reaction rates

• Activation Energy


  The minimum energy which particles need to collide to start a reaction
• Reactions


  1. Collisions take place between particles having sufficient energy

  2. Energy is usually needed to break the relevant bonds in one or either of the reactant molecules
• Collision theory


  • Reactions can only occur when collisions take place between particles having sufficient energy
  • The minimum energy is called the Activation Energy
• Higher concentrations (and pressures)


  More particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions
• Reaction rate


  The change in concentration of a substance in unit time
• Measuring reaction rate


  • Measuring time taken for a cross to disappear due to cloudiness of sulfur
  • Assuming amount of sulfur produced is fixed and constant
• If a question mentions a doubling of concentration/rate then make sure you mention double the number of particles per unit volume and double the frequency of effective collisions
• Higher concentration/temperature/surface area


  Faster the rate (steeper the gradient)
• Graphs of concentration vs time

  • Gradient of the curve is the rate of reaction
  • Initial rate is the rate at the start of the reaction where it is fastest
  • Reaction rates can be calculated from graphs by drawing a tangent to the curve and calculating the gradient
• Catalysts


  • Increase reaction rates without getting used up
  • They do this by providing an alternative route or mechanism with a lower activation energy so more molecules have energy above activation energy
• Heterogeneous catalysis


  Catalyst is in a different phase from the reactants, reaction occurs at the surface of the catalyst
• Homogeneous catalysis


  Catalyst is in the same phase as the reactants, reaction proceeds through an intermediate species
• Techniques to investigate rates of reaction

  • Measurement of the change in volume of a gas
  • Titrating samples of reaction mixture
  • Colorimetry
  • Measurement of change of mass
  • Measuring change in electrical conductivity
• There are several different methods for measuring reactions rates. Some reactions can be measured in several ways
• Maxwell-Boltzmann energy distribution


  Shows the spread of energies that molecules of a gas or liquid have at a particular temperature
• As the temperature increases


  The energy distribution shifts towards having more molecules with higher energies
• Increasing surface area


  Causes collisions to occur more frequently between the reactant particles and this increases the rate of the reaction
• Increasing temperature


  The energy of the particles increases, they collide more frequently and more often with energy greater than the activation energy, so the frequency of successful collisions increases