1.7 Equilibria

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.

A student carried out some acid-base titrations using different reactants.

The indicators and their pH range are given in Table 3.1. 

Table 3.1

In the first titration, they titrated 10.0 cm³ of 0.200 mol dm^-3 HCl against 0.200 mol dm^-3 aqueous ammonia. 

Using Table 3.1, explain why methyl red is a suitable indicator for the first titration but phenolphthalein is not.

In a second titration, the student titrated 25.0 cm³ of 0.200 mol dm^-3 sodium hydroxide against 0.100 mol dm^-3 ethanoic acid solution.

Sketch the graph that would be obtained for this titration.

Using Table 3.1, explain why phenolphthalein is a suitable indicator for the second titration but methyl orange is not.

Explain whether the other two indicators in Table 3.1 are suitable for use in either titration.

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 4.1.

Table 4.1

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 4.2.

Table 4.2

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

Malonic acid is a weak dibasic carboxylic acid with the formula C₃H₄O₄. Draw the displayed structure of malonic acid.

Suggest, with a reason, whether ethanoic acid or malonic acid has a higher pH.

Apart from testing the pH, suggest how equimolar solutions of malonic acid and ethanoic acid may be distinguished.

30.0 of 0.075 mol dm^-3 CH₃COOH with a pH of 2.93 was slowly added to a sample of 10.0 cm³ 0.150 mol dm^–3 KOH with a pH of 13.1. The pH was measured throughout the addition. 

On the following axes, sketch how the pH will change during the addition of a total of 30.0 of 0.075 CH₃COOH. 

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.

This question is about Brønsted-Lowry acids and bases.

A student titrated 0.10 mol dm^-3 acid into a conical flask containing 25.0 cm³ of 0.1 mol dm^-3 of a base, recording the pH with each addition of acid.

The student repeated the procedure using different combinations of acids and bases. 

Identify which curve shown above in Fig. 5.1, is the acid-base combination for the following: 

Identify which indicator given in Table 5.2 would be most suitable for curve Z. Justify your answer. 

Table 5.2

Water is amphoteric and will react with nitric acid and ammonia.