Rates of reaction

• Collision theory


  For a chemical reaction to happen:

  reactant particles must collide with each other

  particles must have sufficient energy for them to react
• Successful collision


  collision that produces a reaction
• Activation energy

  Minimum amount of energy needed for a collision to be successful
• Calculate rate of reaction by measuring change in mass


  Useful for gases such as carbon dioxide but not suitable for gases with a small relative formula mass
• Calculate the rate of reaction by measuring the volume of gas produced

  Measured using a gas syringe / upside down burette / measuring cylinder
• If the reactant’s concentration (solution) / pressure (gas) is increased:

  ->reactant particles become more crowded

  ->frequency of collisions between reactant particles increases

  ->rate of reaction increases
• Does increasing the pressure / concentration increase the mean energy of particles?

  No, it only increases the frequency of collisions and successful collisions
• If the surface area to volume ratio of a reacting solid is increased:

  ->more reactant particles are exposed at the surface

  ->frequency of collisions between reactant particles increases

  ->rate of reaction increases

  ->the mean energy of particles does not change
• If the temperature of the reaction mixture is increased:

  ->reactant particles move more quickly

  ->energy of reactant particles increases

  ->frequency of successful collisions between reactant particles increases

  ->proportion of collision which are successful increases

  ->rate of reaction increases
• Catalyst

  increases the rate of reaction

  does not alter the products of the reaction

  is not chemically changed / used up at the end of the reaction
• Catalysts are specific to the reaction they are used in and not used in other reactions
• Enzymes

  catalysts in biological reactions
• How catalysts work?

  ->provide an alternative reaction pathway with lower activation energy

  ->does not change the frequency of collision

  ->increases the frequency of successful collisions as more particles have energy greater than the activation energy
• Reaction profile

  a chart to show how energy of the reactants and products change during a reaction