Question

The data below show the concentration of N2O5 versus time for the following reaction:

N2O5(g)→NO3(g)+NO2(g)

Time (s)	[N2O5] (mol L−1)
0	1.000
25	0.822
50	0.677
75	0.557
100	0.458
125	0.377
150	0.310
175	0.255
200	0.210

Determine the order of the reaction.

Answer 1

Answer: Rate=K[N2O5] and it's a first order reaction

The best way to find the rate constant for a first order reaction is to graph the natural log of the concentration vs time. The slope is the negative of the rate constant.

Instead, I used the formula

ln (A/Ao) = -kt

and solved for k using 1.000 as Ao and the concentrations you give as A and the times as t.

At 25 sec, I got 7.84 x 10^-3, at 50 sec I got 7.80 x10^-3, then 7.80, then 7.81, then 7.81, then 7.80 (always x 10^3). So, I'd be willing to call the rate constant 7.81 x 10^-3 though you might argue for simply 7.8 x 10^-3 and it's important to know how exactly the time was measured.

The units are sec ^-1