(1)NCERT Class 12 Chemistry Chapter: Chemical Kinetics questions with answers, covering Intext Questions and End Exercise Questions as per latest NCERT 2024–25 syllabus.
🔷 Intext Questions with Answers
Page 102
Q1. For a reaction, A + B → Product, the rate law is: Rate = k[A]^n. If the concentration of A is doubled, and the rate increases four times, what is the order of the reaction with respect to A?
Answer:
Rate ∝ [A]^n
Given: When [A] doubles, rate becomes 4×
So:
2ⁿ = 4 → n = 2
Order = 2
Q2. For a reaction, A + B → Product, rate = k[A]^2[B]. What is the overall order of the reaction?
Answer:
Order w.r.t. A = 2, B = 1
Overall order = 2 + 1 = 3
Page 104
Q3. A first-order reaction is 50% complete in 20 minutes. How long will it take for 90% completion?
Answer:
Use first-order formula:
t₉₀% = (2.303/k) log(100/10) = (2.303/k) × 1
Given t₅₀% = 0.693/k = 20 → k = 0.693/20 = 0.03465
So, t₉₀% = 2.303 / 0.03465 = 66.44 min
Q4. What is the effect of temperature on rate constant k? How can this be represented quantitatively?
Answer:
Rate constant increases with temperature.
Quantitatively, it's given by Arrhenius equation:
k = A e^(-Ea/RT)
Page 106
Q5. In a pseudo first-order hydrolysis of ester in water, the following data were obtained:
| [Ester] mol/L | Time (min) |
|---|---|
| 0.5 | 0 |
| 0.5 | 30 |
| 0.25 | 60 |
| 0.125 | 90 |
Show that the reaction follows first-order kinetics.
Answer:
In first-order, log([R]₀/[R]) ∝ t
-
At 0 min: [R] = 0.5
-
At 30 min: still 0.5
-
At 60 min: 0.25 → [R]₀/[R] = 2
-
At 90 min: 0.125 → [R]₀/[R] = 4
log(2) = 0.301, log(4) = 0.602 → doubling every 30 min
Hence, it follows first-order kinetics.
🔷 End Exercise Questions with Answers
Q1. Define the rate of reaction.
Answer:
Rate of reaction is the change in concentration of reactants or products per unit time.
Q2. What is the difference between average and instantaneous rate of a reaction?
Answer:
-
Average rate: Change in concentration over a time interval.
-
Instantaneous rate: Rate at a specific instant (slope of concentration vs. time curve at that point).
Q3. For a reaction: A → B, the rate is 0.2 mol L⁻¹ s⁻¹. What is the rate of disappearance of A and appearance of B?
Answer:
Rate of disappearance of A = 0.2 mol L⁻¹ s⁻¹
Rate of appearance of B = 0.2 mol L⁻¹ s⁻¹
Q4. A reaction is: 2A + B → C + D. Express the rate in terms of A, B, C, and D.
Answer:
Rate = –(1/2) d[A]/dt = –d[B]/dt = d[C]/dt = d[D]/dt
Q5. In a reaction, A + B → C + D, the concentration of B is doubled. The rate becomes four times. What is the order w.r.t. B?
Answer:
Rate ∝ [B]^n
2ⁿ = 4 → n = 2
Order w.r.t. B = 2
Q6. The half-life of a first-order reaction is 10 min. Calculate rate constant (k).
Answer:
k = 0.693 / t₁/₂ = 0.693 / 10 = 0.0693 min⁻¹
Q7. What will be the units of rate constant for:
(a) Zero-order reaction:
Answer: mol L⁻¹ s⁻¹
(b) First-order reaction:
Answer: s⁻¹
(c) Second-order reaction:
Answer: L mol⁻¹ s⁻¹
Q8. For a reaction: A → B + C, if [A] vs time is a straight line, what is the order?
Answer:
Straight line for [A] vs. t indicates zero-order reaction.
Q9. What is the activation energy of a reaction?
Answer:
The minimum energy required by reactant molecules to undergo a chemical reaction.
Q10. Explain Arrhenius equation and its significance.
Answer:
k = A e^(-Ea/RT)
-
k = rate constant
-
A = frequency factor
-
Ea = activation energy
-
R = gas constant
-
T = temperature
Significance: Shows how k increases with T, and how Ea affects rate.
Q11. A first-order reaction is 40% complete in 50 minutes. What is the rate constant?
Answer:
Let [R]₀ = 100, [R] = 60 (as 40% is used)
Use:
k = (2.303/t) log([R]₀/[R]) = (2.303/50) log(100/60)
= (2.303/50) × log(1.6667) = 0.0187 min⁻¹
Q12. Differentiate between order and molecularity of a reaction.
| Order | Molecularity |
|---|---|
| Experimental concept | Theoretical concept |
| May be zero, fractional or whole | Always whole number |
| Derived from rate law | Based on number of reacting molecules |
| Sum of powers of concentration | Number of molecules involved in elementary step |
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