Key
HOMEWORK for Chapter 13: Chemical Equilibrium
13.5 Le Châtelier’s Principle
1.
(Read pgs. 422 - 432 in the chemistry textbook)
(a) State Le Chatelier’s Principle?
When a stress (change in conditions) is placed on a reaction at
equilibrium, the equilibrium will shift in the direction that
relieves the stress.
(b) What types of changes qualify as “stresses” on chemical equilibrium?
Temperature
Concentration
Volume (pressure)
[for gas reactions only]
(c) What phases of matter are involved in equilibrium?
Gases
Aqueous ions
(d) What phases of matter are NOT involved in equilibrium?
Solids
Liquids
2. How do concentration changes affect chemical equilibrium?
 INCREASE in Concentration:
[reactant]↑ = equilibrium shifts right and [products]↑
[products]↑ = equilibrium shifts left and [reactants]↑
 DECREASE in Concentration :
[reactant]↓= equilibrium shifts left and [products]↓
[products]↓ = equilibrium shifts right and [reactants]↓
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3. Assume that each of the following has come to equilibrium. Predict which way the equilibrium will shift
for each of the following concentration changes. Also, state whether the concentration of reactants and
products will increase or decrease due to the equilibrium shift.
Δ Equilibrium
(a)
PCl5(g)
PCl3(g) + Cl2(g)
some PCl5 is removed
 Equilibrium will shift left to replace the lost PCl5
 [reactants] = ↑
 [products] = ↓
(b)
C2 H4(g) + H2O(g)
C2H5OH (g)
some H2O is added
 Equilibrium will shift right to absorb the added H2O
 [reactants] = ↓
 [products] = ↑
(c)
CaF2 (s)
Ca2+ (aq) + 2 F - (aq)
[Ca2+] increases
 Equilibrium will shift left to absorb the extra Ca2+
 [reactants] = ↑
 [products] = ↓
(d)
2 Li+ (aq) + CO32 - (aq)
Li2CO3 (s)
some Li2CO3 is added
 NO CHANGE – solids are not involved in equilibrium
changes
(e)
CO(g) + H2O(g)
CO2 (g) + H2 (g)
some CO2 is removed
 Equilibrium will shift right to replace the lost CO2
 [reactants] = ↓
 [products] = ↑
(f)
H2O(l)
H+ (aq) + OH - (aq)
some H2O is removed
NO CHANGE – liquids are not involved in equilibrium
changes
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4. How do temperature changes affect chemical equilibrium?
 EXOTHERMIC REACTIONS:
 INCREASE in Temperature:
equilibrium shifts left and [reactants]↑
 DECREASE in Temperature:
equilibrium shifts left and [products]↑
 ENDOTHERMIC REACTIONS:
 INCREASE in Temperature:
equilibrium shifts right and [products]↑
 DECREASE in Temperature:
equilibrium shifts left and [reactants]↑
5. Assume that each of the following has come to equilibrium. Identify whether the reaction is exothermic or
endothermic and then predict which way the equilibrium will shift for the given temperature change. Also,
state whether the concentration of reactants and products will increase or decrease due to the equilibrium
shift.
Δ Equilibrium
(g)
N2(g) + O2(g) + HEAT
2 NO(g)
T is increased
 Reaction is ENDOTHERMIC
 Equilibrium will shift right to absorb extra heat
 [reactants] = ↓
 [products] = ↑
(h)
2 SO2(g) + O2(g)
2 SO3(g) + HEAT
T is increased
 Reaction is EXOTHERMIC
 Equilibrium will shift left to absorb the extra heat
 [reactants] = ↑
 [products] = ↓
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5. Continued:
Δ Equilibrium
(i)
N2 (g) + 3 H2(g)
2 NH3 (aq) + HEAT
T is decreased
 Reaction is EXOTHERMIC
 Equilibrium will shift right to replace the lost heat
 [reactants] = ↑
 [products] = ↓
(j)
PCl5(g) + HEAT
PCl3(g) + Cl2(g)
T is decreased
 Reaction is ENDOTHERMIC
 Equilibrium will shift left to replace the lost heat
 [reactants] = ↑
 [products] = ↓
(k)
N2O4(g)
2 NO2 (g)
Δ H = + 58.0 kJ
T is increased
 Reaction is ENDOTHERMIC (+ ΔH)
 Equilibrium will shift right to absorb the extra heat
 [reactants] = ↑
 [products] = ↓
(l)
3 NO(g)
NO2 (g) + N2O(g) Δ H = ̶ 155.7 kJ T is increased
 Reaction is EXOTHERMIC( ̶ ΔH)
 Equilibrium will shift left to absorb the extra heat
 [reactants] = ↑
 [products] = ↓
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6. How do volume
(pressure) changes affect chemical equilibrium of gas reactions?
