Factors Affecting the Rate of a Reaction (AQA AS Chemistry): Revision Note

Rate of Reaction

Required Practical 3: Factors that 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:

$\mathrm{Rate}\mathrm{of}\mathrm{reaction}=\frac{\mathrm{Change}\mathrm{in}\mathrm{amount}\mathrm{of}\mathrm{reactants}\mathrm{or}\mathrm{products}\left({\mathrm{moldm}}^{-3}\right)}{\mathrm{Time}\left(\mathrm{s}\right)}$

•  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 cyclopropane, the reactant, is decreasing over time as the reaction progresses

 The Disappearing Cross Experiment

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

• It is set up as follows:

• This experiment can be done for several 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, a fixed volume and concentration of sodium thiosulfate reacts with a fixed volume and concentration of hydrochloric acid

• The thiosulfate is warmed to a fixed temperature (preferably in a water bath, which ensures more even heating)

• The acid is added, and the time taken for the cross to disappear is noted

• The experiment is repeated for a range of temperatures

• A plot is carried out of 1/t (proportional to rate) against temperature

• The variables in this experiment are:

  • independent: temperature

  • dependent: time for the cross to disappear

  • control: volumes and concentrations, depth of solution, same cross/observer

• Safety considerations: sulfur dioxide is toxic and an irritant — work in a well-ventilated room or fume cupboard

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

• Another factor that can be investigated using the disappearing cross reaction is changing the concentration of the HCl

  • For that investigation, you would plot a graph of 1/t (rate) against concentration

Related topics

• Collision Theory

• Effect of Temperature on Reaction Rate

• Maxwell–Boltzmann Distributions

• Rate Equations