Reaction Rates

There are many factors that can influence the speed at which a reaction takes place, and these can change with time. In general, it is a function of the state of the system and the nature of the reactants themselves. It is reasonable to assume that, for a reaction to occur, the molecules of the reactants must collide and collide with sufficient energy to initiate a reaction. The number of collisions per unit time can be expected to change and increase with the number of molecules of the reactants available. At higher temperatures the number of collisions is also expected to increase (the molecules are more agitated) and collide with more energy. The rate of reaction also depends on the presence and amount of catalysts or inhibitors. These materials increase or decrease the reaction speed, respectively, although they don't appear in the chemical equation.m (They may be found written above the arrow). The following note presents some of the factors that result in a change in a Reaction Rate

• 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.
    • What is an Effective Collisions? - the fraction of total collisions that actually result in the formation of the product(s).
  • If the concentration of the reactants increases the greater the number of total collisions that can occur.
  • 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 which results in more product being formed.

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
    • What is a Reaction sites? - specific sites on molecules at which reactions occur. More molecules are exposed to collisions with other reacting molecules.
  • 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?
  • What is Temperature (T)? - A measure of the average kinetic energy (KE_avg) of the reacting particles of a substance.
  • Increasing T increases KE_avg of the molecules in the reaction mixture..
  • At higher T, the fraction of molecules with energies which enable the reaction to proceed increases. All chemical reactions require energy to initiate the process of reaction. If molecules do not have this specific amount of energy, then no reaction will occur. That why giving the reacting molecules more energy will increase the number of effective reaction collisions.

Catalysts

• The presence of a catalyst increases the reaction rate.
• What is a Catalyst? - A substance that increases the rate of a reaction but is not consumed in the reaction. It does so by lowering the initial amount of energy required for the reaction to proceed by giving the reaction a different pathway to follow on its way to completion.
  The theory that is used to explain this phenomena is called Transition State Theory It simply infers that energy is required to initiate a reaction mechanism so that molecular bonds can be broken and then remade, but this time with other atoms. A diagram to show this is below
  

  See page 264 of text book for a picture that shows this very same idea.

• Possible ways of lowering the start-up energy of a reaction are as follows:
  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.

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

When a chemical reaction happens, we say that the reactants react to form the products e.g.

HCl	+	NaOH		NaCl	+	H2O
Reactant A		Reactant B		Product C		Product D
hydrogen chloride		sodium hydroxide		sodium chloride		water

So, to alter the rate of a reaction, we need to do something to the reactants. There are 4 main ways to change the rate of a chemical reaction. Remember, an increase in rate means it is going faster:

1	Change the concentration of the reactants (or the pressure, if the reactants are gases).	Increasing the concentration of reactants (making them stronger), increases the rate.
2	Change the temperature of the reactants.	Increasing the temperature of reactants, increases the rate.
3	Change the size of the solid lumps of a reactant.	Using smaller lumps (increasing the surface area), increases the rate.
4	Use a catalyst.	Using a catalyst, usually increases the rate (some catalysts can decrease the rate).

If you had to prove any of these conclusions, what experiments could be conducted and how would you know if the rate has changed?

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