Chemical Equilibria, Le Chatelier’s Principle & Kc (AQA A Level Chemistry)

Ethanol has a great number of uses. For industrial purposes, it can be manufactured via the following reversible reaction. 

C₂H₄ (g) + H₂O (g)   C₂H₅OH (g)    ?H = -46 kJ mol^-1

The optimum pressure for this reaction is between 60 and 70 atm. 

Using Le Chatelier’s principle, state and explain the effect, if any, that increasing the overall pressure would have on the equilibrium yield of ethanol.  

Although Le Chatelier’s principle is used to suggest the best conditions for a reaction, often a compromise has to be made.

i) Use Le Chatelier’s principle to suggest whether a high or low temperature should be used to produce the maximum yield of ethanol in the reaction from part (a).

ii) State a problem which may occur from using this temperature.

Ethanol can be used as a reactant in another equilibrium reaction; the manufacture of ethyl ethanoate.  

CH₃CH₂OH (l)   +   CH₃COOH (l)        CH₃COOCH₂CH₃ (l)   +   H₂O (l)

Give the expression for the equilibrium constant, K_c, for this equilibrium.

A student set up the esterification reaction seen in part (c), adding ethanol and ethanoic acid to a reaction vessel. They set the reaction up in a closed system, at a constant temperature and allowed equilibrium to be reached. 

The reaction was done in a container with a volume of 250 cm³. 

Table 1 below shows the amount of each substance present in the equilibrium mixture. 

Table 1

i) Calculate K_c for this reaction to 2 decimal places.

ii) Deduce the units for K_c. 

Methanol is the first member of the alcohol homologous series. It is an essential industrial chemical, and can be manufactured according to the following reaction: 

CO₂ (g)  +  3H₂ (g)    CH₃OH (g)  +  H₂O (g)    ?H = -91 kJ mol^-1

The conditions used are called compromise conditions, used to find a balance between producing a high yield of product and having manageable and safe conditions.

Give the K_c expression for this equilibrium reaction and deduce the units. 

The reaction in part (a) often uses a copper-based catalyst, meaning that the usual temperature and pressure of the reaction can be reduced. 

State and explain the effect, if any, that using a catalyst for this reaction will have on the yield of methanol which is produced.

State and explain why lowering the temperature of the reaction will contribute to a higher yield of methanol being produced, but why lowering the pressure would contribute to a lower yield of product. 

Excluding any reasons regarding the yield of the product, give a reason why scientists in industry much prefer to use a lower pressure and temperature where possible. 

Some industrial processes involve reversible reactions. If done under the correct conditions, then these reactions will reach a dynamic equilibrium, producing equilibrium mixtures of reactants and products. The conditions of the reaction can be altered and controlled to maximise the product yield. 

Le Chatelier’s principle is a rule which is used to determine how to move the equilibrium position to the left or right, by altering specific conditions of the reaction. 

State Le Chatelier’s principle. 

An example of an equilibrium reaction can be seen between oxygen and nitrogen.

When heated to a high enough temperature, nitrogen will react with oxygen to form nitrogen monoxide, as shown below:

N₂ (g)  +  O₂ (g)      2NO (g)

When the temperature of the system is increased, the yield of NO increases. 

State whether the forward reaction in this system is exothermic or endothermic.

Explain your answer.

State and explain the effect, if any, on the yield of NO produced if the pressure was increased, but the temperature was kept the same. 

A chemist set up a different reaction, shown below:

W (aq)  +  2X (aq)     3Y (aq)  +  Z (aq)

They started with a flask containing 1.75 x 10^-2 mol of an aqueous solution W. They then added 0.050 mol of solution X to the flask, ensured a closed system, and allowed the reaction to reach equilibrium.

Once equilibrium was reached, the reaction mixture contained 0.012 mol of Z.

The overall volume of the reaction mixture was 105 cm³.

i) Give the K_c expression for this reaction.

ii) Calculate a value for K_c.

A reaction mixture was set up in a syringe between dinitrogen tetraoxide gas and nitrogen dioxide gas as shown in the equation below:

N₂O₄ (g)    2NO₂ (g)               ?H = +58 kJ mol^-1

The appearance of the gases is very different; dinitrogen tetraoxide is a colourless gas, whereas nitrogen dioxide is dark brown in colour.

Give the K_c expression for this reaction and deduce the units.

Explain why the reaction mixture turns darker in colour when it is heated.

The reaction which takes place in part (a) has a K_c value of 3.21. A student claims that increasing the temperature of this reaction will increase the value of K_c. 

Is the student correct? Justify your answer.

Using Le Chatelier’s principle, explain what would be seen initially if the plunger of the syringe was pressed and the gases within the syringe were compressed.

During an esterification reaction, methanol and ethanoic acid react together to form the ester, methyl ethanoate, and water as shown below:

CH₃OH (l)  +  CH₃COOH (l)    CH₃COOCH₃ (l)  +  H₂O (l)

A chemist sets up the reaction and allows it to reach dynamic equilibrium at a constant temperature. 

i) State the meaning of the term dynamic equilibrium. 

ii) Give one key condition which must be satisfied for a reversible reaction to reach dynamic equilibrium. 

Once the reaction in part (a) is set up, the students leave it for 24 hours to make sure that it has reached equilibrium.

State how the students could check to make sure that the reaction mixture had reached equilibrium. 

