Kinetics
From Ibstudy
Contents |
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:
- Collide more often
- 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:
- B + C -> D + F (B and C turn into D and F)
- 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.
