Rates of Reaction

The rate of a reaction is a measure of how quickly a reactant is used up, or a product is formed.

  • Collision theory
  • For a chemical reaction to happen:
      * Reactant particles must collide with each other
      * The particles must have enough energy for them to react
  • A collision that produces a reaction is called a successful collision.
  • The activation energy is the minimum amount of energy needed for a collision to be successful. It is different for different reactions.

Colliding particles

  • Two pairs of particles move towards each other
  • There are different ways to determine the rate of a reaction.
      * The method chosen usually depends on the reactants and products involved, and how easy it is to measure changes in them.
  • The mean rate of reaction can be calculated using either of these two equations:
      * Mean rate of reaction=quantity of reactant used time taken
      * Mean rate of reaction=quantity of product formed time taken

Measuring mass

  • The change in mass of a reactant or product can be followed during a reaction.
  • This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small relative formula mass, Mr.
      * The units for rate are usually g/s or g/min.

Measuring volume

  • The change in volume of a reactant or product can be followed during a reaction.
  • This method is useful when a gas leaves the reaction container.
  • The volume of a gas is measured using a gas syringe, or an upside down burette or measuring cylinder.
  • The units for rate are usually cm3 s-1 or cm3 min-1.

Two ways to measure the volume of a gas produced in a reaction

  • Graphs

  • The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time.

  • The graph shows this for two reactions.

  • The steeper the line, the greater the rate of reaction.

  • Faster reactions - where the line becomes horizontal - finish sooner than slower reactions

  • The gradient of the line is equal to the rate of reaction:

  • the steeper the line, the greater the rate of reaction

  • fast reactions - seen when the line becomes horizontal - finish sooner than slow reactions