248 14 · SPEED OF CHEMICAL REACTIONS
●A reaction which possesses a low EAis relatively insensitive to temperature
because most of the reacting molecules already possess the activation energy at
room temperature.●Reactions which are naturally very fast (such as CH 3 • (g)CH 3 • (g)C 2 H 6 (g))
possess activation energies which are virtually zero. As a consequence, the rates of
such reactions show very little dependence upon temperature.Increase in reaction rate at higher
temperature(i)iThe chemical reactions involved in the hardening (‘boiling’)
of an egg roughly double in rate for every 10 °C rise in
temperature. How much faster will these reactions be at
100 °C than at 20 °C?
(ii)Very exothermic reactions are sometimes faster after the
reaction has started, even though the initial reactant
concentrations have fallen – why?Exercise 14E
Factors affecting reaction rate
Zinc reacts with hydrochloric acid to produce hydrogen
gas:Zn(s)2HCl(aq)ZnCl 2 (aq)H 2 (g)Experiments show that the rate of reaction decreases
(i)if zinc powder is replaced by zinc nails and (ii)if the
following are added to the reaction mixture: (a) water,
(b) ice and (c) sodium carbonate. Carefully explain these
effects.Exercise 14F
The temperature of pressurised steam is
above100 °C. A pressure cooker allows
the safe production of pressurized steam,
which then cooks food faster than the
conventional method in which food is
simply immersed in boiling water at
100 °C in an open pot.