Kinetics

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Revision as of 20:09, 4 December 2007

Rates of Reaction

• A rate of reaction for a certain reaction is defined as
  • the rate of decrease in concentration of one of the reactants per unit time, or
  • the rate of increase in concentration of one of the products per unit time.
    • Rates of reaction are therefore measured in mol dm^(-3) s^(-1)

The gradient on a concentration/time graph for a reactant or product involved in a reaction is equal to the rate of reaction. You should be familiar with what these graphs look like -- it is likely that they will show up on the test.

Collision Theory

molecules can react with one another when:

• they hit each other:
  • with the correct collision geometry (the reactive parts get close enough each other to react)
  • and with the required activation energy for that particular reaction

Factors that can change the rate of reaction

Concentration

If there are more molecules in a fixed space, they will collide more often.

• Therefore the rate of reaction will increase with a higher concentration of reactants

Surface area

If the reactants have a higher surface area (eg. they are cut into small pieces) they will have a larger area to react with.

• Therefore the rate of reaction will increase if the reactants have a higher surface area.
  • eg. thin sawdust can practically explode, but wood only burns. Why is this?

Temperature

If the molecules have more kinetic energy, they will:

1. Collide more often
2. Be more likely to possess the required activation energy for the reaction

• Therefore the rate of reaction will increase at higher temperatures

Catalysts

A catalyst can provide an alternate reaction path with lower activation energy or fewer steps. Catalysts are involved in the reaction, but are returned to their original state afterwards.

• If a reaction with lower activation energy is possible, more molecules will have the required activation energy, so more molecules will react quickly.
  • Therefore the presence of a catalyst can increase the rate of reaction.

Rate-determining step

Reactions normally occur in more than one step, for example, the reaction:

A + B + C -> D + E 

may actually happen as a series of reactions:

1. B + C -> D + F (B and C turn into D and F)
2. F + A -> E (the F that has been formed combines with A to form an E)

• Each of these smaller reactions have their own rate.
  • All of the smaller reactions have to be completed for the whole one to be completed.
  • The rate-defining step is the one with the slowest rate.