Chemical Equilibria: Reversible Reactions & Dynamic Equilibrium (CIE AS 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)    ΔH = -46 kJ mol^-1

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

State and explain the effect, if any, that increasing the overall pressure would have on the equilibrium yield of ethanol. Use Le Chatelier’s principle to explain your answer. 

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

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.

K_c =

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

Table 1.1 

Ammonia can be manufactured from nitrogen and hydrogen gases in the Haber process.

The equilibrium is shown below.

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)   ΔH = –91 kJ mol^–1

Write an expression for K_c for this equilibrium. 

K_c = 

A chemist carries out a series of experiments to investigate the conversion of nitrogen and hydrogen into ammonia under different conditions.

The chemist mixes together 10.40 mol N₂ and 22.50 mol H₂ and pressurises the gases so that the total gas volume is 5.00 dm³.

The mixture is allowed to reach equilibrium at constant temperature and without changing the total gas volume. The equilibrium mixture contains 5.60 mol NH₃.

The chemist repeats the experiment several times. In each experiment, the chemist makes one change.

The chemist heats the mixture to a higher temperature at constant pressure.

Explain whether the value of K_c would be greater, smaller or the same.

The chemist increases the pressure of the mixture at constant temperature.

Hydrogen chloride undergoes a reversible reaction with oxygen at 400 °C at in the presence of copper(II) chloride catalyst forming chlorine and water.

When 2.00 mol of HCl are mixed in a sealed container with 0.600 mol of O₂ at 400 °C, 0.700 mol of Cl₂ and 0.700 mol of H₂O are formed.

The total pressure inside the container is 1.15 × 10⁵ Pa.

pHCl = .............................. Pa

pO₂ = .............................. Pa

pCl₂ = .............................. Pa

pH₂O = ................................ Pa

[3]

The reaction is repeated without a catalyst.

State the effect of this on K_p

Ethanoic acid can be reacted with methanoic acid to form methyl ethanoate, and an equilibrium mixture is formed. 

CH₃COOH + CH₃OH CH₃COOCH₃ + H₂O

The reaction is usually carried out in the presence of an acid catalyst.

Write an expression for the equilibrium constant, K_c, for this reaction, clearly stating the units.

K_c =

In an experiment to determine K_c a student placed together in a conical flask 0.20 mol of ethanoic acid, 0.20 mol of methanol, and 0.01 mol of hydrogen chloride catalyst.

The flask was sealed and kept at 25 °C for ten days. After this time, the student titrated all of the contents of the flask with 1.80 mol dm^–3 KOH. At the end-point, 26.30 cm³ of KOH had been used.

The student carried out the following calculations.

[2]

The student performed another titration between the ethanoic acid used in the formation of the ester and the potassium hydroxide solution. The volume of potassium hydroxide added from the burette was recorded. 

Sketch a pH titration curve for this reaction.

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

3A (g) + 2B (g) ⇌ 2C (g)    ΔH = - 65 kJ mol^-1 

The value of K_c for the reaction was 255 mol^-1 dm³ when the equilibrium mixture contained 3.34 mol of A and 4.28 mol of C. 

Calculate the concentration of B in the equilibrium mixture if the volume of the container is 8.00 dm³. Give your answer to 3 significant figures.

Changing the conditions of a closed container can have an effect on the concentrations of the reactants, products and K_c.

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

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

2C (g) ⇌ 3A (g) + 2B (g)

The graph in Fig. 2.1 shows the effect of pressure and temperature on the equilibrium yield of gaseous molecules.

Fig. 2.1

Use Fig. 2.1 to fully explain whether the forward reaction is exothermic or endothermic.

Use Fig 2.1 to explain whether the forward reaction will involve either an increase or decrease in the number of moles of a gas.

The graph shows the relationship between temperature and K_c for a different dynamic equilibrium to produce a gaseous product is shown in Fig 2.2.

Fig 2.2

Use the information in Fig 2.2 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.

At high temperatures, gaseous phosphorus(V) chloride dissociates according to the following equation:

PCl₅ (g) ⇋ PCl₃ (g) + Cl₂ (g)

At 550K, the equilibrium amount of each gas is shown in Table 3.1.

Table 3.1

Calculate the total pressure of the reaction, if K_p = 526 kPa. Give your answers to an appropriate number of significant figures and show your working.

 P = ........................... kPa

Nitrogen dioxide decomposes according to the following equation:

2NO₂ (g) ⇋ 2NO (g) + O₂ (g)

At 700 K, the equilibrium amount of each gas is shown in Table 3.2.

Table 3.2

Calculate the total pressure of the reaction, if K_p = 9.8 x 10^-3 kPa.

P = ................... kPa

Hydrogen iodide can undergo thermal decomposition according to the following equation:

2HI (g) ⇋ H₂ (g) + I₂ (g)

The value of K_p for this reaction at 700 K is 0.0185.

Calculate the total pressure when the partial pressure of hydrogen iodide, at equilibrium, is 133 kPa. Give your answer to the appropriate number of significant figures.

P = .........................

Dinitrogen tetroxide decomposes according to the following equilibrium:

N₂O₄ (g) ⇋ 2NO₂ (g) 

The value of K_p for this reaction at 300 K is 385 kPa.

The equilibrium mixture contained 2.40 moles of nitrogen dioxide after an initial 3.00 moles of dinitrogen tetroxide were heated at 300 K

Calculate the partial pressures of dinitrogen tetroxide and nitrogen dioxide. Give your answer to the appropriate number of significant figures.

p N₂O₄ = ................... kPa

p NO₂ = ................... kPa