Section 1: Le Chatelier’s Principle

1. A mixture of hydrogen and chlorine is left in a pressurised vessel for several hours. The contents were later found to contain hydrogen, chlorine and hydrogen chloride.After several days the vessel contained exactly the same mixture.

Explain these observations ensuring you include in your answer a definition of the term “equilibrium” and an equation for the reaction.You should try to include the following terms in your explanation:
reactants products reversible equilibrium point

H2 (g) + Cl2 (g) 2 HCl (g)

When a reaction is reversible, an equilibrium point can be reached.
At the beginning, only reactants, hydrogen and chlorine, were present in the vessel. These reacted to form some HCl, the product. As the quantity of H2 and Cl2 was decreasing, the rate of the forward reaction was decreasing. However, as more HCl was formed, the rate of the reverse reaction was increasing.When the forward rate = the reverse rate, there will be no further change in the concentrations of either reactants or products. When this occurs, the reaction is said to have reached an equilibrium point.However, both reactions continue to occur at equal rates.

3.1
2. The Contact process is an industrial method for the production of sulphur (VI) oxide from the reversible reaction between sulphur (IV) oxide and oxygen gas. The SO3 is then used to produce sulphuric acid.

2SO2(g) + O2(g) D 2SO3(g) DH = -197 kJ mol-1

i. Describe and explain the factors that would affect the position of equilibrium in this system.
Rxn is exothermic – gives off heat. Therefore heat is a product. Adding heat (product) shifts the reaction to the left to form more reactants. Removing heat (product) – shift right – form more products.
Adding more SO2 or O2 (reactants) – shift right
Add SO3 (product) – shift left
Increase pressure on system – shift right to reduce moles of gas
Dec. pressure on system – shift left to increase moles of gas

ii. In industry the reaction uses a vanadium(V) oxide catalyst. Explain the effect that adding a catalyst would have on the position of equilibrium in this system.
Catalysts increase both fwd and rev rates by lowering the activation energy of both processes. However, this does not affect the position of equilibrium

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3. The Haber process is an important example of an industrial equilibrium, and involves the formation of ammonia (NH3) from its elements.

N2(g) + 3H2(g) D 2NH3(g) DH = -92 kJ mol-1

a) Using Le chattelier’s Principle, predict the optimum conditions to maximise the equilibrium yield for this reaction.
High P – shift right to make less moles of gas
Low T (remove heat) – shift right to make more heat (rxn is exothermic)
Remove NH3 as it is formed, add N2 and H2 as they are used up.

b) The reaction is actually carried out at compromise conditions, primarily for reasons of cost and safety. Identify how the change in reaction conditions influences the equilibrium yield of NH3.
Rxn cannot be cooled efficiently to increase yield, and pressure too high is dangerous. By not being able to optimize these conditions, the equilibrium yield of NH3 is reduced

1. In the following combustion reaction of pentanol, first find how...