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|
Experiment |
[A]/M |
[B]/M |
[C]/M |
Initial rate/arbitrary units |
|
|
1 |
0.01 |
0.02 |
0.02 |
4.0 × 10-5 |
|
|
2 |
0.1 |
0.02 |
0.02 |
3.9 × 10-3 |
|
|
3 |
0.01 |
0.04 |
0.02 |
3.9 × 10-5 |
|
|
4 |
0.1 |
0.02 |
0.06 |
1.2 × 10-4 |
First compare experiments 1 and 2. The concentration of A increase by a factor of ten and the other concentrations are unchanged. This will allow us to work out the order with respect to A. The rate in experiment 1 is 4 × 10-5. If A is zero order we would expect the new rate to remain unchanged at 4 × 10-5. If it is first order it should increase tenfold to 4 × 10-4. If it is second order it should increase by a factor of 100 (102) to 4 × 10-3. The actual result is 3.9 × 10-3. This is clearly closest to second order.
Comparing experiments 1 and 3 gives us the order for B. The concentration of B has doubled (A and C are unchanged), but the rate is almost unchanged from 4 × 10-5 to 3.9 × 10-5. This reactant is zero order.
Finally, comparing experiments 2 and 4 gives us the order for C. The concentration of C increases by 3 whilst A and B remain the same. The rate has changed from 4 × 10-5 to 1.2 × 10-4. This is an increase of exactly three. Where the concentration and rate go up by the same amount the reactant is first order.
We can put this all together in the final rate equation:
|
Rate = k[A]2[C] |