Kinetics
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==Rates of Reaction== | ==Rates of Reaction== | ||
*A '''rate of reaction''' for a certain reaction is defined as | *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 '''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. | **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) | + | ***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. | 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== | ==Collision Theory== | ||
molecules can react with one another when: | molecules can react with one another when: | ||
| - | *they hit each other: | + | **they hit each other: |
**with the correct collision geometry (the reactive parts get close enough each other to react) | **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 | **and with the required '''activation energy''' for that particular reaction | ||
| + | A good way to remember this is the acronym CEO: | ||
| + | **Collision | ||
| + | **Energy | ||
| + | **Orientation | ||
| - | + | ==Factors that can change the rate of reaction== | |
| - | + | ===Concentration=== | |
If there are more molecules in a fixed space, they will collide more often. | 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 | *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. | 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. | *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? | **eg. thin sawdust can practically explode, but wood only burns. Why is this? | ||
| - | + | ===Temperature=== | |
If the molecules have more kinetic energy, they will: | If the molecules have more kinetic energy, they will: | ||
#Collide more often | #Collide more often | ||
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*Therefore the rate of reaction will increase at higher temperatures | *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. | 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. | *If a reaction with lower activation energy is possible, more molecules will have the required activation energy, so more molecules will react quickly. | ||
Current revision as of 08:24, 30 August 2010
Contents |
[edit] 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.
[edit] 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
A good way to remember this is the acronym CEO:
- Collision
- Energy
- Orientation
[edit] Factors that can change the rate of reaction
[edit] 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
[edit] 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?
[edit] 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
[edit] 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.
[edit] 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.
