Chemical Kinetics
What you must understand
Define reaction rate in terms of concentration change over time and relate it to stoichiometric coefficients. Derive rate laws from experimental data, distinguishing zero-, first-, and second-order behaviour, and determine reaction order by the method of initial rates or by graphing. Know integrated rate laws and half-lives for first order. Understand collision theory and the Arrhenius equation, including the roles of activation energy, temperature, and orientation. Explain catalysis, including homogeneous, heterogeneous, and enzyme catalysis, and interpret energy profiles with and without a catalyst.
Traps to watch for
- Confusing rate constant k with the equilibrium constant K.
- Assuming reaction order equals stoichiometric coefficient; order must be determined experimentally.
- Forgetting that a catalyst lowers activation energy but does not change the thermodynamics or the equilibrium constant.
- Using the wrong units for k, which depend on overall order.
- Misreading a rate-concentration graph: first order is linear in rate versus concentration, zero order is flat.
Worked example
Doubling the concentration of A doubles the rate, and doubling B has no effect. The rate law is therefore rate = k[A], first order in A and zero order in B, with overall order one. If the rate constant is 0.020 per second at a given temperature, the half-life is ln2 divided by k, about 34.7 seconds, independent of starting concentration.
Rule of thumb
Compare experiments where only one concentration changes; the exponent is the ratio of rate change to concentration change.