NEET Chemical Kinetics — Set 14

Question 1

A first-order reaction has a half-life of 35 minutes. What fraction of the reactant remains after 105 minutes?

Accepted Answer

Number of half-lives = $ \frac{105}{35} = 3 $. Fraction remaining = $ \left( \frac{1}{2} \right)^3 = \frac{1}{8} $.

Question 2

A first-order reaction has a rate constant of $ 0.0462 \, \text{min}^{-1} $. What is the time required for 75% completion?

Accepted Answer

75% completion means 25% remains, $ \frac{[\text{R}]}{[\text{R}]_0} = 0.25 $, $ \frac{[\text{R}]_0}{[\text{R}]} = 4 $. $ t = \frac{2.303}{k} \log \frac{[\text{R}]_0}{[\text{R}]} = \frac{2.303}{0.0462} \log 4 = \frac{2.303 \times 0.602}{0.0462} \approx 30 \, \text{min} $.

Question 3

For the reaction $ 2\text{A} \to \text{B} + \text{C} $, the rate of formation of C is $ 0.05 \, \text{mol L}^{-1} \text{s}^{-1} $. What is the rate of disappearance of A?

Accepted Answer

Rate = $ -\frac{1}{2} \frac{\Delta[\text{A}]}{\Delta t} = \frac{\Delta[\text{C}]}{\Delta t} $. Given: $ \frac{\Delta[\text{C}]}{\Delta t} = 0.05 \, \text{mol L}^{-1} \text{s}^{-1} $. $ -\frac{\Delta[\text{A}]}{\Delta t} = 2 \times 0.05 = 0.1 \, \text{mol L}^{-1} \text{s}^{-1} $.

NEET Chemistry: Chemical Kinetics — Practice Set 14

Q1. A first-order reaction has a half-life of 35 minutes. What fraction of the reactant remains after 105 minutes?

Q2. A first-order reaction has a rate constant of \( 0.0462 \, \text{min}^{-1} \). What is the time required for 75% completion?

Q3. For the reaction \( 2\text{A} \to \text{B} + \text{C} \), the rate of formation of C is \( 0.05 \, \text{mol L}^{-1} \text{s}^{-1} \). What is the rate of disappearance of A?

Q4. The molecularity of the elementary step \( \text{H}_2 + 2\text{ICl} \to 2\text{HCl} + \text{I}_2 \) is:

Q5. A first-order reaction has a half-life of 18 minutes. What percentage of the reactant remains after 54 minutes?

Q6. A reaction’s rate constant doubles when the temperature increases from 20°C to 30°C. What is the activation energy (\( R = 8.314 \, \text{J mol}^{-1} \text{K}^{-1} \))?

Q7. A zero-order reaction takes 120 s to reduce the concentration from \( 0.06 \, \text{mol L}^{-1} \) to \( 0.03 \, \text{mol L}^{-1} \). What is the rate constant?

Q8. A first-order reaction has a rate constant of \( 0.0139 \, \text{min}^{-1} \). What is the half-life?

Q9. A reaction has the rate law \( \text{Rate} = k[\text{A}][\text{B}] \). If the concentration of A is tripled and B is reduced to one-third, what happens to the rate?

Q10. A second-order reaction has a rate constant of \( 0.02 \, \text{L mol}^{-1} \text{min}^{-1} \) and an initial concentration of \( 0.4 \, \text{mol L}^{-1} \). What is the half-life?