The Rate and Extent of Chemical Change

• When measuring the rate of reaction, we need to look carefully at the units used
• Units for rate of reaction

  • Volume in cm^3 per minute
  • Mass in g per second
• First graph - volume of carbon dioxide being produced

  Goes up
• Second graph - mass being lost


  Goes down
• Finding the rate of reaction at a particular point

  1. Draw a tangent

  2. Calculate the gradient of the tangent
• The steeper the tangent, the faster the rate of reaction
• Different rates of reaction at different points in the reaction
• Ways to follow a reaction

  • Loss of mass
  • Collecting the gas produced
  • Observing color change
• Reaction between sodium thiosulfate and hydrochloric acid requires careful handling due to health and safety concerns
• As the reaction progresses

  The cross becomes less visible
• The same person should measure the rate of reaction to avoid differences in observations
• Collecting gas using an inverted measuring cylinder


  Gas moves from conical flask through delivery tube into measuring cylinder
• Errors can be introduced from gas already present in the measuring cylinder before the experiment starts
• Powdered calcium carbonate

  Faster rate of reaction than large marble chips
• Successful collisions


  Collisions between particles that result in a reaction
• Higher temperature


  Faster rate of reaction
• Surface area

  More surface area means more particles available to react, leading to more successful collisions
• Higher pressure or concentration


  Faster rate of reaction
• Catalyst


  Something that makes a reaction easier to happen by lowering the activation energy
• Activation energy


  The energy barrier that reactants must overcome to form products
• Catalysts lower the activation energy, making the reaction easier to occur
• Reversible reaction


  A reaction that can go in both the forward and reverse directions
• Endothermic reactions absorb energy, exothermic reactions release energy
• Le Chatelier's principle: if a system in equilibrium is subjected to a change, the system will shift to counteract the change and restore equilibrium