Experiment [BF3] [NH3] Initial rate, M/s
1 0.250 0.250 0.2130
2 0.250 0.125 0.1065
3 0.200 0.100 0.0682
4 0.350 0.100 0.1193
5 0.175 0.100 0.0596
where m and n are the orders with respect to BF3 and NH3, respectively. We must use the experimental data above to determine these quantities. First, we will look for two experiments in which [BF3] is constant, and [NH3] is changed, and we will look at the effect of the reaction rate when [NH3] is varied. We can use experiments 1 and 2 for this purpose: [BF3] is constant, and [NH3] is changed. The ratio of the rates is
So, when we double [NH3], the rate of the reaction doubles. This means that the order of the reaction with respect to NH3 must be 1, so we set n=1 in the above rate law.
Now, we find the value of a. We want to find two experiments where NH3 is constant and BF3 is varied. Experiments 4 and 5 will do nicely; in these experiments, the ratio of BF3 concentrations is
so we notice that when the concentration of BF3 is doubled, the reaction rate doubles. We conclude that m = 1, and the reaction is first order in BF3 also.
