Everything Science Grade 12

CHAPTER 3. REACTION RATES 3.7

k 1

k 2

=

[C][D]

[A][B]

or, if the constants k 1 and k 2 are simplified to a singleconstant, the equation becomes:

kc=

[C][D]

[A][B]

A more general form ofthe equation for a reaction at chemical equilibrium is:

aA + bB� cC + dD

where A and B are reactants, C and D are products and a, b, c, and d are the coefficients
of the respective reactants and products. A more general formula for calculating the
equilibrium constant istherefore:

Kc=

[C]c[D]d

[A]a[B]b

It is important to note that if a reactant or a product in a chemical reaction is in either
the liquid or solid phase, the concentration stays constant duringthe reaction. There-
fore, these values can be left out of the equation to calculate Kc. For example, in the
following reaction:

C (s) + H 2 O (g)� CO (g) + H 2 (g)

Kc=

[CO][H 2 ]

[H 2 O]

The constant Kcis affected by temperature and so, if the values of Kcare being com-
pared for different reactions, it is important that all the reactions have taken place at the
same temperature. Kcvalues do not have units. If you look at the equation, the units
all cancel each other out.