Name: ____________________________________ Date: ________________ Mods: ____________
Ch. 15 - Writing Equilibrium Expressions
What is chemical equilibrium?
Chemical equilibrium is the condition which occurs when a reaction is ________________________,
meaning that a reaction can move in both the forward and reverse directions. During a reversible
reaction, the concentration of the reactants ___________________ over time while the concentration
of products __________________ over time. At a certain point, the concentration of reactants and the
concentration of products no longer changes (remain constant over time); this in when equilibrium is
achieved. When a reaction reaches equilibrium, the reaction may appear to ____________ due to the
fact that there is no change in concentration, however the reaction is still in a process called dynamic
equilibrium – reactant molecules are becoming product molecules at the same rate as the reverse is
occurring (there is a constant reaction occurring, with no net change in concentrations).
N2O4 (g)


2 NO2 (g)
Figure 1: Concentration vs. Time
Figure 2: Rate vs. Time
[N2O4] decreases as [NO2] increases
Once equilibrium is achieved, both
concentrations remain constant (no
net change)
• the rate of the forward reaction decreases
• the rate of the reverse reaction increases
• at equilibrium, the rates of forward and
reverse reactions are equal
Writing Equilibrium Expressions
A reaction that has reached equilibrium can be described with an equation known as an equilibrium
expression. An equilibrium expression relates the concentration of products divided by the
concentration of reactants to an equilibrium constant (Keq). When writing an equilibrium expression
only aqueous and gaseous substances are included.
For the general reaction:
aA + bB
Keq = products =
reactants
[C]c[D]d
[A]a[B]b
cC + dD
In the equation CaCO3 (s)
CaO (s) + CO2 (g), the equilibrium expression would only include CO2
because the other substances are both solids. The equilibrium expression is Keq = [CO2]. The reason
that solids and liquids are not included in the equilibrium expressions is that their concentration cannot
be measured – they are pure substances, not mixtures.
Example:
Write the equilibrium expression for the reaction: N2 (g) + 3 H2 (g)
2 NH3 (g)
Write the equilibrium expressions for the following reactions:
1) 2 NO2 (g)
N2O4 (g)
2) H2 (g) + I2 (g)
3) CaSO4 (s)
7) CO (g) + 2 H2 (g)
2 HI (g)
Ca2+ (aq) + SO42- (aq)
4) Mg (s) + 2 Ag+ (aq)
5) C (s) + H2O (g)
6) CO2 (aq) + H2O (l)
Mg2+ (aq) + 2 Ag (s)
CO (g) + H2 (g)
H+ (aq) + HCO3– (aq)
8) 2 POCl3 (g)
2 PCl3 (g) + O2 (g)
9) Sn (s) + 2 CO2 (g)
10) 2 HBr (g)
CH3OH (g)
SnO2 (g) + 2 CO (g)
H2 (g) + Br2 (l)
11) O2 (g) + NO2 (g)
O3 (g) + NO (g)
12) Fe (s) + H2O (g)
FeO (s) + H2 (g)
Balance the equations and write the equilibrium expressions for the following reactions:
13) ____ O3 (g)
____ O2 (g)
14) ____NO (g) + ____Cl2 (g)
15) ____ CO (g)
16) ____ SO3 (g)
____NOCl (g)
____ C (s) + ____ O2 (g)
____ SO2 (g) + ____ O2 (g)
17) ____ HCl (g) +____ O2 (g)
____ H2O (l) + ____Cl2 (g)
18) ____ Sb2S3 (g) + ____ H2 (g)
____ Sb (s) +____ H2S (g)
Calculating Equilibrium Constants
Equilibrium describes a situation in which the rate of the forward reaction is equal to the rate of the
reverse reaction. In other words, products are made from reactants at the same speed that reactants
are formed from products. When a reaction has reached these conditions the ratio of the
concentration of products to concentration of reactants is equal to a value called the equilibrium
constant (K).
A small value for an equilibrium constant indicates that the concentration of products is small and the
concentration of reactants is large. In this scenario, we say that the equilibrium lies to the
_____________ because there are more reactants. If the equilibrium constant is large, this indicates
that there is a high concentration of products and a low concentration of reactants. In this situation, we
say the equilibrium lies to the ______________ because there are more products.
Large K = Eq. lies to the right
Small K = Eq. lies to the left
Small equilibrium constant (K)
Large equilibrium constant (K)
Example:
Equilibrium lies to the…
left
right
Description
Larger concentration of reactants
Larger concentration of products
Write the equilibrium expression for the reaction: N2O4 (g)
2 NO2 (g)
Use the equilibrium concentrations to determine the equilibrium constant (K).
At equilibrium, [N2O4] = 0.65 M and [NO2] = 0.012 M.
Use the information below to calculate the Keq for the following reactions
and tell whether the equilibrium lies to the left or right.
1) H2 (g) + Cl2 (g)
2 HCl (g)
At equilibrium, [H2] = 0.42 M, [Cl2] = 0.075 M, and [HCl] = 0.95 M.
2) N2 (g) + 3 H2 (g)
2 NH3 (g)
At equilibrium, [N2] = 0.34 M, [H2] = 0.13 M, and [NH3] = 0.19 M.
3) 2 NO (g) + O2 (g)
2 NO2 (g)
At equilibrium, [NO] = 2.4 x 10–3 M, [O2] = 1.4 x 10–4 M, and [NO2] = 0.95 M.
4) C (s) + CO2 (g)
2 CO (g)
At equilibrium, [CO2] = 8.3 x 10–6 M and [CO] = 5.4 x 10–5 M.
5) 2 NO (g) + Br2 (g)
2 NOBr (g)
At equilibrium, [NO] = 0.5 M, [Br2] = 0.25 M, and [NOBr] = 3.5 M.
6) 2 Fe (s) + 3 H2O (g)
Fe2O3 (s) + 3 H2 (g)
At equilibrium, [H2O] = 1.0 M and [H2] = 4.5 M.
7) CaCO3 (s)
CaO (s) + CO2 (g)
At equilibrium [CO2] = 4.0 x 10–3 M
8) PCl5 (g)
PCl3 (g) + Cl2 (g)
At equilibrium, [PCl5] = 0.25 M, [PCl3] = 9.7 x 10–4 M, and [Cl2] = 3.2 x 10–3 M.
9) CO (g) + 3 H2 (g)
CH4 (g) + 3 H2O (g)
At equilibrium, [CO] = 4.0 M, [H2] = 2.8 M, [CH4] = 0.75 M, and [H2O] = 0.12 M.
10) 2 SO2 (g) + O2 (g)
2 SO3 (g)
At equilibrium, [SO2] = 2.4 x 10–2 M, [O2] = 6.4 x 10–2 M, and [SO3] = 8.2 x 10–8 M