5 Kinetics

• Reactions can only occur when collisions take place between particles having sufficient energy. The energy is usually needed to break the relevant bonds in one or either of the reactant molecules.This minimum energy is called the activation energy.
• The activation energy, EA, is defined as the minimum energy which particles need to collide to start a reaction.
• Reaction curve:

  A) reactants

  B) products

  C) Activation energy

  D) Enthalpy change

  
• The Maxwell-Boltzmann energy distribution curve shows the spread of energies that molecules of a gas or liquid have at a particular temperature
• In a Maxwell-Boltzmann distribution curve:

  • The energy distribution should go through the origin because there are no molecules with no energy
  • The mean energy of the particles is not at the peak of the curve
  • Emp this is the most probable energy
  • The area under the curve represents the total number of particles present
  • The distribution is not symmetrical
  • Only a few particles have energy greater than the EA
  • The energy distribution should never meet the x axis, as there is no maximum energy for molecules
• Maxwell-Boltzmann Distribution Curve:

  A) Kinetic energy

  B) Number of molecules

  C) Most probable energy

  D) Mean particle energy

  E) Activation energy

  
• How can a reaction go to completion if few particles have energy greater than EA?

  Particles can gain energy through collisions
• Increasing temperature effect on maxwell-boltzmann distribution curve:

  • The distribution shifts towards having more molecules with higher energies
  • Both the most probable energy and mean energy shift to higher energy values, although the number of molecules with those energies decrease.
  • The molecules have a wider range of energies than at lower temperatures.
  • The total area under the curve should remain constant because the total number of particles is constant
• Increase in temperature effect on the Maxwell-Boltzmann distribution curve:

  A) Activation energy

  B) higher

  C) Lower

  D) Mean particle energy

  
• The rate of reaction is defined as the change in concentration of a substance in unit time. The usual unit is moldm-3s-1
• The initial rate is the rate at the start of the reaction where it is fastest.
• Reaction rates can be calculated from graphs of concentration of reactants or products, by drawing a tangent to the curve and calculating the gradient of the tangent.
• Complete the diagram:

  A) rate of reaction

  B) concentration

  
• 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.
• 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 successful collisions.
• Note: 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.
• Effect of increasing concentration/pressure on the Maxwell-Boltzmann Distribution curve:

  • The shape of the energy distribution curves does not change (i.e. the peak is at the same energy) so the most probable energy and mean energy do not change.
  • They curves will be higher, and the area under the curves will be greater because there are more particles.
  • More molecules have energy > activation energy (although not a greater proportion)
• Different volumes of the same initial concentrations will have the same initial rate of reaction (if other conditions are the same) but will end at different amounts
• The higher the concentration/ temperature/ surface area the faster the rate of reaction (steeper the gradient)
• At higher temperatures the energy of the particles increases. The pariticles collide more frequently and with energy greater than the activation energy. More collisions result in a reaction.
• In a maxwell-boltzmann distribution curve, as the temperature increases, the graph shows that a significantly bigger proportion of particles have energy greater than the activation energy, so the frequency of successful collisions increases.
• 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 the rate of reaction without getting used up by providing an alternative route or mechanism with a lower activation energy.
• Complete the graph labels:

  A) without

  B) with

  
• If the activation energy is lower, more particles will have energy > EA, so there will be a higher frequency of successful collisions. The reaction will be faster.
• Complete the graph labels:

  A) with catalyst

  B) without catalyst

  C) activation

  D) more

  E) greater