• Concentration
• Surface Area
• Temperature
• Catalysts

Concentration

• As the concentration of the reactants increases, the reaction rate increases.
• WHY?
  • According to the collision theory, the rate of a reaction is directly proportional to the number of effective collisions per second between the reactant molecules.
    • Effective Collisions - the fraction of total collisions that actually result in the formation of the product(s).
  • If the concentration of the reactants increases (i.e. particles per given volume) the greater the number of total collisions.
  • The greater the frequency of total collisions, the greater the frequency of effective collisions.
  • If the frequency of effective collisions increases, so does the reaction rate.

Surface Area

• As the surface area of the reactants increases, the reaction rate increases.
• WHY?
  • Increasing the surface area of the reactants results in a higher number of reaction sites.
    • Reaction sites - specific sites on molecules at which reactions occur.
  • Increasing the number of reaction sites increases the number of total collisions.
  • The greater the frequency of total collisions, the greater the frequency of effective collisions.
  • If the frequency of effective collisions increases, so does the reaction rate.

Temperature

• As the temperature of a system increases, the reaction rate increases.
• WHY?
  • Temperature (T) - A measure of the average kinetic energy (KE_avg) of the particles of a substance.
  • Increasing T increases KE_avg.
  • At higher T, the fraction of molecules with energies greater than the activation energy (E_a) increases.
    • Activation Energy (E_a) - the energy level that must be overcome for a reaction to occur.

	James Clark Maxwell
Ludwig Boltzmann
A Maxwell-Boltzmann diagram shows the distribution of molecules at differing kinetic energy levels in a gas sample.  At higher temperature, a larger fraction of molecules has kinetic energy equal to or greater than the activation energy (Ea) for the reaction than at lower temperature.  In other words, a greater proportion of molecules have enough kinetic energy to participate in the reaction.

Catalysts

• The presence of a catalyst increases the reaction rate.
• Catalyst - A substance that increases the rate of a reaction but is not consumed in the reaction.  It does so by lowering the activation energy of a reaction.
• Possible ways of lowering the E_a of a reaction:
  1. Increases the frequency of collisions between the reactant molecules.
  2. Changes the relative orientation of the reactant molecules.
  3. Donates electron density to the reactant molecules.
  4. Reduces the intramolecular bonding within the reactant molecules.
  5. Provides an alternate pathway or mechanism for the reaction.

For equilibrium reactions, both the forward and reverse reaction rates are affected by the catalyst. i.e. the Ea for both directions is decreased. Therefore, the equilibrium constant is not changed by the presence of a catalyst. The relative concentrations of the reactants and products is not changed.

• Examples of common catalysts:
  • Platinum
  • Nickel
  • Manganese Dioxide (MnO₂)

Next:  "Activation Energy"