Chemical Kinetics
Chemical Kinetics
Questions target rate expressions, order, integrated rate laws, half-life, Arrhenius behaviour and collision theory.
Core rules
- Rate = k[A]^m[B]^n; order is experimental, molecularity is theoretical.
- Zero order: [A]=[A]₀−kt, t½=[A]₀/2k, k unit mol L⁻¹ s⁻¹.
- First order: k=(2.303/t)log([A]₀/[A]), t½=0.693/k, independent of concentration.
- Arrhenius: k=Ae^(−Ea/RT); log(k₂/k₁)=Ea(T₂−T₁)/(2.303R T₁T₂).
- A catalyst lowers Ea, speeds both directions and does not change ΔH, ΔG or K.
Worked example A first-order reaction is 75% complete in 60 min. Find k and t½. [A]/[A]₀=0.25. k=(2.303/60)log(1/0.25)=(2.303/60)(0.602)=0.0231 min⁻¹. t½=0.693/0.0231=30 min. The 75% point is two half-lives, consistent.
Traps
- Treating order as the coefficients in a balanced equation.
- Units of k: first order s⁻¹, second order mol⁻¹ L s⁻¹.
- For zero order, t½ depends on initial concentration.
- Arrhenius plot slope is −Ea/2.303R if log k is plotted.