Calculating Rates of ReactionsOxford AQA IGCSE Chemistry: Revision notes

The rate of a reaction is a measure of how quickly reactants are used up or how quickly products are formed during a chemical reaction. A fast reaction has a high rate; a slow reaction has a low rate.

Rate of reaction can be calculated using either of these equations:

• Rate = amount of reactant used ÷ time
• Rate = amount of product formed ÷ time

The "amount" can be measured in grams (mass), cm³ (volume, for a gas), or mol/dm³ (concentration), depending on what is easiest to measure in the experiment. Time is usually measured in seconds.

Graphs of reaction progress usually plot the amount of product formed (or reactant used up) on the y-axis against time on the x-axis.

• A steeper gradient (slope) means a faster rate of reaction
• As the reaction proceeds, reactants are used up, so the graph line typically becomes less steep over time
• The graph becomes flat (horizontal) when the reaction has finished, because no more product is being formed
• The gradient at any point on the curve gives the rate of reaction at that instant (instantaneous rate)

Comparing two reaction graphs, the one with the steeper initial gradient has the faster initial rate of reaction. If a graph reaches its final flat plateau sooner, that reaction finished more quickly, though the total amount of product may be the same if both reactions used the same starting amounts of reactant.