Question 1

The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time. If the concentration of a reactant decreases from 0.8 M to 0.2 M in 3 seconds, what is the average rate of reaction?

Accepted Answer

Average rate = Δ[concentration]/Δtime = (0.8 - 0.2) M / 3 s = 0.6 M / 3 s = 0.2 M/s.

Question 2

For a first-order reaction, the integrated rate law is given by ln[A] = ln[A]₀ - kt. If the half-life of a first-order reaction is 10 minutes, what will be the half-life if the initial concentration is doubled?

Accepted Answer

For first-order reactions, half-life is independent of initial concentration and depends only on the rate constant k.

Question 3

The activation energy (Eₐ) of a reaction is the minimum energy required for reactants to form products. How does increasing the temperature of a reaction affect the number of collisions with energy ≥ Eₐ?

Accepted Answer

Higher temperature increases the kinetic energy of molecules, causing more collisions to exceed the activation energy threshold.

Question 4

A catalyst is a substance that increases the rate of reaction by lowering the activation energy. Which of the following statements about catalysts is correct?

Accepted Answer

Catalysts lower activation energy by offering an alternative reaction mechanism but remain unchanged and do not affect ΔH or equilibrium position.

Question 5

For a zero-order reaction, the integrated rate law is [A] = [A]₀ - kt. If a zero-order reaction has a rate constant of 0.05 M/s and initial concentration of 2 M, how long will it take for the concentration to become zero?

Accepted Answer

Using [A] = [A]₀ - kt, when [A] = 0: 0 = 2 - (0.05)t, so t = 2/0.05 = 40 seconds.

Question 6

The rate constant for a first-order reaction is 0.693 min⁻¹. What is the half-life of this reaction?

Accepted Answer

For a first-order reaction, t₁/₂ = 0.693/k = 0.693/0.693 = 1 minute.

Question 7

If the concentration of a reactant decreases from 1.0 M to 0.25 M in 20 minutes for a zero-order reaction, what is the rate constant?

Accepted Answer

For zero-order: k = Δ[A]/Δt = (1.0 - 0.25)/20 = 0.75/20 = 0.0375 M min⁻¹.

Question 8

The activation energy of a reaction is 50 kJ/mol. If the temperature is increased from 300 K to 310 K, by what factor does the rate constant increase? (R = 8.314 J mol⁻¹ K⁻¹)

Accepted Answer

Using ln(k₂/k₁) = (Eₐ/R)(1/T₁ - 1/T₂), the ratio k₂/k₁ ≈ 2.0 for this 10 K temperature change.

Question 9

For a reaction with rate law: rate = k[A]²[B], what is the overall order of the reaction?

Accepted Answer

Overall order = sum of individual orders = 2 + 1 = 3rd order.

Question 10

A second-order reaction has a rate constant of 0.5 L mol⁻¹ s⁻¹. If the initial concentration is 2 M, what is the time required for the concentration to drop to 0.5 M?

Accepted Answer

For second-order: t = 1/k × (1/[A]ₜ - 1/[A]₀) = 1/0.5 × (1/0.5 - 1/2) = 2 × 1.5 = 1.5 seconds.

Question 11

Which of the following correctly relates to the integrated rate law for a first-order reaction?

Accepted Answer

The integrated first-order rate law is ln[A] = ln[A]₀ - kt, which is the linear form for kinetic analysis.

Question 12

A catalyst increases the rate of a reaction by lowering which of the following?

Accepted Answer

A catalyst provides an alternative reaction pathway with lower activation energy, accelerating the reaction without being consumed.

Question 13

For the reaction 2A + B → C, the rate law is: rate = k[A][B]². If [A] is doubled and [B] is halved, the rate changes by a factor of:

Accepted Answer

New rate/Old rate = (2[A])¹ × (0.5[B])² / ([A] × [B]²) = 2 × 0.25 = 0.5.

Question 14

For a first-order reaction, if the rate constant k = 0.693 min⁻¹, what is the half-life of the reaction?

Accepted Answer

For first-order reactions, t₁/₂ = 0.693/k = 0.693/0.693 = 1 min (using the standard relationship where half-life depends only on k, not concentration).

Question 15

A reaction has an activation energy of 50 kJ/mol. If the rate constant doubles when temperature increases from 300 K to 310 K, which statement is correct?

Accepted Answer

Using the Arrhenius equation ln(k₂/k₁) = (Eₐ/R)(1/T₁ - 1/T₂), a k doubling (ln 2 ≈ 0.693) over 10 K requires Eₐ ≈ 52.9 kJ/mol, not 50 kJ/mol.

Question 16

For the reaction 2A + B → C, the rate law is determined to be: Rate = k[A]²[B]. If [A] is doubled and [B] is halved, by what factor does the reaction rate change?

Accepted Answer

New rate = k(2[A])²(0.5[B]) = k × 4[A]² × 0.5[B] = 2 × k[A]²[B] = 2 × (original rate).

CAPF AC Chemical Kinetics

Chemical Kinetics — Practice Questions