1.5 Kinetics

• Define rate of reaction.

  The change in the concentration of reactants or products per unit time (state the units if given!)
• Describe the graph below. [3]

  Between A-B the rate is fast and constant because there are many particles present at the start of the reaction.

  At B-C the concentration of at least one reactant falls so the rate decreases.

  When one of the reactants is completely used up the graph levels off since the concentration doesn't change.

  
• Define activation energy.

  The minimum energy required for particles to react.
• Before a reaction can occur what two things must happen?

  • Particles must collide with enough energy to overcome activation energy.
  • Particles must collide in the correct orientation.
• Why do most collisions not result in a reaction?

  They do not have the activation energy required for a successful collison.
• List five factors that can affect the rate of a reaction.

  1. Surface Area.
  2. Temperature.
  3. Pressure.
  4. Concentration.
  5. Catalyst.
• How does an increased surface area increase the rate of reaction?

  It results in more successful collisions because the surface area increases meaning that more solid particles are exposed to the other reactants.
• How does an increased temperature increase the rate of reaction? [2]

  It increases the energy of all particles resulting in more successful collisions increasing the number of particles with activation energy.
• How does an increased pressure increase the rate of reaction?

  It results in more successful collisions because the number of particles per unit volume increases
• How does an increased concentration increase the rate of reaction?

  It results in more successful collisions because the number of particles per unit volume increases.
• What is a catalyst and how does it increase the rate of reaction?

  A catalyst increases the rate of reaction without being chemically changed.

  It increases the rate of reaction by providing an alternative pathway of lower activation energy.
• Which of the 5 factors has the greatest effect on the rate of reaction?

  Temperature as it has two reasons for increasing the rate of a reaction:

  1. It increases the energy of all particles resulting in more successful collisions.
  2. It increases the number of particles with activation energy.
• How do we recognise if a catalyst is in a reaction?

  A catalyst appears in the reaction steps but not in the overall equation as it is chemically unchanged at the end of the reaction (used but reformed).
• What is the catalyst in this reaction? Why?

  V2O5 as it's used as a reactant in the initial step but reformed in the final step so must be a catalyst.

  
• Why do reactions between solids usually occur slowly, if at all?

  The particles are in a fixed position and not free to move so the chances of successful collisions is very low.
• Fill in the following table:

  A) No effect

  B) No effect

  C) No effect

  D) Increases

  E) Increases

  F) No effect

  G) Increases

  H) Increases

  I) No effect

  J) Decreases

  K) Increases

  L) Increases

  
• What does a Maxwell-Boltzmann distribution curve show?

  It shows the molecular energies present in a gas.
• Suggest why a small increase in temperature can lead to a large increase in the reaction rate between colliding particles.

  A small temperature increase results in a greater amount of particles that are at or past the activation energy resulting in a increase in the reaction rate between colliding particles.
• Give one reason why the reaction between hydrogen and chlorine is very slow at room temperature.

  The activation energy required for this reaction is high so fewer successful collisions will occur
• In a Maxwell-Boltzmann distribution curve why does the curve start at the origin?

  Because there are no molecules with zero energy
• In the following equations, suggest two reasons, other than an increase in the reaction rate, why these equations suggest that hydrogen bromide is a catalyst.

  H2O2 + HBr --> HBrO + H2O

  HBrO + H2O2 --> H2O + O2 + HBr

  Reason 1: Hydrogen bromide reacts in step 1.

  Reason 2L Hydrogen bromide is reformed in step 2.
• Fill in this table:

  A) Decreases

  B) Decreases

  C) Increases

  D) No change

  E) Decreases

  F) Increases

  G) Increases

  H) Decreases

  I) No change

  J) Increases