Summary
Summary ALEVEL CHEMISTRY - Initial Rates and Clock Reactions Practical Notes
- Module
- Unit 3.1.8 - Thermodynamics
- Institution
- AQA
ALEVEL CHEMISTRY - Initial Rates and Clock Reactions Practical Notes
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Initial Rates and Clock ReactionsThe initial rate is the rate at the start of the reaction.
The initial rate of the reaction can be calculated by drawing a tangent at T=0 and calculating the
gradient of the time. Y/X is the initial rate.
The initial rates method is used to work out the order of reaction for each reactant.
Unlike continuous monitoring the reaction does not need to be monitored all the way through -
enough data needs to be collected to work out the initial rate.
Initial Rates Method
1. Carry out the reaction and monitor its progress. Draw a concentration time graph.
2. Repeat the experiment using different initial concentrations of one reactant.
3. Draw another concentration-time graph.
4. Calculate the initial rate for each experiment.
5. Look at how different concentrations affect different rates. Use this to work out the order
of reaction.
If doubling the concentration has no effect on the rate. The reaction order is 0 in
respect to X.
If doubling the concentration increases the rate by the same factor of 2. The reaction
order is 1 in respect to X.
If doubling the concentration increases the rate by a factor of 4. The reaction order is
2 in respect to X.
Clock Reactions
In clock reactions the initial rate can be estimated. The reactions involve how the time taken to
reach a fixed concentration of a product changes as the concentration of the reactants
changes. It follows the principle that before a certain reactant (limiting reactant) is used up
there is a sudden increase in the concentration of the product. This usually leads to an easily
observable end point such as a colour change.
In clock reactions there are often two successive reactions. The end point is achieved when one
limited reactant runs out, resulting in a sudden colour change.
By repeating the experiment several times, varying the concentration of a reactant e.g. I– , (
keeping the other reactants at constant concentration )you can determine the order of reaction
with respect to that reactant.
Assumptions Made
The concentration of each reactant does not change significantly over the time period of
the clock reaction.
Temperature is constant.
When end point is seen reaction does not proceed further than it.
Iodine Clock Reaction
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