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Chemical Kinetics

Chemistry Unit 5
25 min read
IAT Advanced
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1. Core Concept

Chemical kinetics studies the speed of reactions. While thermodynamics tells us if a reaction will occur (ΔG < 0), kinetics tells us how fast it happens and how (the mechanism).

2. Rate of Reaction

Rate is the change in concentration of a reactant/product per unit time.

For aA + bB → cC + dD:

Rate = -(1/a)(d[A]/dt) = -(1/b)(d[B]/dt) = +(1/c)(d[C]/dt) = +(1/d)(d[D]/dt)
Rate = k[A]x[B]y
Rate Law: Order = x + y (must be determined experimentally).

3. Factors Influencing Rate of a Reaction

Effect of Concentration (Rate Law)

According to the collision theory, higher concentration means more particles in a given volume, leading to a higher frequency of effective collisions.

  • The exact dependence is given by the experimentally determined Rate Law.
  • Increasing the concentration of a zero-order reactant does not change the rate.

Effect of Temperature

For most chemical reactions, the rate increases roughly 2 to 3 times for every 10°C rise in temperature.

  • Higher temperature means higher kinetic energy.
  • A significantly larger fraction of molecules now possesses energy greater than the activation energy (Ea).

Effect of Catalyst & Surface Area

Catalyst: Alters the rate by providing an alternative reaction pathway/mechanism with a lower activation energy. It does not alter ΔG or equilibrium state, but helps achieve equilibrium faster.

Surface Area: For heterogeneous reactions (e.g., solid reactants), finely divided powders react much faster than large lumps because the exposed surface area for collisions is enormously increased.

4. Integrated Rate Equations

Order Integrated Equation Half-life (t1/2)
0 [A] = [A]0 - kt [A]0 / 2k
1 ln[A] = ln[A]0 - kt 0.693 / k
2 1/[A] = 1/[A]0 + kt 1 / (k[A]0)
1st Order Note: t1/2 is independent of initial concentration [A]0. Radio-active decay always follows 1st order kinetics!

5. Temperature Dependence

k = A e-Ea/RT
Arrhenius Equation. A is the pre-exponential factor.
log(k2/k1) = (Ea / 2.303R) [ (T2 - T1) / (T1T2) ]
Two-Temperature form to find Activation Energy.

6. Collision Theory

Rate = P · ZAB · e-Ea/RT
ZAB = Collision frequency. P = Steric Factor (geometry/orientation constraint).

7. Conceptual Insights

Order vs Molecularity

Feature Order of Reaction Molecularity of Reaction
Definition Sum of concentration powers in rate law Number of reacting species in elementary step
Values Can be 0, fraction, or integer Always a positive whole number (1, 2, rarely 3)
Nature Determined experimentally Theoretical concept

Mechanism & Slowest Step

In complex (multi-step) reactions, molecularity has no meaning for the overall reaction. The overall rate is governed by the Rate Determining Step (RDS), which is the slowest step in the mechanism.

8. Common Mistakes

  • Rate Constant (k) Units: They depend heavily on order! Units of k = (mol/L)1-n s-1.
  • Arrhenius Exponent Sign: The exponent is negative (-Ea/RT). Higher Temperature T makes the exponent less negative, causing k to increase exponentially.

9. IAT Exam Focus Points

  • Half-life Dependencies: For an nth order reaction, t1/2 ∝ 1/[A]0n-1. If halving initial concentration doubles t1/2, it's 2nd order.
  • Pseudo-First Order: E.g., Hydrolysis of esters or inversion of cane sugar in water. Water is in huge excess, so its concentration doesn't change, making it pseudo-1st order.
  • Graph Identification:
    • Plot [A] vs t forms straight line → Zero Order.
    • Plot ln[A] vs t forms straight line → First Order.
    • Plot 1/[A] vs t forms straight line → Second Order.

10. Practice Mock Test

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Chemical Kinetics

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