Rates of reaction · Year 13
The rate of a reaction is how fast reactants are used up or products are made. It can be followed by measuring a gas given off, a loss in mass, or how long a solution takes to go cloudy. Reactions happen when particles collide with enough energy (the activation energy), so anything that makes successful collisions more frequent makes a reaction faster: higher concentration or pressure, a larger surface area, a higher temperature or a catalyst.
Key facts
- Mean rate = quantity of product formed ÷ time taken (or quantity of reactant used ÷ time). Units: cm³/s, g/s or mol/s.
- On a graph of product against time, the gradient is the rate. The steepest part is the fastest; a flat line means the reaction has stopped.
- The rate at one moment is the gradient of the tangent to the curve at that time.
- Collision theory: particles must collide with at least the activation energy to react.
- Higher concentration (or gas pressure) and larger surface area give more frequent collisions. Higher temperature gives more frequent and more energetic collisions.
- A catalyst speeds up a reaction by giving a pathway with a lower activation energy. It is not used up. Enzymes are biological catalysts.
- In the disappearing-cross experiment, rate is proportional to 1 ÷ time.