Rates of reaction: factors and collision theory

Reactions happen when particles collide with at least a minimum amount of energy, called the activation energy. Anything that makes successful collisions happen more often increases the rate. There are two ways: make collisions more frequent, or give more particles enough energy to react.

Temperature is the only factor that also increases the energy of collisions.

A catalyst speeds up a reaction without being used up, so it can be used again. It works by providing a different reaction pathway with a lower activation energy, so more collisions are successful. Different reactions need different catalysts. Enzymes are biological catalysts. A catalyst isn't included in the word or symbol equation, because it isn't a reactant.

• Volume of gas produced: collect it in a gas syringe or an upturned measuring cylinder.
• Loss of mass: if a gas escapes, the mass on a balance falls.
• Colour change or cloudiness: in the "disappearing cross" experiment, sodium thiosulfate reacts with hydrochloric acid to form a sulfur precipitate. Time how long it takes for a cross under the flask to disappear.

If 60 cm³ of gas is made in 30 s, the mean rate is $60 \div 30 = 2$ cm³/s. Units can be g/s or cm³/s (and mol/s at higher tier). Practise with our rate of reaction questions.

On a graph of product against time, the gradient shows the rate. The line is steepest at the start, when there are most reactant particles, and flattens when a reactant runs out and the reaction stops.

• A steeper line means a faster reaction.
• The same final amount of product means the same quantity of the limiting reactant was used.
• The rate at one moment is the gradient of a tangent to the curve at that time (higher tier).

In the AQA practical you investigate how concentration affects rate, either by timing the disappearing cross with different concentrations of sodium thiosulfate, or by measuring the volume of gas produced. Keep temperature, volumes and the size of the flask the same. Work in a well-ventilated room, because the thiosulfate reaction gives off sulfur dioxide.

• Saying particles collide "harder" or "with more force" when concentration increases. Concentration changes frequency, not energy.
• Saying a catalyst "gives particles more energy". It lowers the activation energy.
• Forgetting "per unit time" or "more frequent" in collision explanations: just "more collisions" may not get the mark.

1. Explain why powdered marble reacts faster with acid than marble chips.
2. 0.48 g of magnesium reacts in 40 s. Calculate the mean rate.
3. Explain, using collision theory, why increasing the temperature increases the rate.

Answers: 1. Powder has a larger surface area, so more particles are exposed and collisions are more frequent. 2. $0.48 \div 40 = 0.012$ g/s. 3. Particles move faster, so they collide more frequently, and more of the collisions have energy equal to or greater than the activation energy.