Kinetics

• Reactions can only occur when collisions take place between particles having sufficient energy. This energy is called the activation energy.
• The activation energy is the minimum energy required for the colliding particles to react, a successful collision.
• Concentration of reactants in solution, pressure of reacting gasses, temperature, surface area of solid reactants, and presence of a catalyst all affect the rate of a chemical reaction.
• Factors affecting rate of reaction: Concentration

  • If concentration increases, there are more particles of that reactant present.
  • This leads to more successful collisions between reactant particles in a given period of time, increasing the rate of reaction.
• Factors affecting rate of reaction: Pressure

  • If pressure increases, the particles are forced closer together.
  • Leading to more successful collisions between reactant particles in a given period of time, increasing the rate of reaction.
• Factors affecting rate of reaction: Temperature

  • If temperature increases, the particles gain more kinetic energy (move faster).
  • Leading to more frequent and successful collisions between reactant particles in a given period of time, increasing the rate of reaction.
• Factors affecting rate of reaction: Surface area

  Increasing the surface area of solid reactants (by grinding) increases the exposed surface of the reactants. This increases the number of successful collisions in a given period of time which increases the rate of reaction.
• Factors affecting rate of reaction: Presence of a catalyst

  • A catalyst increases the rate of reaction without being used up.
  • A catalyst provides an alternative reaction pathway of lower activation energy. Leading to more successful collisions in a given period of time and so the rate of reaction increases.
• Explain why most collisions do not lead to a reaction.

  Most collisions don't lead to a reaction because reactant particles need both sufficient energy (activation energy) and the correct orientation to break bonds and form new ones, which is a requirement for a successful collision. 
• A Maxwell–Boltzmann distribution is a plot of the number of gaseous molecules against the energy they have at a fixed temperature. A single plot on the graph shows the distribution of molecular energies at a constant temperature.
• As the Maxwell–Boltzmann distribution represents the energy of the reactant molecules there will be an energy value on the x-axis which is the activation energy.

  The blue area represents all the reactant molecules which have energy above the activation energy, so will successfully collide.

  The green area represents a catalyst which increases the rate of reaction by providing an alternative reaction route with lower activation energy, so the area is greater but the shape is the same.

  
• Maxwell–Boltzmann distribution at different temperatures:

  • Lower temperature distributions are moved to the left and the peak is higher.
  • Higher temperature distributions are moved to the right and the peak is lower.

  
• A small increase in temperature can cause a large increase in the rate of reaction as there is a significant increase in the number of molecules with enough energy to undergo a successful collision.
• The rate of reaction is the change in concentration of a reactant or product in a given period of time.
• Maxwell–Boltzmann distribution at an increased concentration:

  • Curve retains the same basic shape. This means that the most probable energy of the molecules remains the same so the peak should get higher but at the same energy value on the horizontal axis.
  • More total reactant molecules at the same temperature, the overall area under the curve increases. This increases the number of reactant molecules which have enough energy to undergo a successful collision.
• • Increased surface area = more particles exposed = greater collision frequency
  • Increased concentration = particles closer together = greater collision frequency
  • Increased temperature = particles have more energy and move faster = greater collision frequency and more collisions are successful = double effect on rate
  • Adding a catalyst = lower activation energy = more collisions are successful
• A catalyst is a substance that increases the rate of a chemical reaction without being changed in chemical composition or amount. Catalysts work by providing an alternative reaction route of lower activation energy.