Rates of reaction · GCSE Chemistry

Measuring rate

The rate of a reaction can be found by measuring how quickly a reactant is used up or a product is formed.
Mean rate = quantity of reactant used ÷ time taken, or quantity of product formed ÷ time taken.
Units are often g/s or cm³/s. (Higher) Rate can also be measured in mol/s.
On a graph of product against time, a steeper gradient means a faster rate. The line levels off when the reaction stops.
(Higher) The rate at a particular time is the gradient of a tangent drawn to the curve at that time.

Chemical reactions can only happen when reacting particles collide with enough energy: at least the activation energy.
The rate depends on how often particles collide (frequency) and how many collisions have enough energy.

Concentration (or pressure of gases): more particles in the same volume, so collisions are more frequent.
Surface area: breaking a solid into smaller pieces exposes more particles, so collisions are more frequent.
Temperature: particles move faster, so they collide more often and more of the collisions have enough energy. This has the biggest effect.
Catalysts: they speed up reactions without being used up.

A catalyst changes the rate of a reaction but is not used up. It is not included in the word equation.
It provides a different reaction pathway with a lower activation energy, so more collisions are successful.
Different reactions need different catalysts. Enzymes are biological catalysts.

Gas method: react magnesium ribbon with hydrochloric acid of different concentrations, and measure the volume of hydrogen collected in a gas syringe at regular intervals.
Colour change method: react sodium thiosulfate with hydrochloric acid in a flask over a cross drawn on paper. Time how long it takes for the cloudy sulfur to hide the cross.
Keep other variables the same: temperature, volumes and mass or length of magnesium.

Rate of reaction: How quickly reactants are used up or products are formed.
Collision theory: Reactions happen when particles collide with enough energy.
Activation energy: The minimum energy particles need to react when they collide.
Catalyst: A substance that speeds up a reaction without being used up.
Surface area: The total area of the surface of a solid that is exposed to other reactants.
Tangent: A straight line that touches a curve at one point; its gradient gives the rate at that time.
Gas syringe: Apparatus used to measure the volume of gas produced.