Many reactions occur over a number of steps. The slowest of these steps determines the rate of the reaction and is called the "rate determining step".
A. Rate Expression (or Rate Law)
For a given reaction:
aA + bB cC + dD (where a, b, c & d are the stoichiometric
number - for balancing)
Rate = k [A]x [B]y (x & y have nothing to do with a, b, c or d)
(k is called the "rate constant")
{using square brackets around a substance, [A], means the concentration of "A" in mol dm-3}
The rate is always the initial rate and is found by monitoring the concentration of one of the species. If [C] is being monitored, the rate is equal to .
This means we can rewrite the rate expression as:
= k [A]x [B]y
We can only find the values of "x" and "y" by doing experiments.
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D-558-2 launch from B-29 mothershipIn these experiments we first measure the rate with certain known concentrations. Then we vary one concentration while keeping the other constant and measure the rate again.
If we double [A] and the rate doesn't change x = 0
If we double [A] and the rate also doubles x = 1 (because 2 = 21)
If we double [A] and the rate increases 4 times x = 2 (because 4 = 22)
We then keep [A] constant while varying the concentration of the other species and use the same logic.
Eg
Experiment [A] [B] Rate
1 0.1 mol dm-3 0.1 mol dm-3 0.02 mol dm-3 s-1
2 0.2 mol dm-3 0.1 mol dm-3 0.04 mol dm-3 s-1
3 0.1 mol dm-3 0.2 mol dm-3 0.08 mol dm-3 s-1
Analysis:
Using Exp 1 & 2, [A] doubles while [B] is constant. The rate also doubles. Therefore x = 1.