🔷 NCERT Exercise Questions with Answers – Chemical Kinetics
Q1. Define the rate of a reaction.
Answer:
Rate of a reaction is the change in concentration of a reactant or product per unit time.
It is expressed as:
Rate = –Δ[R]/Δt for reactants
Rate = +Δ[P]/Δt for products
Q2. Define average rate and instantaneous rate of a reaction.
Answer:
-
Average rate: Change in concentration over a finite time interval:
Average rate = Δ[R]/Δt -
Instantaneous rate: Rate at a particular instant (t → 0), i.e., slope of the tangent on the concentration vs. time curve.
Instantaneous rate = d[R]/dt
Q3. For the reaction:
A + B → C + D,
If rate = 0.2 mol L⁻¹ s⁻¹, what is the rate of:
(a) Disappearance of A, (b) Appearance of C?
Answer:
Since the stoichiometry is 1:1:1:1,
-
Rate of disappearance of A = 0.2 mol L⁻¹ s⁻¹
-
Rate of appearance of C = 0.2 mol L⁻¹ s⁻¹
Q4. For the reaction:
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. A reaction is first-order w.r.t. B. If [B] is doubled, how does the rate change?
Answer:
Rate ∝ [B]
If [B] doubles, then rate doubles.
Q6. A first-order reaction is 50% complete in 30 minutes. Calculate the rate constant.
Answer:
For first-order:
k = 0.693 / t₁/₂ = 0.693 / 30 = 0.0231 min⁻¹
Q7. For a reaction:
A → B + C
The plot of [A] vs time is a straight line. What is the order of reaction?
Answer:
A straight line [A] vs time indicates zero-order reaction.
Q8. What is the effect of temperature on the rate constant?
Answer:
Rate constant increases exponentially with temperature. This is explained by the Arrhenius equation:
k = Ae^(-Ea/RT)
Higher temperature → more molecules have energy ≥ Ea → faster rate.
Q9. What is activation energy?
Answer:
Activation energy (Ea) is the minimum energy required by the reactant molecules to successfully form products.
Q10. Define order and molecularity. Differentiate between the two.
Answer:
| Order | Molecularity |
|---|---|
| Experimentally determined | Theoretical concept |
| Can be 0, fractional, or whole | Always a whole number |
| Sum of powers in rate law | Number of molecules in elementary step |
| Overall kinetics concept | Mechanism concept |
Q11. Derive the integrated rate equation for a first-order reaction.
Answer:
For first-order:
Rate = –d[R]/dt = k[R]
Integrating:
∫d[R]/[R] = –∫k dt
ln[R] = –kt + ln[R]₀
So,
ln([R]₀/[R]) = kt
or
k = (2.303/t) × log([R]₀/[R])
Q12. The decomposition of ammonia on a platinum surface is a zero-order reaction. What are the units of rate constant?
Answer:
Units of rate constant for zero-order = mol L⁻¹ s⁻¹
Q13. For a reaction:
A → B,
if [A] decreases from 0.6 M to 0.3 M in 20 minutes, find the rate assuming zero-order.
Answer:
Zero-order:
Rate = –Δ[A]/Δt = (0.6 – 0.3)/20 = 0.015 mol L⁻¹ min⁻¹
Q14. For the reaction:
N₂O₅ → 2NO₂ + ½ O₂
If the rate of decomposition of N₂O₅ is 6.2 × 10⁻³ mol L⁻¹ s⁻¹, find rate of formation of NO₂ and O₂.
Answer:
From stoichiometry:
Rate of NO₂ = 2 × 6.2 × 10⁻³ = 1.24 × 10⁻² mol L⁻¹ s⁻¹
Rate of O₂ = ½ × 6.2 × 10⁻³ = 3.1 × 10⁻³ mol L⁻¹ s⁻¹
Q15. A reaction is:
2A → Products
Rate = k[A]²
If [A] = 0.5 mol L⁻¹, k = 0.2 L mol⁻¹ s⁻¹
Find rate of reaction.
Answer:
Rate = k[A]² = 0.2 × (0.5)² = 0.2 × 0.25 = 0.05 mol L⁻¹ s⁻¹
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