Chemical Kinetics is crucial because it links reaction rates with concentration, temperature, and mechanisms. In CBSE boards and competitive exams (JEE/NEET), questions from kinetics frequently test your understanding of rate laws, half-life concepts, Arrhenius behavior, and how fast equilibria/slow steps determine overall reaction rates.
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Marking
Q1. For a first-order reaction A → products, of A reacts in . The half-life is approximately:
Q2. Initial rate data for the reaction are: Exp‑1: ; Exp‑2: ; Exp‑3: . The rate law and value of the rate constant are:
Q3. Consider the Lindemann mechanism:
(activation/deactivation) and . Given , , and , the observed rate can be written as . The value of (in ) is approximately:
Q4. For the reaction with , let , and . The time required for to fall to is approximately:
Q5. Two elementary reactions follow the Arrhenius equation with parameters:
Reaction I: ; Reaction II: . Which statement correctly describes which reaction is faster at and at ?
Reaction I is faster at ; Reaction II is faster at
Reaction II is faster at both and
Reaction I is faster at both and
Reaction II is faster at ; Reaction I is faster at
Q6. A reactant decomposes by first-order kinetics: products. If and after , what is after ? (Use )
Q7. The initial rate data for the reaction products are:
Experiment 1:
Experiment 2:
Experiment 3:
Which of the following gives the correct rate law and the value of the rate constant ?
Q8. For consecutive first-order reactions with and , the time at which is maximum is given by . What is (in s)?
Q9. Two different pathways X and Y for the same elementary step have Arrhenius parameters:
Pathway X:
Pathway Y:
Which statement is correct regarding their rate constants and at and ?
at both and
at both and
at , but at
at , but at
Q10. For the reversible first-order reaction starting with pure at , the concentration approaches its equilibrium value as . The time required for to reach the midpoint between and is:
Q11. A first-order reaction products has half-life . If the initial concentration is , how long will it take for to reach ? (Use and .)
Q12. The rate constant for a reaction is at and at . Using the Arrhenius equation , the activation energy (in ) is approximately:
Q13. Consider the mechanism:
(1) ; (fast equilibrium, )
(2) products ; (slow)
If step (1) is fast equilibrium and step (2) is rate-determining, which expression represents the overall rate law?
Q14. For the reaction products with rate law , initial concentrations are and and . Using the integrated relation , the time required for to fall to is closest to:
Q15. Assertion (A): If the observed activation energy for an overall reaction is negative (), the measured rate constant decreases with increasing temperature.
Reason (R): An apparent negative can arise when a fast exothermic pre-equilibrium produces an intermediate whose equilibrium concentration falls as temperature rises, so the effective rate constant for the overall process decreases with . (Use .)
Both A and R are true but R is not the correct explanation of A.
A is true but R is false.
Both A and R are true and R is the correct explanation of A.
A is false but R is true.