Chemistry (university) · Part 06
Chemical kinetics
How fast reactions go: rate laws, half-lives, activation energy, mechanisms and catalysts.
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- Reaction rateReaction rate is how fast a reactant is used up or a product appears: change in concentration divided by time. Units are usually mol/(L·s). Rates are positive, and they slow as reactants are used up.
- Rate law and reaction orderA rate law links rate to concentrations: rate = k[A]ᵐ[B]ⁿ. The exponents m and n are the orders, found by experiment, not from the balanced equation. k is the rate constant. The overall order is m + n.
- Method of initial ratesTo find orders, run several experiments with different starting concentrations and measure the initial rate. Change one concentration at a time. Compare how the rate changes with how the concentration changed.
- First-order integrated rate lawFor a first-order reaction, ln[A] = ln[A]₀ − kt. A plot of ln[A] against time is a straight line with slope −k. Equivalent form: [A] = [A]₀e^(−kt).
- Half-lifeThe half-life (t½) is the time for a reactant's concentration to fall to half. For a first-order reaction, t½ = 0.693/k, and it is the same no matter how much you start with. Zero and second order half-lives do change.
- Collision theoryFor two molecules to react, they must collide with enough energy and with the right orientation. Only a small fraction of collisions do both. More concentration, higher temperature and a bigger surface raise the number of effective collisions.
- Activation energy and the Arrhenius equationActivation energy (Ea) is the minimum energy needed for a collision to react. The Arrhenius equation, k = Ae^(−Ea/RT), shows k grows with temperature and falls as Ea rises. A is the frequency factor.
- Reaction mechanismA mechanism is the series of simple steps (elementary steps) that make up an overall reaction. Intermediates form in one step and are used in another. The slowest step, the rate-determining step, controls the overall rate.
- CatalystA catalyst speeds up a reaction without being used up. It gives the reaction a new path with a lower activation energy. It does not change ΔH or the final equilibrium position, only how fast equilibrium is reached.