Topic 10 – Equilibrium I
In order to develop their practical skills, students should be encouraged to carry out a range of practical experiments related to this topic. Possible
experiments include investigating equilibrium systems, such as iron(III) – thiocyanate, or the effect of temperature on the equilibrium between
[Co(H2O)6]2+ and [CoCl4]2–. Mathematical skills that could be developed in this topic include deriving an expression for the equilibrium constant.
1. know that many reactions are readily reversible and that they can reach dynamic equilibrium where:
Give the definition of dynamic equilibrium.
Dynamic equilibrium is where the rateforward = ratebackward, and the concs of reactants and products remain constant.
2. be able to predict/justify the effect of a change in temp, conc or pressure on a homogeneous system in eq
In the formation of ammonia, the equation for this reaction is C H + H 2 O ( g ) ⇌ C O(g )+ 3 H 2( g ), with
4 (g )
−1
∆ r H=+210 kJmo l . Typical conditions for this reaction are a temperature of 1000K, and a pressure of
30atm. A chemist recommends lowering the temperature to 800K to save energy. Explain two cons of making
this change.
The decrease in temperature would cause a decrease in the rate of reaction because fewer molecules will have an
energy greater than or equal to the activation energy for the reaction. Furthermore, the decrease in temperature will
cause the position of equilibrium to shift to the left as the forward reaction is endothermic.
The manufacturer decides to keep the temperature at 1000K, but to increase the pressure to 50atm. This only
causes a slight increase in energy usage. Explain the main disadvantage of making this change in pressure.
The increase in pressure causes the p.o.e. to shift to the left as there are fewer gas moles on the reactant side.
3. evaluate data to explain the necessity to reach a compromise between the yield and the rate of reaction
This graph shows how the yield of ammonia from the Haber process changes with
temperature and pressure. N 2 ( g) +3 H 2 ( g ) ⇋2 N H 3 ( g ), ∆ r H=+ ve.
Explain why a temperature of 450oC is used.
A lower temperature results in a higher yield as the forward reaction is endothermic,
this also reduces costs as less energy is needed. However, a higher temperature would
give a faster rate of reaction so a compromise is found with a good rate and good yield.
Explain why a pressure of 200atm is used. Why is an iron catalyst used?
A higher pressure gives a higher yield as there are fewer moles of gas on the right hand side of the reaction, and a
higher pressure gives a higher rate of reaction. However, a high pressure is expensive to maintain as special
equipment is needed and is more dangerous, so 200atm is a compromise between good yield and cost/safety. A
catalyst is used to increase the rate of reaction without changing the position of equilibrium.
4. be able to express Kc, for homogeneous/heterogeneous systems, in terms of equilibrium concentrations
Give an expression for Kc for the following reaction: 2 S O 2 ( g )+O 2 ( g) ⇌2 S O 3( g ). State the unit.
2
[ S O3 ] , mol
K c= 2
-1
dm3
[ S O 2 ] × [ O2 ]
In order to develop their practical skills, students should be encouraged to carry out a range of practical experiments related to this topic. Possible
experiments include investigating equilibrium systems, such as iron(III) – thiocyanate, or the effect of temperature on the equilibrium between
[Co(H2O)6]2+ and [CoCl4]2–. Mathematical skills that could be developed in this topic include deriving an expression for the equilibrium constant.
1. know that many reactions are readily reversible and that they can reach dynamic equilibrium where:
Give the definition of dynamic equilibrium.
Dynamic equilibrium is where the rateforward = ratebackward, and the concs of reactants and products remain constant.
2. be able to predict/justify the effect of a change in temp, conc or pressure on a homogeneous system in eq
In the formation of ammonia, the equation for this reaction is C H + H 2 O ( g ) ⇌ C O(g )+ 3 H 2( g ), with
4 (g )
−1
∆ r H=+210 kJmo l . Typical conditions for this reaction are a temperature of 1000K, and a pressure of
30atm. A chemist recommends lowering the temperature to 800K to save energy. Explain two cons of making
this change.
The decrease in temperature would cause a decrease in the rate of reaction because fewer molecules will have an
energy greater than or equal to the activation energy for the reaction. Furthermore, the decrease in temperature will
cause the position of equilibrium to shift to the left as the forward reaction is endothermic.
The manufacturer decides to keep the temperature at 1000K, but to increase the pressure to 50atm. This only
causes a slight increase in energy usage. Explain the main disadvantage of making this change in pressure.
The increase in pressure causes the p.o.e. to shift to the left as there are fewer gas moles on the reactant side.
3. evaluate data to explain the necessity to reach a compromise between the yield and the rate of reaction
This graph shows how the yield of ammonia from the Haber process changes with
temperature and pressure. N 2 ( g) +3 H 2 ( g ) ⇋2 N H 3 ( g ), ∆ r H=+ ve.
Explain why a temperature of 450oC is used.
A lower temperature results in a higher yield as the forward reaction is endothermic,
this also reduces costs as less energy is needed. However, a higher temperature would
give a faster rate of reaction so a compromise is found with a good rate and good yield.
Explain why a pressure of 200atm is used. Why is an iron catalyst used?
A higher pressure gives a higher yield as there are fewer moles of gas on the right hand side of the reaction, and a
higher pressure gives a higher rate of reaction. However, a high pressure is expensive to maintain as special
equipment is needed and is more dangerous, so 200atm is a compromise between good yield and cost/safety. A
catalyst is used to increase the rate of reaction without changing the position of equilibrium.
4. be able to express Kc, for homogeneous/heterogeneous systems, in terms of equilibrium concentrations
Give an expression for Kc for the following reaction: 2 S O 2 ( g )+O 2 ( g) ⇌2 S O 3( g ). State the unit.
2
[ S O3 ] , mol
K c= 2
-1
dm3
[ S O 2 ] × [ O2 ]
