CHEMICAL EQUILIBRIUM
Closed system - a system in which mass is conserved inside the system but energy can enter or leave the system
freely
Open system - a system in which both energy and matter can be exchanged between the system and its
surroundings
Le Châtelier's principle - 'When an external stress (change in pressure, temperature or concentration) is applied
to a system in dynamic chemical equilibrium, the equilibrium point will change in such a way as to counteract the
stress'
Dynamic equilibrium - a reversible reaction in a closed system where the reactions (forward & reverse) take place
continually and at equal rates. The concentration of the reactants & products remain constant.
DYNAMIC EQUILIBRIUM
(Evaporation of water molecules from a beaker)
A closed system is one from which no substances can escape. Initially the water
evaporates rapidly and the level of water drops but as the number of water vapour
molecules increase condensation begins. The rate of condensation increases until
the rate of condensation is equal to the rate of evaporation. Once this occurs
the quantity of liquid remains constant and it appears as if the evaporation has
stopped. A dynamic equilibrium has been established in which two opposing
processes (evaporation and condensation) continue but at the same rate.
CHEMICAL EQUILIBRIUM
A chemical system is reversible if the products of the reaction can change back into the reactants.
H2 (g) + I2 (g) ↔ 2HI (g) ΔH = – 13kJ/mol (negative value indicates reaction is exothermic)
Forward reaction: H2 (g) + I2 (g) 2H (g) Exothermic
Reverse reaction: 2HI (g) H2(g) + I2(g) Endothermic
● At t = 0, there is lots of H2 and I2, so the forward reaction rate is high.
● Forward reaction slows down as H2 and I2 gets used up.
● At t = 0 there is no HI, so the reverse reaction (2HI → H2 + I2) has not happened
● At t = X, equilibrium has been reached therefore the reverse reaction rate equals the forward reaction rate.
Equilibrium does not mean that the number of reactants = the number of products.
The rate that reactants form products equals the rate that products form reactants.
1
, LE CHATELIER’S PRINCIPLE
3 factors will influence chemical equilibrium:
1. Temperature
2. Concentration
3. Pressure
Method of explanation of equilibrium shift in terms of Le Chatelier’s Principle:
1. Identify the external change to the system (external stress)
2. State that the system will oppose the change as predicted by Le Chatelier’s Principle
3. Decide whether the forward or the reverse reaction will be favoured
4. Discuss the result of this reaction being favoured (eg. colour change)
Concentration:
When the concentration of one substance increases/decreases, the system will favor the reverse/forward reaction in
which the product that is in excess is used in order to reinstill equilibrium
Example:
Explain, using Le Chatelier’s Principle, how the yield of the product will be a ected by the addition of O2 to the
equilibrium. 2SO2(g) + O2(g) ↔ 2SO3(g)
● Stress: Increase in concentration of O2 gas in a system
● Le Chatelier’s Principle predicts that the system will respond to counteract the stress and so increase the rate of the forward reaction
● The forward reaction is favoured
Temperature:
When the temperature of one reaction increases/decreases, the system will favour the forward/reverse reaction
which counteracts the increase/decrease in temperature. The reaction is determined through the use of exothermic
vs endothermic information
Exothermic vs Endothermic reactions
Exothermic reaction - Energy (heat) is released when the reaction is favoured (∆H < 0)
Endothermic reaction - Energy (heat) is absorbed when the reaction is favoured (∆H > 0)
Example:
Use Le Chatelier’s Principle to explain the colour change observed when the temperature of the
pink solution is increased.
● Stress: Increased temperature of the system
● Le Chatelier’s Principle predicts that the system will respond to counteract the stress and so decrease the temperature
of the system
● The rate of the forward and reverse reactions will increase (the kinetic energy of the reactants and products increase)
● The reverse reaction is favoured
● Resulting in a decrease in concentration of products and the colour hanging from pink to blue
● Increase in temperature always increases the rate of both the forward and the reverse reaction but the rate of one
increases more.
