7.4 PREDICTING THE DIRECTION OF A REACTION
The Reaction quotient, Q

If a chemical system begins with reactants only, it is obvious that the reaction will
initially proceed to the right, toward products.

However, if reactants and products are both initially present, the direction in which the
reaction proceeds is usually less obvious. What we can do is substitute the
concentrations into the equilibrium law expression to produce a trial value, called a
reaction quotient, Q.
 We can think of Q as being similar to K, with the difference being that K is calculated
using concentrations at equilibrium, whereas Q may or may not be at equilibrium. The
same mathematical equation is used for calculating K and Q.

1.
2.
3.
The result of such a trial calculation must be one of three possible situations:
Q = K, system is at equilibrium
Q > K, system must shift to the left (toward reactants) to reach equilibrium.
Q < K, system must shift to the right (toward products) to reach equilibrium.
Example: The following reaction occurs in a closed container at 445°C. The equilibrium
constant, K, is 0.20.
2 HI(g) ↔ H2(g) + I2(g)
Is the system at equilibrium in each of the following cases? If not, predict the direction in
which the reaction will proceed to reach equilibrium.
(a) [HI(g)] = 0.14 mol/L
[H2(g)] = 0.04 mol/L
[I2(g)] = 0.01 mol/L
2 HI(g)
0.14 M
Q
= [H2][I2]
[HI]2
↔
H2(g) +
0.04 M
= (0.04)(0.01)
(0.14M)2
I2(g)
0.01 M
= 0.0204
Q < K system will shift to the right to yield more products
(b) [HI(g)] = 0.20 mol/L
2 HI(g)
0.20 M
Q
[H2(g)] = 0.15 mol/L
↔
H2(g) +
0.15 M
[I2(g)] = 0.09 mol/L
I2(g)
0.09 M
= [H2][I2] = (0.15)(0.09)
= 0.3375
2
2
[HI]
(0.20M)
Q > K system will shift to the left to yield more reactant
FACTORS INFLUENCING EQUILIBRIUM POSITION
Le Châtelier’s Principle
 When a chemical system at equilibrium is disturbed by a change in a property, the
system adjusts in a way that opposes the change.
Concentration Changes

 reactant concentration will shift the reaction eq’m to the right forming more products
( reactants, shifts )

 products will shift the eq’m to the right as well ( products, shifts )
Ex)
PCl5 (g)  PCl3 (g) + Cl2 (g)

If PCl5 is added, equilibrium will shift to the

If Cl2 is added, equilibrium will shift to the
right
left
to form more products
to form more reactants
Temperature Changes

The energy in a chemical equilibrium equation can be treated as though it were a
reactant or a product.
Endothermic reaction:
Exothermic reaction:
reactants + energy ↔ products
reactants ↔ products + energy

In either situation, the equilibrium shifts to minimize the change. If the system is
cooled, the system tries to “warm” itself and the equilibrium shifts in the direction that
produces heat.

If heat is added, the equilibrium shifts in the direction that absorbs heat.
Ex)
Co(H2O)62+  CoCl42- + energy
(violet)
(pink)

 temp causes eq’m to shift to the , forming the colour violet

 temp caused eq’m to shift to the , forming the colour pink
Pressure Changes (Gases only)

If the volume of the vessel containing the reaction mixture is decreased, the overall
pressure is increased. Le Châtelier’s principle suggests that the system will react in a
way that resists the change (ie. the way that reduces the pressure).
Ex)
N2 (g) + 3H2 (g)  2NH3 (g)

If the volume  (pressure ), the equilibrium will shift , which decreases the # of gas
molecules in the container, therefore reducing pressure

If the volume  (pressure ), the eq’m will shift , which increases the # of gas
molecules, therefore increasing pressure
 A system with an equal number of gas molecules on each side of the equation will not
shift after a change in volume, since no shift can change the pressure in the vessel.
Changes that do not Affect the Position of Equilibrium Systems
Adding Catalysts

A catalyst decreases the time required to reach the equilibrium position, but does not
affect the final position of equilibrium (relative concentrations of reactants and
products)

The presence of a catalyst in a chemical reaction system lowers the activation energy
for both forward and reverse reactions by an equal amount, so the equilibrium
establishes much more rapidly but at the same position as it would without the catalyst.
Adding Inert Gases

If volume is held constant, the concentrations and partial pressures of the gases do not
change, even though we have increased the total pressure, therefore no change in
eq’m

If volume is allowed to increase, the concentrations as well as the partial pressures, all
decrease. Therefore the eq’m will shift in the direction that increases pressure
Ex)
PCl5 (g)

PCl3 (g)
+
Cl2 (g)
Which direction will the equilibrium shift if you:
a) add He but keep the volume constant? no change
b) add He but keep total pressure constant?  V, shift 
Ex)
For the reaction below, which direction would the equilibrium to shift?
CH4(g) + 2H2S(g) + energy ↔ CS2(g) + 4H2(g)
Ex)
(a) Decrease the concentration of dihydrogen sulfide.
LEFT
(b) Increase the pressure on the system.
LEFT
(c) Increase the temperature of the system.
RIGHT
(d) Increase the concentration of carbon disulfide.
LEFT
(e) Decrease the concentration of methane.
LEFT
(f) Addition of a catalyst
NO CHANGE
Consider the following equilibrium system in a closed container:
Ni(s) + 4 CO(g)  Ni(CO)4(g)
ΔH = - 161 kJ
In which direction will the equilibrium shift in response to each change, and what will be
the effect on the indicated quantity?
Change
(a) add Ni(s)
(b) raise temperature
Direction
of Shift
( ; ; or
no change)
RIGHT
LEFT
Effect on
Quantity
Ni(CO)4(g)
Effect
(increase,
decrease,
or no change)
INCREASE
Ni
INCREASE
(c) add CO(g)
RIGHT
amount of Ni(s)
DECREASE
(d) remove Ni(CO)4(g)
RIGHT
CO(g)
DECREASE
(e) decrease in volume
RIGHT
Ni(CO)4(g)
INCREASE
(f)
RIGHT
CO(g)
DECREASE
Ni(CO)4
DECREASE
lower temperature
(g) remove CO(g)
LEFT