Rates of Reaction

• COLLISION THEORY

  Reactions take place when particles collide with a certain amount of

  energy.

  The minimum amount of energy needed for the particles to react is called

  the activation energy, and is different for each reaction.

  The rate of a reaction depends on two things:

  the frequency of collisions between particles

  the energy with which particles collide.

  If particles collide with less energy than the activation energy, they will not react.

  The particles will just bounce off each other.
• Chemical reactions occur when particles of reactant collide

  with enough energy to react.
• Changing rate of reaction:

  Anything that increases the number of successful collisions between reactant particles will speed up a reaction.
• Factors that effect rate of reaction

  increased temperature

  increased concentration of dissolved

  reactants, and increased pressure of gaseous

  reactants

  increased surface area of solid reactants

  use of a catalyst.
• Speed of reactions


  Reactions do not proceed at a steady rate. They start off at a certain speed, then get

  slower and slower until they stop.

  As the reaction progresses, the concentration of reactants decreases.

  This reduces the frequency of collisions between particles and so the reaction slows

  down.
• Rate of reaction

  Rate of reaction = product made / time
• Exothermic reactions


  Release energy and get hot
• Endothermic reactions


  Absorb energy and get cold.
• Catalysts


  Catalysts provide an alternative lower energy pathway in a reaction.

  This lowers the activation energy of a reaction.

  More particles that collide will overcome activation energy.

  So the rate of reaction increases.
• Effect of temperature on rate of reaction:

  At higher temperatures, molecules move faster and have more

  kinetic energy.

  So more molecules collide with more energy (to overcome activation

  energy) and more often.
• 'Disappearing cross' - practical to investigate the effect of temperature on rate of reaction:


  Method

  • Warm 10cm3 of dilute sodium

  thiosulphate solution in a water bath (or

  over a gentle Bunsen burner flame).

  • Take the temperature with a

  thermometer.

  • Place the flask over a black cross

  drawn on a piece of paper.

  • Add 10cm3 of dilute hydrochloric acid

  and immediately start a stopwatch.

  • Time how long it takes for the cross to

  ‘disappear’.

  • Repeat the experiment at a minimum of

  5 different temperatures.