● For an exothermic reaction decreasing the temperature will always favour the forward reaction, but if the temperature is too
low the reaction rate decreases and the reaction may not occur
(collisions between molecules may not occur as the energy of molecules may be lower than activation energy)
2
Closed system - a system in which mass is conserved inside the system but energy can enter or leave the system
freely
Open system - a system in which both energy and matter can be exchanged between the system and its
surroundings
Le Châtelier's principle - 'When an external stress (change in pressure, temperature or concentration) is applied
to a system in dynamic chemical equilibrium, the equilibrium point will change in such a way as to counteract the
stress'
Dynamic equilibrium - a reversible reaction in a closed system where the reactions (forward & reverse) take place
continually and at equal rates. The concentration of the reactants & products remain constant.
DYNAMIC EQUILIBRIUM
(Evaporation of water molecules from a beaker)
A closed system is one from which no substances can escape. Initially the water
evaporates rapidly and the level of water drops but as the number of water vapour
molecules increase condensation begins. The rate of condensation increases until
the rate of condensation is equal to the rate of evaporation. Once this occurs
the quantity of liquid remains constant and it appears as if the evaporation has
stopped. A dynamic equilibrium has been established in which two opposing
processes (evaporation and condensation) continue but at the same rate.
CHEMICAL EQUILIBRIUM
A chemical system is reversible if the products of the reaction can change back into the reactants.
H2 (g) + I2 (g) ↔ 2HI (g) ΔH = – 13kJ/mol (negative value indicates reaction is exothermic)
Forward reaction: H2 (g) + I2 (g) 2H (g) Exothermic
Reverse reaction: 2HI (g) H2(g) + I2(g) Endothermic
● At t = 0, there is lots of H2 and I2, so the forward reaction rate is high.
● Forward reaction slows down as H2 and I2 gets used up.
● At t = 0 there is no HI, so the reverse reaction (2HI → H2 + I2) has not happened
● At t = X, equilibrium has been reached therefore the reverse reaction rate equals the forward reaction rate.
Equilibrium does not mean that the number of reactants = the number of products.
The rate that reactants form products equals the rate that products form reactants.
1
, LE CHATELIER’S PRINCIPLE
3 factors will influence chemical equilibrium:
1. Temperature
2. Concentration
3. Pressure
Method of explanation of equilibrium shift in terms of Le Chatelier’s Principle:
1. Identify the external change to the system (external stress)
2. State that the system will oppose the change as predicted by Le Chatelier’s Principle
3. Decide whether the forward or the reverse reaction will be favoured
4. Discuss the result of this reaction being favoured (eg. colour change)
Concentration:
When the concentration of one substance increases/decreases, the system will favor the reverse/forward reaction in
which the product that is in excess is used in order to reinstill equilibrium
Example:
Explain, using Le Chatelier’s Principle, how the yield of the product will be a ected by the addition of O2 to the
equilibrium. 2SO2(g) + O2(g) ↔ 2SO3(g)
● Stress: Increase in concentration of O2 gas in a system
● Le Chatelier’s Principle predicts that the system will respond to counteract the stress and so increase the rate of the forward reaction
● The forward reaction is favoured
Temperature:
When the temperature of one reaction increases/decreases, the system will favour the forward/reverse reaction
which counteracts the increase/decrease in temperature. The reaction is determined through the use of exothermic
vs endothermic information
Exothermic vs Endothermic reactions
Exothermic reaction - Energy (heat) is released when the reaction is favoured (∆H < 0)
Endothermic reaction - Energy (heat) is absorbed when the reaction is favoured (∆H > 0)
Example:
Use Le Chatelier’s Principle to explain the colour change observed when the temperature of the
pink solution is increased.
● Stress: Increased temperature of the system
● Le Chatelier’s Principle predicts that the system will respond to counteract the stress and so decrease the temperature
of the system
● The rate of the forward and reverse reactions will increase (the kinetic energy of the reactants and products increase)
● The reverse reaction is favoured
● Resulting in a decrease in concentration of products and the colour hanging from pink to blue
● Increase in temperature always increases the rate of both the forward and the reverse reaction but the rate of one
increases more.
● For an exothermic reaction decreasing the temperature will always favour the forward reaction, but if the temperature is too
low the reaction rate decreases and the reaction may not occur
(collisions between molecules may not occur as the energy of molecules may be lower than activation energy)
2
