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 particles 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 usual unit for reaction rate is mol dm-3s-1
• Initial rate


  The rate at the start of the reaction where it is fastest
• Measuring reaction rates


  1. Plot concentration of reactant vs time

  2. Gradient of curve is the rate of reaction
• 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
• At higher temperatures


  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
• Comparing rate curves


  1. Need to calculate/compare initial moles of reactants to distinguish between different finishing volumes

  2. Different volumes of the same initial concentrations will have the same initial rate (if other conditions are the same) but will end at different amounts
• In the experiment between sodium thiosulfate and hydrochloric acid, we usually measure reaction rate as 1/time. The time is the time taken for a cross placed underneath the reaction mixture to disappear due to the cloudiness of the sulfur
• This is an approximation for rate of reaction as it does not include concentration. We can use this because we can assume the amount of sulfur produced is fixed and constant