Kinetics

• Activation energy (EA)


  The minimum energy which particles need to collide to start a reaction
• Maxwell-Boltzmann energy distribution


  Shows the spread of energies that molecules of a gas or liquid have at a particular temperature
• The energy distribution should go through the origin because there are no molecules with no energy
• The energy distribution should never meet the x axis, as there is no maximum energy for molecules
• The mean energy of the particles is not at the peak of the curve
• The area under the curve represents the total number of particles present
• A few have low energies because collisions cause some particles to slow down
• Only a few particles have energy greater than the EA
• Most molecules have energies between the two extremes but the distribution is not symmetrical (normal)
• Emp


  The most probable energy (not the same as mean energy)
• As the temperature increases


  The distribution shifts towards having more molecules with higher energies
• The total area under the curve should remain constant because the total number of particles is constant
• At higher temperatures the molecules have a wider range of energies than at lower temperatures
• At higher temps both the Emp and mean energy shift to higher energy values, although the number of molecules with those energies decrease
• Reaction rate


  The change in concentration of a substance in unit time
• The initial rate is the rate at the start of the reaction where it is fastest
• At higher concentrations (and pressures)


  There are more particles per unit volume and so the particles collide with a greater frequency and there will be a higher frequency of effective collisions
• If concentration increases, the shape of the energy distribution curves do not change (i.e. the peak is at the same energy) so the Emp and mean energy do not change
• The curves will be higher, and the area under the curves will be greater because there are more particles
• More molecules have energy > EA (although not a greater proportion)
• Increasing surface area


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


  Increases reaction rates without getting used up by providing an alternative route or mechanism with a lower activation energy
• As the temperature increases

  The energy of the particles increases, the particles collide more frequently and more often with energy greater than the activation energy, so more collisions result in a reaction
• As the temperature increases, a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases
• With a lower activation energy


  More particles have energy greater than the activation energy
• Reaction progress


  1. Reactants

  2. Activation energy without catalyst

  3. Activation energy with catalyst

  4. Products