
- D. A product
Show answer
Answer: C — $\mathrm{IO^-}$ is called an intermediate because it is formed during the course of the reaction but does not appear in the overall balanced equation.
NCERT Class 12 Chemistry Ch 3 ("called as an intermediate since it is formed during the course of the")
According to NCERT, the unstable intermediate formed in $\mathrm{H_2 + I_2}$ exists for a very short time and then
- A. decomposes back only into $\mathrm{H_2}$ and $\mathrm{I_2}$
- B. is trapped by the catalyst permanently
- C. breaks up to form two molecules of hydrogen iodide
- D. is stabilised as a permanent product
Show answer
Answer: C — NCERT states that the intermediate exists for a very short time and then breaks up to form two molecules of hydrogen iodide.
NCERT Class 12 Chemistry Ch 3 ("collide to form an unstable intermediate (Fig. 3.6). It exists")
In the reaction $\mathrm{C_4H_9Cl + H_2O \rightarrow C_4H_9OH + HCl}$ used to illustrate average rate, the products formed are
- A. $\mathrm{C_4H_9OH}$ and $\mathrm{HCl}$
- B. $\mathrm{C_4H_9OH}$ and $\mathrm{H_2}$
- C. $\mathrm{C_4H_9Cl}$ and $\mathrm{H_2O}$
- D. $\mathrm{C_4H_8}$ and $\mathrm{HCl}$
Show answer
Answer: A — The equation given is $\mathrm{C_4H_9Cl + H_2O \rightarrow C_4H_9OH + HCl}$, so butyl alcohol and hydrogen chloride are the products. Butyl chloride and water are the reactants.
NCERT Class 12 Chemistry Ch 3 ("C4H9Cl + H2O ® C4H9OH + HCl")
Which aspect of atoms and molecules does collision theory ignore, according to NCERT?
- A. their activation energy
- B. their kinetic energy
- C. their collision frequency
- D. their structural aspect
Show answer
Answer: D — NCERT states that collision theory ignores the structural aspect of atoms and molecules by treating them as hard spheres.
NCERT Class 12 Chemistry Ch 3 ("molecules to be hard spheres and ignores their structural aspect. You")
According to NCERT, increasing the temperature **or** decreasing the activation energy will result in
- A. an increase in the rate of the reaction and an exponential increase in the rate constant
- B. a decrease in the rate of the reaction
- C. no change in the rate constant
- D. a change in the order of the reaction only
Show answer
Answer: A — NCERT states that increasing the temperature or decreasing the activation energy will result in an increase in the rate of the reaction and an exponential increase in the rate constant.
NCERT Class 12 Chemistry Ch 3 ("increase in the rate constant.")
Why can a reaction involving the simultaneous collision of more than three molecules effectively not occur in one step?
- A. Because the rate constant would be negative
- B. Because the molecules would be destroyed on collision
- C. Because the probability of such a simultaneous collision is very small
- D. Because energy is not conserved in such a collision
Show answer
Answer: C — The probability that more than three molecules can collide and react simultaneously is very small, so such an overall reaction must take place in more than one step.
NCERT Class 12 Chemistry Ch 3 ("The probability that more than three molecules can collide and")
A plot of $\ln[\mathrm{R}]$ against $t$ for a reaction is a straight line with a negative slope. This tells us that the reaction is:
- A. pseudo first order only
- B. of order which cannot be decided from this plot
- C. first order
- D. zero order
Show answer
Answer: C — Only a first order reaction gives a straight line when $\ln[\mathrm{R}]$ is plotted against $t$, because $\ln[\mathrm{R}]=-kt+\ln[\mathrm{R}]_0$. A zero order reaction gives a straight line for $[\mathrm{R}]$ versus $t$, not $\ln[\mathrm{R}]$ versus $t$.
NCERT Class 12 Chemistry Ch 3 ("we get a straight line with slope = -k and intercept equal to")
In the hydrolysis of butyl chloride, $[\mathrm{C_4H_9Cl}]$ is $0.0905\ \mathrm{mol\ L^{-1}}$ at $50\ \mathrm{s}$ and $0.0820\ \mathrm{mol\ L^{-1}}$ at $100\ \mathrm{s}$. The average rate over this interval is
- A. $1.58 \times 10^{-4}\ \mathrm{mol\ L^{-1}\ s^{-1}}$
- B. $1.70 \times 10^{-4}\ \mathrm{mol\ L^{-1}\ s^{-1}}$
- C. $1.90 \times 10^{-4}\ \mathrm{mol\ L^{-1}\ s^{-1}}$
- D. $8.5 \times 10^{-5}\ \mathrm{mol\ L^{-1}\ s^{-1}}$
Show answer
Answer: B — Average rate $= \dfrac{0.0905 - 0.0820}{100 - 50} = \dfrac{0.0085}{50} = 1.70 \times 10^{-4}\ \mathrm{mol\ L^{-1}\ s^{-1}}$.
NCERT Class 12 Chemistry Ch 3 ("0.100 0.0905 0.0820 0.0741")
NCERT concludes that in collision theory the criteria for an effective collision, and hence the rate of a chemical reaction, are together determined by
- A. the molar masses of the colliding species
- B. collision frequency and temperature alone
- C. the equilibrium constant and the Gibbs energy
- D. activation energy and proper orientation of the molecules
Show answer
Answer: D — NCERT states: 'in collision theory activation energy and proper orientation of the molecules together determine the criteria for an effective collision and hence the rate of a chemical reaction.'
NCERT Class 12 Chemistry Ch 3 ("Thus, in collision theory activation energy and proper orientation of")
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