Changes that Affect the Rate of Reaction

Specification Point 7.4:
  • Explain the effects on rates of reaction of changes in temperature, concentration, surface area to volume ratio of a solid and pressure (on reactions involving gases) in terms of frequency and/or energy of collisions between particles.

Effect of Temperature

Rate Temperature, Edexcel GCSE Chemistry

Diagram showing the effect of temperature on particles

  • With an increase in the temperature, the rate of reaction will increase.
  • This is because the particles will have more kinetic energy than the required activation energy, therefore there will be more successful collisions per second.
  • They are also moving around faster when at a higher temperature, so the number of collisions also increases.

Effect of Concentration

Rate Concentration, Edexcel GCSE Chemistry

Diagram showing increase in concentration of solution

  • With an increase in the concentration of a solution, the rate of reaction will increase.
  • This is because there will be more reactant particles in a given volume, producing a higher number of collisions per second.

Effect of Surface Area

Rate Surface Area, Edexcel GCSE Chemistry

Diagram showing surface area to volume ratio of a cube

  • With an increase in the surface area of a solid reactant, the rate of reaction will increase.
  • This is because more surface area of the particles will be exposed to the other reactant, producing a higher number of collisions per second.
  • For the diagram shown above, the large cube has a surface area of 2 x 2 x 6 = 24cm2.
  • When broken up into 8 smaller cubes, the new surface area is 1 x 1 x 6 x 8 = 48cm2.
  • Therefore the same mass of substance has doubled its surface area by being broken up into smaller pieces.

Effect of Pressure

Rate Pressure, Edexcel GCSE Chemistry

Diagram showing an increase in pressure

  • With an increase in pressure of reactions with gaseous reactants, the rate of reaction will increase.
  • This is because there will be more reactant particles in a given volume of gas, producing a higher number of collisions per second.

Interpreting Reaction Rate Graphs

Specification Point 7.5:
  • Interpret graphs of mass, volume or concentration of reactant or product against time.
  • Up until now we have looked at defining reaction rate and the different methods of measuring rate and recording data as well as the effects of changing reaction conditions on the rate of reaction.
  • Data recorded in rate studies are 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 rate of reaction and the steeper the gradient of the line then the faster the rate of reaction.
  • The gradient of a straight line is given by the equation:

  • Often a curved graph is obtained or a graph which starts out as a straight line but then curves to form a horizontal line as the reaction peters out, usually due to one of the reactants running out.
  • The curved section signifies that the relationship between rate and the factor being measured is not directly proportional, so the rate of reaction is different along each point the curve.
  • For a curve graph a tangent must be drawn to calculate the change in x and y.
  • Place a ruler on the point being studied and adjust its position so the space on either side of the point between the ruler and curve are equal.

Diagram showing how to calculate values for change in x and y using a tangent

Example

A company is developing a new fabric that is designed to be resistant to acidic conditions for use in umbrellas in areas that experience acid rain. The proposed formula for the fabric contains nickel, a transition metal. The initial rate of reaction between nickel and two concentrations of sulphuric acid was studied as part of testing. In both cases the rate of reaction was analysed by the amount of hydrogen gas produced, which was measured using a gas syringe:

Ni + H2SO4 → NiSO4 + H2

The following graph was plotted from the results obtained:

Graph Reaction Rate Example Question, Edexcel GCSE Chemistry
  • The initial rate of reaction for the lower concentration of acid was calculated.
  • A tangent was drawn at the beginning of the green reacting curve.
  • From the tangent the rate of reaction is thus 60cm3 / 5s = 12cm3 s-1.
  • You can also tell just by looking at a graph whether the data was produced from measuring the reactant being used up or the product forming.
  • A reaction rate graph based on measurements of a product being formed will have a positive correlation.

Rate Graphs of Products Formed

  • Initial amount of product is 0.
  • Positive correlation of the graph indicates the amount of product increases as the reaction progresses.
  • A reaction rate graph based on measurements of a reactant being used up will have a negative correlation.
Line Graph Reactant Lost, Edexcel GCSE Chemistry
Curve Graph Reactant Lost, Edexcel GCSE Chemistry
  • Initial amount of reactant is at its maximum value.
  • Negative correlation of the graph indicates the amount of reactant decreases as the reaction progresses.

Mass / Volume

Graph Rate Mass & Volume, Edexcel GCSE Chemistry

Graph showing the effect of mass / volume  on the rate of reaction

  • Compared to a reaction with less mass / volume of reactant, the graph for the same reaction with a higher mass / volume has a steeper gradient at the start and becomes horizontal sooner.
  • This shows that with increased mass / volume of a reactant, the rate of reaction increases.

Concentration / Pressure

Graph Rate Conc & Pressure, Edexcel GCSE Chemistry

Graph showing the effect of concentration of a solution on the rate of reaction

  • Compared to a reaction with low concentration / pressure, the graph for the same reaction at a higher concentration / pressure has a steeper gradient at the start and becomes horizontal sooner.
  • This shows that with increased concentration / pressure of a reactant, the rate of reaction increases.

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.