264 14 · SPEED OF CHEMICAL REACTIONS
In an experiment carried out at 65 °C, the concentration of
N 2 O 5 changed with time as follows:
[N 2 O 5 ]/mol dm^3 Time after start of reaction/h
.10 10 ^2 0 (i.e. at start)
8.6 10 ^2 0.50
7.3 10 ^2 1.0
6.3 10 ^2 1.5
5.4 10 ^2 2.0
4.6 10 ^2 2.5
3.9 10 ^2 3.0
3.4 10 ^2 3.5
2.9 10 ^2 4.0
(i) What is the overall order of reaction?
(ii)Plot [N 2 O 5 (g)] against time. Join the points with a smooth
curve.(iii)From your graph, find the initial rate of reaction.
(iv)Calculate the rate constant for the reaction at 65 °C and
state its units.(v)What is the half-life of N 2 O 5 at this temperature?
14.6.The hydrolysis of bromomethane,
CH 3 Br(l)OH(aq)CH 3 OH(aq)H 2 O(l)follows the rate expression:
rate of reaction k[CH 3 Br(l)][OH(aq)]The rate constant kwas found to be 3.0 10 ^4 mol^1 dm^3 s^1
at 300 K.
(i)With excess OH(aq), the reaction was found to be pseudo
first order. What does this mean? What would be the re-
action order if the bromomethane (and not the OH(aq))
was in excess?(ii)In an experiment [OH(aq)] 0 1.0 10 ^2 mol dm^3 and
[CH 3 Br(l)] 0 1.0 10 ^4 mol dm^3. What is the value of
(a) the initial rate of reaction, (b) the pseudo-first-order
rate constant and (c) the pseudo-reaction half-life, at 300 K
at this hydroxide concentration?14.7.The reaction of chlorine oxide (ClO•) radicals with
nitrogen dioxide,
ClO•(g)NO 2 (g)N 2 (g)ClONO 2 (g)N 2 (g)
is an important ‘sink’ for chlorine oxide radicals in the atmo-
sphere. (The nitrogen absorbs the excess energy of colliding
molecules and acts as a type of catalyst.) The reaction was stud-
ied in the laboratory and showed the following dependence
upon the concentrations of the reactants at 298 K:(i) Write down the rate expression for the reaction.
(ii)Calculate an average value of k(298 K).
(iii)Is the rate expression consistent with the formation of
chlorine nitrate being a single-stage reaction? Explain.14.8.Draw an energy profile, similar to Fig. 14.7, for an
endothermicreaction.14.9.Show, starting from the equation [A]t[A] 0 ekt, that
the time (symbolized t0.99) it takes for a first-order reaction to
use up 99% of the reactant is given by the expressiont0.994.605
k
(HintAftert0.99seconds, the ratio of initial to actual concentra-
tions is 1/100.)14.10.The reaction:
2CO(g)O 2 (g)2CO 2 (g)
is catalysed by powdered platinum in ‘catalytic converters’.
Draw sketches (similar to those found in Fig. 14.12) which show
a possible mechanism for the reaction. For simplicity, represent
carbon monoxide as CO.Initial concentration/mol dm^3Experiment [ClO(g)] 0 [NO 2 (g)] 0 [N 2 (g)] 0 Initial rate of
reaction/
mol dm^3 s^1
11 .0 10 ^5 2.0 10 ^5 3.0 10 ^5 3.5 10 ^4
20 .5 10 ^5 2.0 10 ^5 3.0 10 ^5 1.8 10 ^4
31 .0 10 ^5 4.0 10 ^5 3.0 10 ^5 7.1 10 ^4
40 .5 10 ^5 2.0 10 ^5 6.0 10 ^5 3.6 10 ^4The graphical determination of the Arrhenius activation energy is discussed in Appendix 14 on the
website.