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THERMODYNAMICS AND THE EXTENT OF REACTION 159

chemistry (see Chapter 6) we write a dissociation constant, but in complexation equi-
libria we write a formation constant.
In fact, an equilibrium constant is only ever useful when we have carefully defined
the chemical process to which it refers.

The reaction quotient

It is well known that few reactions (other than inorganic redox reactions) ever reach
completion. The value of ξ (eq) is always less than one.
The quotient of products to reactants during a reaction is
The mathematical sym-
[products] bol means the ‘pi
ν
(4.48)
Q = ν
[reactants] product’, meaning the
terms are multiplied
which is sometimes called the reaction quotient. The values of ν together, so (2,3,4)
are the respective stoichiometric numbers. The mathematical value is 2 × 3 × 4 = 24.
of Q increases continually during the course of reaction because
of the way it relates to concentrations during reaction. Initially,
the reaction commences with a value of Q = 0, because there is no product (so the
numerator is zero).

Aside

−3
A statement such as ‘K = 0.4 mol dm ’ is wrong, although we find examples in a
great number of references and textbooks. We ought, rather, to say K = 0.4 when the
equilibrium constant is formulated (i) in terms of concentrations, and (ii) where each
−3
concentration is expressed in the reference units of mol dm . Equilibrium constants
such as K c or K p are mere numbers.

Why does the voltage of a battery decrease to zero?

Relationships between K and G O

The voltage of a new torch battery (AA type) is about 1.5 V. After
the battery has powered the torch for some time, its voltage drops,
The battery produces
which we see in practice as the light beam becoming dimmer. If ‘power’ W (energy per
further power is withdrawn indefinitely then the voltage from the
unit time) by passing
battery eventually drops to zero, at which point we say the battery
a current through a
is ‘dead’ and throw it away. resistor. The resister in
A battery is a device for converting chemical energy into electri- atorch is thebulb
cal energy (see p. 344), so the discharging occurs as a consequence filament.
of chemical reactions inside the battery. The reaction is complete