The order with respect to SO 3 is 2, since the rate quadruples while the concentration of
SO 3 doubles (with the concentration of H 2 O remaining constant) between trials 1 and 2.
The order with respect to H 2 O is 1, as the rate triples as the concentration of H 2 O triples
(with the concentration of SO 3 remaining constant) between trials 1 and 4.
X can be calculated by plugging the values from trial 3 into the rate expression. First,
however, calculate the rate constant, k, by plugging in the known values from trial 1, 2, or
- For instance:
Trial 4: 0.039 = k[0.1]^2 [0.03]: k = 130
To calculate X, plug in the values of rate and [H 2 O] for trial 3, using k = 130:
One can also arrive at this answer without first calculating the rate constant by noting
that the concentration of H 2 O is doubled on going from trial 1 to trial 3. Since the
reaction is first order in H 2 O, we would expect the rate to have doubled from the change
in [H 2 O] alone. The fact is, however, that the rate has been increased 18 times (0.013 × 18
= 0.234), and so the remaining factor of 9 (18 = 9 × 2) in the increase has to come from the
change in [SO 3 ]. We know that the reaction is second order in SO 3 , and so a threefold
increase in [SO 3 ] would give us the overall increase we are looking for. Therefore the
concentration of SO 3 is 3 × 0.1 = 0.3.
3
The order of the reaction is the sum of the exponents in the rate expression: in this case,
2 + 1 = 3.
B.
130
For calculations, see solution to part A.
C.
0.156 units
Substitute 0.2 instead of 0.1:
D.
