Factors Affecting the Rate of a Reaction (AQA A Level Chemistry)

Rate of Reaction

REQUIRED PRACTICAL 3

Factors which affect the rate of a chemical reaction

• During a reaction, the reactants are used up and changed into the products

• The rate of a chemical reaction can be calculated using the following equation:

• As a reaction proceeds, the concentration of the reactants is decreasing and the concentration of the products is increasing

• Because of this, the rate of the reaction is not the same throughout the reaction but changes

• The rate of reaction during the reaction can be calculated from a concentration-time graph 

• A tangent will have to be drawn at a certain point on the graph, to calculate the rate at that specific time

• The following graph is an example of cyclopropane being converted to propene

  • The concentration of cyclopropene, the reactant, is decreasing over time as the reaction progresses

The rate of reaction at 3 different concentrations of cyclopropane is calculated by drawing tangents at those points in the graph

The Disappearing Cross Experiment

• A simple experiment which can be done to determine how rate of reaction is affected by concentration is the disappearing cross experiment

• It is set up as follows:

The Disappearing Cross Experiment

• This experiment can be done for a number of different reactions, but taking the following reaction as an example:

Na₂S₂O₃ (aq) + 2HCl (aq)  → 2NaCl (aq) + H₂O (l) + SO₂ (g) + S (s)

• In this reaction, sodium thiosulphate reacts with hydrochloric acid

• The key product which allows this experiment to work, is the sulfur which is a solid, and causes the solution to become opaque

• There are actually two factors which can be investigated using the disappearing cross reaction:

  • Changing the temperature

  • Changing the concentration of the HCl

• You can plot graphs of concentration against time if the concentration of HCl was changed, or plot 1/t against temperature if the temperature was changed

  • You plot 1/t because the rate is proportional to 1/time and the graph you want is rate versus temperature