Rates of reaction

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.

Worked examples

Example 1

Marble chips react with hydrochloric acid and 60 cm³ of carbon dioxide is collected in 40 s. What is the mean rate of reaction?

  1. Mean rate = volume of gas ÷ time. rate = 60 ÷ 40
  2. Work it out and give the unit. = 1.5 cm³/s

Answer: 1.5 cm³/s

Example 2

The graph shows the volume of gas from a reaction. A tangent is drawn at 20 s through (0 s, 18 cm³) and (40 s, 58 cm³). Find the rate at 20 s, and describe how the rate changes.

  1. The rate at 20 s is the gradient of the tangent there.
  2. Gradient = change in volume ÷ change in time. (58 − 18) ÷ (40 − 0) = 40 ÷ 40 = 1 cm³/s
  3. The curve is steepest at the start and gets less steep: the rate falls as the acid is used up. When the line is flat, the reaction has stopped.

Answer: 1 cm³/s; the rate falls until the reaction stops

Example 3

Explain, using collision theory, why powdered marble reacts faster with acid than marble chips of the same mass.

  1. Breaking the solid into small pieces exposes much more surface.
  2. More particles of marble are in contact with the acid, so collisions happen more often.
  3. More frequent collisions means more successful collisions each second: the rate is faster. The total amount of gas is the same.

Answer: larger surface area, so more frequent collisions