Page 384 - Physical chemistry understanding our chemical world

KINETIC DEFINITIONS 351

Table 8.1 Selection of rate constants k
◦
Reaction Phase Temperature/ C k/ units a
First-order reactions
Gas 320 2 × 10 −5
SO 2 Cl 2 → SO 2 + Cl 2
Cyclopropane → propene Gas 500 6.71 × 10 −5
ž −4
C 2 H 6 → 2CH 3 Gas 700 5.36 × 10
=CHCl + HCl Gas 780 4.4 × 10 −3
ClH 2 C–CH 2 Cl → CH 2
Second-order reactions
−
−
ClO + Br → BrO + Cl − Aqueous 25 4.2 × 10 −7
−
|
CH 3 COOC 2 H 5 + NaOH → CH 3 CO 2 Na + C 2 H 5 OH Aqueous 30 1.07 × 10 −2
|
H 2 + I 2 → 2HI Gas 400 2.42 × 10 −2
2NO 2 → 2NO + O 2 Gas 300 0.54
H 2 + 2NO → N 2 + 2H 2 O Gas 700 145.5
2I → I 2 Gas 23 7 × 10 9
−
+
H + OH → H 2 O Aqueous 25 1.35 × 10 11
a For first-order reactions, k has the units of s . For second-order reactions, k has units of dm mol s .
3
−1
−1 −1
bleach because the concentration term ‘[ClO ]’ in Equation (8.3)
−
Care: A ‘rate constant’
has increased.
is written as a lower
case k in contrast to the
SAQ 8.1 Consider the reaction between ethanoic acid more familiar ‘equilib-
and ethanol to form the pungent ester ethyl ethanoate rium constant’, which
and water: is writtenasanupper
case K.
CH 3 COOH + CH 3 CH 2 OH −−−→ CH 3 COOC 2 H 5 + H 2 O
Write an expression for the rate of this reaction in a similar form to that in
Equation (8.3), assuming the reaction proceeds in a single step as written.

SAQ 8.2 Write an expression for the rate of the reaction Cu 2+ (aq) +
4NH 3(aq) → [Cu(NH ) 4 ] 2+ (aq) , assuming that the reaction proceeds in a
3
single step as written.

Why does the bleaching reaction eventually stop?

Calculating rates and rate constants

When cleaning in the kitchen with a pool of bleach on tables and surfaces, there
comes a time when the bleaching action seems to stop. We might say that the bleach
is ‘exhausted’, and so pour out some more bleach from the bottle.
When thinking about reaction kinetics, we need to appreciate that reactions involve
chemical changes, with reactants being consumed during a reaction, and products