3.1.5 Kinetics

• Rate of reaction is the change of concentration/amount of a reactant or product per unit time.
• Reactions can only occur when collisions take place between particles with sufficient energy (activation energy) and correct orientation.
• Rate of reaction = amount of reactant used or product made / time
• Activation energy is the minimum amount of energy needed for a reaction to occur.
• Maxwell-Boltzmann distribution shows the energy in gas particles.
• The Maxwell-Boltzmann distribution curve has number of molecules on the y-axis and kinetic energy on the x-axis. The graph starts at (0,0) as no particles have zero kinetic energy,
• The area under a Maxwell-Boltzmann curve is equal to the total number of molecules.
• The peak on the Maxwell-Boltzmann curve represents the most likely energy of a particle in the sample.
• The mean energy the particles have is slightly right to the peak on the Maxwell-Boltzmann curve.
• The temperature of a system affects the shape of the Maxwell-Boltzmann curve.

  • As temperature increases, the peak moves further right along the x-axis and is lower.
  • The area under the curve is the same, but the area under curve beyond the activation energy increases.
• The temperature of a system affects the shape of the Maxwell-Boltzmann curve.

  • As temperature decreases, the peak shifts to the left and is higher.
  • The area under the curve is the same, but the area under curve beyond the activation energy decreases.
• How does increased temperature affect rate of reaction?

  • particles have higher KE
  • particles move around faster, so higher frequency of successful collisions as particles collide with high energy that may be above their activation energy
  • increases rate of reaction
• How does increased pressure affect rate of reaction?

  • greater number of particles in a given volume so particles are closer together
  • higher frequency of collisions
  • increases rate of reaction
• How does increased concentration affect rate of reaction?

  • more particles in a given volume
  • leads to higher frequency of collisions
  • increases rate of reaction
• How does a catalyst affect rate of reaction?

  • provides an alternate reaction pathway that has a lower activation energy
  • increases rate of reaction
• Catalysts are chemically unchanged (not used up) during the reaction and speed up the rate of reaction.
• Catalysts affects the shape of the Maxwell-Boltzmann curve.

  • activation energy shifts to left
  • this increases area under curve beyond activation energy
• How can rate of reaction be measured during experiments?

  how long it takes for precipitate to form

  • draw a cross on paper and place under a conical flask, time how long it takes for cross to disappear

  amount of mass lost

  • place reaction on balance and measure mass loss as gas lost for reactions that produce a gas

  volume of gas produced

  • measure amount of gas produced using a gas syringe, measure over a specified time
• Explain why a small increase in temperature causes a large increase in the rate of reaction.

  • many more particles have energy equal to or above activation energy
  • greater frequency of successful collisions
• At fixed temperatures the average kinetic energy of gases is constant.