When the equilibrium was reached, the equilibrium moles of each component in the mixture were as follows:

    - Ethanoic acid = 0.378

    - Methanol = must be calculated 

    - Methyl ethanoate = 0.716

    - Water = 1.08

K_c for the reaction was 7.21.  

i) Calculate the number of moles of methanol at equilibrium.

ii) State why there are no units for K_c in this reaction. 

Adding more ethanoic acid to the reaction mixture will increase the yield of the ester produced. 

Use Le Chatelier’s principle to explain the above statement. 

The following dynamic equilibrium was reached at temperature, T, in a closed container.

2 X (g) + Y (g)  2 Z (g)         ΔH = - 65 kJ mol^-1

The value of K_c for the reaction was 75.0 mol^-1 dm³ when the equilibrium mixture contained 2.97 mol of Y and 5.38 mol of Z.

i) Give the definition of dynamic equilibrium.

ii) Write an expression for K_c for the reaction.

iii) Show how the units for K_c are mol^-1 dm³.

Calculate the concentration of X in the equilibrium mixture if the volume of the container is 12.00 dm³. Give your answer to 3 significant figures.

If the conditions for a closed container are changed, it can have an effect on the concentrations of the reactants, products and K_c.

State the effect, if any, on the concentration of Y at equilibrium if temperature, T, is decreased and give a reason for your answer.

Calculate the equilibrium constant for the following reaction at temperature, T. 

2 Z (g)  2 X (g) + Y (g)

A 0.680 mol sample of SO₃ is introduced into a 3.04 dm³ reaction container and allowed to reach equilibrium at temperature T. 32% of the SO₃ had decomposed. 

Calculate the value for K_c in this reaction, giving your answer to 2 significant figures. 

2 SO₃ (g)  2 SO₂ (g) + O₂ (g)     ΔH = + 196 kJ mol^-1

The size of the container for the reaction in part (a) is increased. State the effect if any on the equilibrium constant, K_c, and the position of equilibrium. Justify your answer.

The temperature of the reaction in part (a) is increased. State the effect, if any, on the equilibrium constant, K_c, and the position of equilibrium. Justify your answer.

If the value of the equilibrium constant, K_c, is 2.7 x 10^-2 at temperature T1 for the reaction:

2 SO₃ (g)  2 SO₂ (g) + O₂ (g)           

i) Calculate the equilibrium constant, K_c, for the reaction:
4 SO₂ (g) + 2 O₂ (g)  4 SO₃ (g)
Give your answer to 2 decimal places.

ii) State the units for K_c for the reaction in part (i).

A mixture of 1.32 moles of E, 1.49 moles of F and 0.752 moles of G were placed into a 5.0 dm³ container at temperature, T, and allowed to reach equilibrium. At equilibrium, the number of moles of E was 1.86.

Calculate the value of the equilibrium constant, K_c, to 3 significant figures. 

2 E (g)  2 F (g) + G (g)    ΔH = -143 kJ mol^-1

The value of K_c for the reaction in part (a) at a different temperature, T1, is 2.98 mol dm^-3. Comment on the relationship between the concentration of the reactant E and products F and G with regards to K_c.

Reactants G and H react together to form products J and K according to the equation

3G + H  4J + K

A beaker contained 35 cm³ of 0.18 mol dm^-3 of an aqueous solution of G. 

8.41 x 10^-3 moles of H and 3.1 x 10^-3 moles of J were also added to the beaker. The equilibrium mixture contained 4.1 x 10^-3 moles of G.

Calculate the number of moles of H, J and K at equilibrium.

For the reaction in part (c) write the expression for the equilibrium constant, K_c, and state the units.

Diesters are compounds often used as synthetic lubricants for machinery such as compressors. The reaction below shows the formation of a diester from propanoic acid and propane-1,3-diol.

2 CH₃CH₂COOH + HOCH₂CH₂CH₂OH  C₉H₁₆O₄ + 2 H₂O

At equilibrium the reaction mixture contained 3.25 moles of CH₃CH₂COOH, 1.15 moles of HOCH₂CH₂CH₂OH, 1.18 moles of C₉H₁₆O₄.

The value for K_c at temperature, T, is 1.29.

Give the units for K_c.

Calculate the concentration of water in the reaction mixture in part (a) at equilibrium. Give your answer to 3 significant figures.

A student deduced that in order to calculate the value of K_c for the reaction in part (a) you must work out the concentrations using the overall volume. 

Justify whether the student is correct.

The forward reaction in part (a) is slightly exothermic. At a different temperature, T1, the value for K_c increases to 22.78. 

State whether the new temperature, T1, is higher or lower than the original temperature. Justify your answer.

The graph in Figure 1 shows the effect on pressure and temperature on the equilibrium yield of gaseous molecules.

Figure 1

Use Figure 1 to fully explain whether the forward reaction is exothermic or endothermic

Use Figure 1 to fully explain whether the forward reaction will involve either an increase or decrease in the number of moles of a gas.

The graph to show the relationship between temperature and Kc for a different dynamic equilibrium to produce a gaseous product is shown in Figure 2.

Figure 2

Use the information in Figure 2 to establish whether the forward reaction is exothermic or endothermic. Justify your answer.

Gaseous products can be manufactured by the direct combination of reactants.

Explain why it is important for a chemist to know the value of K_c, at a given temperature, for such a reaction.