Chemistry
Chemical reaction rate
What is rate
Reaction rate: the change in reactant concentration per unit of time.
v = ±Δc / Δt
Unit of measurement: mol/(l·s).
The rate is affected by: the nature of the reactants, concentration, temperature, catalyst, surface area and pressure (for gases).
| Factor | Effect on rate |
|---|---|
| C ↑ | increases |
| T ↑ (10 °C) | 2-4 times ↑ |
| Catalyst | increases (Ea ↓) |
| Inhibitor | decreases |
| Surface ↑ | increases |
Laws
- Law of mass action: for aA + bB → product, v = k·[A]ᵃ·[B]ᵇ. k is the rate constant.
- Van 't Hoff rule: for every 10 °C the rate increases 2-4 times.
- Formula: v₂ = v₁·γ^((t₂ − t₁) / 10).
Catalyst and inhibitor
- A catalyst lowers Ea, so the reaction speeds up.
- An inhibitor slows the reaction down.
- A catalyst does not shift the equilibrium. It remains unchanged at the end of the reaction.
Also worth knowing
- A solid is not included in the equation (in heterogeneous reactions).
- k does not depend on concentration; it depends on temperature and the catalyst.
- A homogeneous reaction takes place in one phase (gas-gas). A heterogeneous one takes place in different phases (solid-gas).
- A crushed substance has a larger surface, so it reacts faster.