Download Chemical Equilibrium II

Chemistry 12
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Chemical Equilibrium II
Figure 1:
The net change in reactant and
product concentrations is zero at
equilibrium for the reaction:
Figure 2:
Dynamic Equilibrium: The rates of
the forward and the reverse reactions
are equal. Even though the net
changes in concentrations are zero,
the rates are not zero.
The Equilibrium Expression
This is a mathematical expression that relates the concentrations of
reactants and products as a function of the equilibrium. This was first
proposed as the Law of Mass Action by the Norwegian chemists Cato
Guldberg (1836-1902) and Peter Waage (1833-1900).
Consider the following general equilibrium equation:
aA +
bB

cC
+ dD
When a reaction is at equilibrium, the rates of these two reactions are
identical, thus no ____ (macroscopic) change is observed. However,
individual components are actively being transformed at the
microscopic level. Guldberg and Waage showed that the rate of the
reaction in either direction is proportional to what they called the
“active masses” of the various components:
Rate of forward reaction = ________________
Rate of reverse reaction = ________________
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At equilibrium:
Rearranging the above equation:
Where ____ is known as the ___________ ___________
Note that the equilibrium expression can be expressed by
concentrations in terms of _________ for aqueous solutions or
_________________ for gases (although for the purposes of
Chemistry 12, we will not be using partial pressures)
Some rules to follow when writing equilibrium expressions:
“____________” concentrations in equilibrium expressions:
Substances whose concentrations undergo no significant change
in a chemical reaction do not appear in equilibrium constant
expressions. Two general cases:
i) when the substance is also the _______ – eg. dissociation
of acetic acid (_______ acts as a solvent):
CH3COOH(aq) + H2O(l) 
CH3COO-(aq) +
H3O+(aq)
ii) when the substance is a _______ or a ________ phase
Eg 1. in solubility equilibria:
CaF(s) Ca2+(aq) + 2F–(aq)
Eg 2. in heterogeneous reactions:
Fe3O4(s) + 4H2(g)  4H2O(g) + 3Fe(s)
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The Significance of Keq
Equilibrium constants can be very large or very small. The magnitude
of Keq provides important information about the composition of an
equilibrium mixture.
Consider the reaction of carbon monoxide and chlorine gas at 100C –
a toxic gas called phosgene (COCl2) is formed and used for
manufacturing certain polymers and insecticides:
CO(g) + Cl2(g)  COCl2(g)
Keq = [COCl2] = 1.49 x 108
[CO][Cl2]
In order for Keq to be so large, the
numerator of the equilibrium expression
(product concentrations) must be much
_______ than the denominator (reactant
concentrations). Thus, an equilibrium
mixture of all three gases is primarily pure
_____. We say that this equilibrium ____
____________________. Likewise, a
very ______ Keq value signifies an
equilibrium mixture containing mostly
________ (equilibrium lies to the _____
or ________ side).
(diagram taken from Chemistry: The
Central Science, 9th edition pg. 582)