250 14 · SPEED OF CHEMICAL REACTIONS
Catalysts are important in industry. For example, iron is used to catalyse the produc-
tion of ammonia from nitrogen and hydrogen, and nickel is used to speed up the
conversion of vegetable oils to margarines. For more about catalysis, see page 262.Reaction rate expressions
More about the variation of rate with concentration
At a fixed temperature the rates of chemical reactions
A B C... products(where A, B and C are the reactants) are usually found to follow the general expression
rate of reaction k[A]x[B]y[C]z...wherekis called the rate constantand the square brackets represent the concentra-
tions (in mol dm^3 ) of the reactants which are raised to the powers of x,y,z... The
concentration of reactants will begin to fall as soon as the reactants are mixed.
Therefore, this equation defines the rate of the reaction at one instant– as [A], [B]
and [C] continuously fall with time, so does the rate of reaction.
The powers x,y andzare said to be the individual orders of reaction for
reactants A, B and C, respectively. The overall order of reaction is the sum of the indi-
vidual orders x,y, and z...
overall order x y z...Reactions with an overall order of one are said to be ‘overall first-order’;
reactions with an overall order of two are said to be ‘overall second-order’; reactions
with an overall order of zero are said to be ‘overall zeroth order’ and so on.
The values of x,yandz(etc.) are decided by experiment – they cannot be reliably pre-
dicted by simply looking at the chemical equation for the reaction.14.3
Fig. 14.9Effect of temperature on the
catalytic activity of the enzyme malt
amylasewhen hydrolysing starch to
glucose.Rusting is the reaction of iron with
oxygen in the presence of water. The rate
of rusting is controlled by the surface
area of the iron exposed to the oxygen,
by the concentration of water vapour in
the air, and by temperature. Rusting
proceeds slower at lower temperatures,
but salt (often used on the roads in cold
climates) acts as a catalyst and causes
cars to rust faster.