Chemical Kinetics
Easy Overview
Some reactions happen in a flash. Others take years — like rust forming. Chemical kinetics is the study of speed. How fast does a reaction go? What affects its pace? And most importantly — how do we figure that out?
Rate of reaction
The rate is just how fast reactants disappear or products appear. Like watching ice melt. Rate = change in concentration / change in time. But here's the thing — the rate isn't constant. As reactants get used up, the reaction slows down, like eating a pizza — fast at first, slower as you finish.
Rate law and order
You can't look at a chemical equation and know the rate law. You have to measure it experimentally. The rate law is rate = k [A]^m [B]^n. The exponents m and n define the order. Order doesn't have to match the coefficients. Chemistry is sneaky like that.
Integrated rate laws
These let you calculate concentrations at any time. Zero order: concentration drops linearly. First order: drops exponentially. Second order: 1/concentration increases linearly. Radioactive decay is first order — that's how carbon dating works.
Half-life
The time it takes for half of the reactant to be used up. For first order reactions, half-life is constant — always the same, no matter how much you start with. For zero order, half-life depends on initial concentration. For second order, half-life depends on concentration too.
Arrhenius equation and activation energy
Molecules need a minimum amount of energy to react — like needing a running start to jump over a fence. That minimum is activation energy (Ea). The Arrhenius equation tells you how temperature affects rate. Higher temperature = more molecules have enough energy = faster reaction. Roughly, every 10°C increase doubles the rate.
Key Points
- •Rate = -d[R]/dt = +d[P]/dt
- •Rate law determined experimentally, not from balanced equation
- •First order: t1/2 = 0.693 / k (independent of concentration)
- •Zero order: t1/2 = [R]0 / 2k
- •Arrhenius: k = Ae^(-Ea/RT)
- •Activation energy is the energy barrier for reaction
- •Catalyst lowers activation energy without being consumed
Practice Questions
- The half-life of a first order reaction is 100 s. Calculate the rate constant.
- What is activation energy? Explain with a potential energy diagram.
- Derive the integrated rate equation for a first order reaction.
- How does temperature affect reaction rate? Explain using the Arrhenius equation.