Topic 9 - kinetics

• Activation Energy


  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
• Activation Energy (EA)


  The minimum energy required for a reaction to occur
• Reaction rate


  The change in concentration of a substance in unit time
• 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)
• The higher the concentration/ temperature/ surface area the faster the rate (steeper the gradient)
• In the experiment between sodium thiosulfate and hydrochloric acid we usually measure reaction rate as 1/time where 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
• Increasing surface area will cause successful collisions to occur more frequently between the reactant particles and this increases the rate of the reaction
• Catalysts


  Increase reaction rates without getting used up
• Catalysts


  Provide an alternative route or mechanism with a lower activation energy
• If the activation energy is lower


  More particles will have energy > EA, so there will be a higher frequency of effective collisions. The reaction will be faster
• As the temperature increases


  The energy of the particles increases. They collide more frequently and more often with energy greater than the activation energy. More collisions result in a reaction
• Increasing pressure has limited effect on the rate of heterogenous catalysed reactions because the reaction takes place on surface of the catalyst. The active sites on the catalyst surface are already saturated with reactant molecules so increasing pressure wont have an effect
• Heterogeneous catalysis


  The reaction occurs at the surface of the catalyst
• Active site


  The place where the reactants adsorb on to the surface of the catalyst
• Industrially catalysts speeds up the rate allowing lower temperature to be used (and hence lower energy costs) but have no effect on equilibrium
• Catalysed reactions can occur at lower temperature so less fuel needed and fewer emissions from fuels
• Catalysed reaction enables use of an alternative process with higher atom economy so meaning fewer raw materials needed and less waste products are produced