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Chemistry · Unit 7

Reaction Kinetics: MDCAT Notes and MCQs

Definitions

  • Chemical kinetics: study of rates of reactions, factors affecting them and reaction mechanisms.
  • Rate of reaction = change in concentration per unit time; unit mol dm−3 s−1. Rate decreases as reaction proceeds. Instantaneous rate = slope of tangent; average rate over an interval.
  • Rate equation / rate law: Rate = k[A]m[B]n. m and n are found experimentally (not from stoichiometry). Overall order = m + n.

Order & units of k

OrderUnits of kExample
0mol dm−3 s−1Photochemical H2 + Cl2, decomposition of NH3 on hot Pt/W
1s−1Radioactive decay, decomposition of N2O5
2dm3 mol−1 s−12HI → H2 + I2; 2NO2 → 2NO + O2
3dm6 mol−2 s−12NO + O2 → 2NO2

Half-life of a first-order reaction is independent of initial concentration: t½ = 0.693/k. In general t½ ∝ 1/an−1. The slowest step of a mechanism is the rate-determining step.

Factors affecting rate

Nature of reactants; concentration (more collisions); surface area (powder reacts faster); temperature (rate roughly doubles for every 10 K rise — more molecules exceed Ea); catalyst; light (photochemical).

Activation energy & activated complex

  • Activation energy (Ea): minimum extra energy colliding molecules need to react (energy barrier).
  • Activated complex: unstable, short-lived, high-energy arrangement of atoms at the top of the energy barrier (transition state); can go to products or back to reactants.
  • Lower Ea → more effective collisions → faster reaction. A catalyst lowers Ea by providing an alternative pathway; it does not change ΔH.
  • Arrhenius equation: k = A e−Ea/RT — k increases with temperature and decreases with Ea.

Rate constant (k)

k = rate when all concentrations are unity (specific rate constant). It depends on temperature and catalyst, not on concentration. Larger k = faster reaction.

PMDC learning outcomes for this unit
  • Chemical Kinetics: 7.1 Define chemical kinetics.
  • Chemical Kinetics: 7.2 Explain the terms: rate of reaction, rate equation.
  • Factors affecting rate of reaction: 7.3 Explain qualitatively factors affecting rate of reaction.
  • Order of Reaction: 7.4 Give the order with respect to the reactant, write the rate law for reaction.
  • Order of Reaction: 7.5 Explain the meaning of the term "activation energy" and "activated complex".
  • Order of Reaction: 7.6 Relate the ideas of activation energy and the activated complex to the rate of reaction.
  • Rate Constant: 7.7 Describe the role of the rate constant in the theoretical determination of reaction rate.

Reaction Kinetics MCQs with answers

Practise and track your score →
Q1
The unit of rate constant for a first-order reaction is:
As⁻¹
Bdm³ mol⁻¹ s⁻¹
Cdm⁶ mol⁻² s⁻¹
Dmol dm⁻³ s⁻¹
Show answer
Answer: A (s⁻¹). For first order, rate = k[A] → k has units s⁻¹.
Q2
The half-life of a first-order reaction:
AIs independent of initial concentration
BIncreases with concentration
CIs zero
DDecreases with concentration
Show answer
Answer: A (Is independent of initial concentration). t½ = 0.693/k — no concentration term.
Q3
For Rate = k[A]²[B], the overall order of reaction is:
A4
B3
C1
D2
Show answer
Answer: B (3). Overall order = 2 + 1 = 3.
Q4
A catalyst increases the rate of reaction by:
ALowering the activation energy
BShifting the equilibrium
CIncreasing the energy of products
DIncreasing ΔH
Show answer
Answer: A (Lowering the activation energy). It provides an alternative path with lower Ea.
Q5
The activated complex is:
AA catalyst
BThe final product
CA stable intermediate
DThe high-energy arrangement at the top of the energy barrier
Show answer
Answer: D (The high-energy arrangement at the top of the energy barrier). It is the transition state that exists momentarily at maximum energy.
Q6
The order of a reaction is determined:
AFrom the balanced equation
BFrom ΔH
CFrom molecularity
DExperimentally
Show answer
Answer: D (Experimentally). Order must be found by experiment.
Q7
The rate constant k depends on:
AVolume of container
BConcentration of reactants
CTemperature
DTime
Show answer
Answer: C (Temperature). k = Ae^(−Ea/RT) depends on temperature (and catalyst), not concentration.
Q8
A rise of 10 K in temperature generally:
ARoughly doubles the rate
BStops reaction
CDoes not affect rate
DHalves the rate
Show answer
Answer: A (Roughly doubles the rate). The fraction of molecules exceeding Ea roughly doubles.
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