Rates of Reaction

7.1 Core Practical: Investigate the effects of changing the conditions of a reaction on the rates of chemical reactions by: measuring the production of a gas (in the reaction between hydrochloric acid and marble chips) and observing a colour change (in the reaction between sodium thiosulfate and hydrochloric acid) 
7.2 Suggest practical methods for determining the rate of a given reaction 
  • Rates of reactions can be measured using the amount of product used, or amount of product formed over time:

                           Rate of reaction = amount of reactant used

                                                                               Time

                           Rate of reaction = amount of product formed

                                                                              Time

  • Quantity of reactant or product can be measured by the mass in grams or by a volume in cm^3
  • Units of rate of reaction may be given as g/s or cm^3/s
  • (HT only) use quantity of reactants in terms of moles and therefore, units for rate of reaction in mol/s
7.3 Explain how reactions occur when particles collide and that rates of reaction are increased when the frequency and/or energy of collisions is increased
  • Chemical reactions only occur when the reacting particles collide with enough energy, the minimum amount of energy required is called the activation energy.
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  
  • Increasing the temperature increases the rate of reaction. As increasing temperature increases the speed of the moving particles, so they collide more frequently and energetically.
  • Increasing pressure in reacting gases increases the rate of reaction, as it increases the frequency of collisions.
  • Increasing concentration of reacting solutions increases the rate of reaction, as it increases the frequency of collisions.
  • Increasing the surface area of solid reactants increases the rate of reaction, as it increases the frequency of collisions.
7.5 Interpret graphs of mass, volume or concentration of reactant or product against time
  • To find the rate of reaction graphically
    • Draw tangents to curves and use the slope of the tangent as a measure of the rate of reaction
    • (HT only) calculate the gradient of a tangent to the curve on these graphs as a measure of rate of reaction at a specific time
  • Gradient = rate of reaction, therefore use this information to interpret any given graph, therefore a steeper line means a greater increase/decrease in rate
7.6 Describe a catalyst as a substance that speeds up the rate of a reaction without altering the products of the reaction, being itself unchanged chemically and in mass at the end of the reaction 
  • Catalysts are substances that speed up chemical reactions without being changed or used up during the reaction.
7.7 Explain how the addition of a catalyst increases the rate of a reaction in terms of activation energy 

  • Catalysts decrease the activation energy; this increases the proportion of particles with energy to react.
  • Catalysts provide a different pathway for a chemical reaction that has a lower activation energy.
7.8 Recall that enzymes are biological catalysts and that enzymes are used in the production of alcoholic drinks
  • Enzymes act as catalysts in biological systems
  • Yeast is the enzyme used in the production of ethanol as it is fermented, ethanol is in alcoholic drinks
7.8 Recall that enzymes are biological catalysts and that enzymes are used in the production of alcoholic drinks
  • Enzymes act as catalysts in biological systems
  • Yeast is the enzyme used in the production of ethanol as it is fermented, ethanol is in alcoholic drinks