Rate Graphs (Edexcel GCSE Chemistry: Combined Science)

• Data recorded in rate studies is used to plot graphs to calculate the rate of a reaction
• Time is normally plotted on the x-axis with the concentration of the reactant or product on the y-axis
• A number of measurements should be taken to provide a complete set of data
• If the relationship between the factor being measured and the amount produced is directly proportional (i.e. if the concentration of a reactant doubles the rate also doubles) then the resulting graph will be a straight line graph going through the origin
• The gradient of the line is equal to the initial rate of reaction and the steeper the gradient of the line then the faster the rate of reaction

Initial Rate Graphs

An initial rate graph for the formation of a product shows a straight line with a positive correlation starting from the origin

• A reaction rate graph based on measurements of a reactant being used up will have a negative correlation

An initial rate graph for a reactant shows a straight line with a negative correlation starting from the y-axis

Rate Graphs Until Completion

• Plotting a graph until the completion of the reaction shows how the rate changes with time
• The concentration of a reactant or product may be plotted against time and will produce one of the following curves:

The concentration of a reactant or product against time

• Over time the rate of reaction slows as the reactants are being used up so the line becomes less steep and eventually becomes horizontal, indicating the reaction has finished
• Often it is more practical to measure a property like mass of volume against time
• You can plot more than one run of a variable on the same graph making it easier to see how the variable influences the rate
  • For Example, plotting the effect of concentration on a reaction between acid and marble chips:

This graph shows how the mass/volume of a product changes over time for a high concentration and a low concentration

• Drawing a tangent to the slope allows you to show the gradient at any point on the curve
• The volume of a gaseous product would increase to a maximum over time, so the line levels out indicating the reaction is over
• Since the volume and mass would be proportional, this could also be a graph of mass of product versus time