Page 383 - Physical chemistry understanding our chemical world

350 CHEMICAL KINETICS

We ignore the We soon discover that a ‘strong’ bleach cleans the surfaces faster
complication here than a more dilute bleach. The reason is that ‘strong’ bleaches are
that solution-phase in fact more concentrated, since they contain more ClO ions per
−
ClO − is in equilibrium unit volume than do ‘weaker’ bleaches.
with chlorine.
We will consider the chemical reaction between the hypochlo-
rite ion and coloured grease to form a colourless product P (the
‘bleaching’ reaction) as having the following stoichiometry:
This reaction could be
one of the steps in ClO + grease −−→ P (8.1)
−
a more complicated
series of reactions, We wish to know the rate at which this reaction occurs. The rate
in a so-called multi- is defined as the number of moles of product formed per unit time.
step reaction.If this We define this rate according to
reaction is the rate-
determining step of [product]
the overall compli- rate = dt (8.2)
cated series, then this
rate law still holds; As far as equations like Equation (8.2) are concerned, we tend to
see p. 357.
think of a chemical reaction occurring in a forward direction, so the
product in Equation (8.2) is the chemical at the head of the arrow
in Equation (8.1). Consequently, the concentration of product will
always increase with time until the reaction reaches its position of
We define the rate of
equilibrium (when the rate will equal zero). This explains why the
reaction as the speed at
which a chemical con- rate of reaction always has a positive value. The rate is generally
−3 −1
version proceeds from cited with the units of mol dm s , i.e. concentration change
start to its position per second.
of equilibrium, which The numerical value of the rate of reaction is obtained from a
explains why the rate rate equation, which is obtained by first multiplying together the
is sometimes written concentrations of each reactant involved in the reaction. (Before
as dξ/dt,where ξ is the we do this, we must be sure of the identities of each reactant – in
extent of reaction.
a complicated multi-step reaction, the reacting species might differ
from those mentioned in the stoichiometric equation.) The follow-
ing simple equation defines exactly the rate at which the reaction
in Equation (8.1) occurs:
We formulate the rate
of a reaction by multi- −
plying therateconstant rate = k[ClO ][grease] (8.3)
of the reaction by the
concentrationofeach where the constant of proportionality k is termed the rate constant.
reactant, i.e. by each The value of k is generally constant provided that the reaction
species appearing at is performed at constant temperature T . Values of rate constant
the tail end of the are always positive, although they may appear to be negative in
arrow. We can only do some of the more complicated mathematical expressions. Table 8.1
this if the reaction is contains a few representative values of k.
elementary (proceeds
in a single step) We see from Equation (8.3) that the reaction proceeds faster (has
a faster rate) when performed with a more concentrated (‘strong’)