Rates of reaction 2.2

• Rates of reaction


  1. Rate is fastest at the start; the concentration of reactants is at their highest

  2. Rate slows down as the reaction proceeds; the concentration of reactants has decreased

  3. Rate is zero when reaction stops; one of the reactants has been used up
• Calculating rates


  1. Measure the amount of a reactant used up, or amount of a product formed, over a period of time

  2. Draw a graph of the results and the gradient of this line gives the initial rate of reaction

  3. If the line is a curve, draw a tangent at t=0 to find the gradient
• Factors affecting rates of reaction

  • Concentration of a solution (pressure of a gas)
  • Surface area of a solid
  • Temperature
  • Catalyst
  • Light (in some reactions)
• Collision theory


  • For a chemical reaction to take place, reacting molecules must collide effectively
  • The reaction rate is a measure of how frequently effective collisions occur
  • For a collision to be effective the molecules must collide in the correct orientation and have sufficient energy
  • The minimum energy needed is called the activation energy
• Activation energy


  The minimum energy needed for a collision to be effective
• Energy profiles


  Diagrams that compare the enthalpy of the reactants with the enthalpy of the products

  
• In an exothermic reaction

  The products have less energy than the reactants and the excess energy is lost from the reaction as heat, so ΔH is negative
• In an endothermic reaction


  The energy of the products is more than the energy of the reactants and heat is taken in from the surroundings, so ΔH is positive
• Enthalpy change of reaction


  ΔH = Eaf - Eab
• Increasing the concentration of reactants

  Increases the rate of reaction
• For a gaseous reaction, increasing the pressure


  Is the same as increasing the concentration
• For a solid, reducing the particle size


  Increases the surface area and has the same effect as increasing concentration
• Increasing the temperature of reactants

  Increases the rate of reaction
• Boltzmann energy distribution curve

  Shows that at higher temperatures, more molecules have energy greater than the activation energy

  
• Catalyst


  • Increases the rate of a chemical reaction without being used up in the process
  • Catalysts work by providing a different reaction pathway for the reaction with a lower activation energy

  
• Types of catalyst


  • Homogeneous catalyst (in the same phase as the reactants)
  • Heterogeneous catalyst (in a different state from the reactants)
• Catalysts lower the activation energy of a reaction, saving energy costs and benefiting the environment by reducing fossil fuel use and CO2 emissions
• Studying rates of reaction

  1. Change in gas volume

  2. Change in gas pressure

  3. Change in mass

  4. Change in colour
• There are two specified practical tasks: using a gas collection method and studying an 'iodine clock' reaction
• Rate


  = change in concentration / time