Effect of pressure, volume, temperature and catalysts

Pressure and volume affect the rate only in a closed system containing gases: if the volume decreases n times, the pressure increases n times and the concentration of every gas also increases n times. For a one-step reaction we substitute into the rate expression to see how many times the rate changes: for example, for 2A + B → … if the concentrations rise 3 times, the rate changes by 3²·3 = 27 times. The Van’t Hoff rule: for every 10 °C rise in temperature the rate increases roughly 2–4 times; this number is the temperature coefficient γ, v₂ = v₁·γ^((t₂ − t₁)/10). A catalyst increases the rate and is not consumed in the reaction: it lowers the activation energy, the minimum energy particles need in order to react. Biological catalysts of protein nature are called enzymes; they are damaged by heating and lose their action. Substances that slow a reaction are called inhibitors. A catalyst makes a reaction faster but does not change how much product can finally be obtained from it.