ZIMSEC A Level · Chemistry · Form 6
Rate and collision theory
Rate = change in concentration ÷ time. Reactions need collisions with energy ≥ activation energy and the correct orientation. The Boltzmann distribution shows that at a higher temperature many more molecules exceed the activation energy; a catalyst lowers the activation energy.
Rate equations
Rate = k[A]ᵐ[B]ⁿ, where m and n are the orders (found by experiment, not from the equation) and k is the rate constant.
- Zero order: rate independent of concentration.
- First order: rate doubles when concentration doubles; constant half-life (t½ = 0.693/k).
- Second order: rate quadruples when concentration doubles.
Worked example: Doubling [A] doubles the rate; doubling [B] quadruples the rate. Write the rate equation and the overall order.
Rate = k[A][B]²; overall order 3
Mechanisms
The slowest step (rate-determining step) involves the species in the rate equation.
Catalysts
Homogeneous (same phase, e.g. Fe²⁺/Fe³⁺ in the I⁻/S₂O₈²⁻ reaction) and heterogeneous (different phase, e.g. iron in the Haber process; adsorption onto the surface). Enzymes are biological catalysts.
The Arrhenius equation (extension)
k = Ae^(−Ea/RT): a higher temperature or lower activation energy increases k.
Check yourself
- What are the units of k for a first-order reaction?
- A reaction has a constant half-life. What is its order?
- How does a heterogeneous catalyst work?
Show answers
- s⁻¹
- First order.
- Reactants adsorb onto its surface, bonds weaken, reaction occurs, and products desorb.
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