 INCREASE in Volume = decrease in pressure
equilibrium shifts in whichever direction will increase the number
of moles of gas
 DECREASE in Volume = increase in pressure
equilibrium shifts in whichever direction will decrease the number
of moles of gas
7. Assume that each of the following has come to equilibrium in a closed container. In each case state
whether the given change will increase or decrease the pressure and then predict which way the equilibrium
will shift. Also, state whether the concentration of reactants and products will increase or decrease due to
the equilibrium shift.
Δ Equilibrium
(a)
2 SO2(g) + O2(g)
2 SO3(g)
volume of container is increased
 Pressure will decrease
 Equilibrium will shift left to increase the moles of gas
 [reactants] = ↑
 [products] = ↓
(b)
N2 (g) + 3 H2(g)
2 NH3 (aq)
volume is of container is decreased
 Pressure will increase
 Equilibrium will shift right to decrease the moles of gas
 [reactants] = ↓
 [products] = ↑
(d)
CO(g) + H2O(g)
CO2 (g) + H2 (g)
volume is of container is decreased
NO CHANGE – same number of moles of gas on each side
(c)
2 NO2(g)
2NO (g) + O2(g)
volume is of container is decreased
 Pressure will increase
 Equilibrium will shift left to decrease the moles of gas
 [reactants] = ↓
 [products] = ↑
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8.
Ozone is converted to ozone, O3, by the energy provided from an electric spark.
3 O2(g)
+
HEAT
2 O3(g)
For each of the following changes at equilibrium, indicate whether the equilibrium shifts towards product or
reactant or does not shift:
EQULIBRIUM SHIFT
(a) adding O2(g)
right, towards the products
(b) adding O3(g)
left, towards the reactants
(c) raising the temperature
right, towards the products
(d) decreasing the volume of the container
right, towards the products
(e) adding a catalyst
no change
9. Ammonia is produced by reacting nitrogen gas and hydrogen gas.
N2(g)
+
3 H2(g)
2 NH3(g) + 92 kJ
For each of the following changes at equilibrium, indicate whether the equilibrium shifts towards product or
reactant or does not shift:
EQULIBRIUM SHIFT
(a) removing N2(g)
left, towards the reactants
(b) adding NH3(g)
left, towards the reactants
(c) lowering the temperature
right, towards the products
(d) adding H2(g)
right, towards the products
(e) increasing the volume of the container
left, towards the reactants
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10. Hydrogen chloride can be made by reacting hydrogen gas and chlorine gas.
H2(g)
+
Cl2(g) + HEAT
2 HCl(g)
For each of the following changes at equilibrium, indicate whether the equilibrium shifts towards product or
reactant or does not shift:
EQULIBRIUM SHIFT
(a) adding H2(g)
right, towards the products
(b) removing HCl(g)
right, towards the products
(c) increasing the temperature
right, towards the products
(d) adding a catalyst
no change
(e) remove Cl2(g)
left, towards the reactants
11. When heated, carbon reacts with water to produce carbon monoxide and hydrogen.
C(s)
+
H2O (g) + HEAT
CO(g)
+ H2(g)
For each of the following changes at equilibrium, indicate whether the equilibrium shifts towards product or
reactant or does not shift:
EQULIBRIUM SHIFT
(a) increasing the temperature
right, towards the products
(b) adding C(s)
no change
(c) removing CO2 (g)
right, towards the products
(d) adding H2O(g)
right, towards the products
(e) decreasing the volume of the container
left, towards the reactants
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