14 · SPEED OF CHEMICAL REACTIONSextremely rapidly in an effort to use the odd electron to form a covalent bond. For
example, the reaction,CH 3 • (g) CH 3 • (g)C 2 H 6 (g)is extremely fast.
- Concentration (or for gases, pressure)
Increasing the concentration of reactants increases the chances of collisions between
reactant molecules. In other words, increasing concentration increases the number
of collisions between molecules per second. This increases the reaction rate. The con-
centration of a gas increases with its partial pressure. So, we can increase the rate of
gaseous reactions by increasing the partial pressure of the reactants.
Where one of the reactants is a solid, increasing the surface area of the solid also
increases the chance of the particles of reactants colliding. For example, powdered
carbon burns in oxygen faster than lumps of carbon:C(s) O 2 (g)CO 2 (g)This is why, in power stations, coal is pulverized before allowing it to burn. Fine
powders can sometimes react extremely rapidly. For example, stringent safety
regulations need to be applied to grain and flour factories, since fine grain and flour
can burn explosively fast in air (Exercise 14D).246
Fig. 14.4Variation of reaction
rate with temperature for
reactions which are (a) very
sensitive, (b) fairly sensitive,
(c)insensitiveto changes in
temperature. (The rate
constant is the reaction rate
with the reactants at a
concentration of
1.0 mol dm^3 .)
Surface area
A cube of marble 1 cm 1 cm 1 cm is cut into two smaller pieces along the dotted lines
as shown:(i)iWhat is the increase in surface area after cutting?
(ii)How does this help explain why marble powder reacts more rapidly with hydrochloric acid
than marble blocks?Exercise 14D
- Temperature
Increasing the temperature of a reaction mixture increases the rate of the chemical
reaction. (This happens whether the reaction is exothermic or endothermic.) This
increase is often dramatic (Fig. 14.4).
Although the number of collisions per second between reactant molecules rises
with temperature, calculations show that this makes only a small contribution to the
increase of reaction rate with temperature. The accepted explanation for the increase
in reaction rate involves a key idea in chemical kinetics, that of activation energy.
Think of a chemical reaction in which molecules of M and N form the product
molecule MN in a single step. In order for a collision between molecules to result in
chemical change, old bonds need to be broken and new ones formed. Bond destruc-
tion and bond formation require a redistribution of the bonding electrons within