Exam code: 9701
Explain what is meant by the term dynamic equilibrium.
A general reaction exists in the following dynamic equilibrium.
3A + 2B ⇌ C2 + D
State the condition required for this equilibrium to be established.
The gases nitrogen dioxide, NO2 (g), and dinitrogen tetroxide, N2O4 (g), exist in the following dynamic equilibrium.
2NO2 (g) ⇌ N2O4 (g)
An increase in pressure shifts this equilibrium to the right. Explain why.
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During the manufacture of sulfuric acid in the Contact process, sulfur dioxide, SO2, is oxidised to sulfur trioxide, SO3.
2SO2 (g) + O2 (g) ⇌ 2SO3 (g) ΔH = −197 kJ mol−1
State the effect of an increase in temperature on the rate of this reaction.
State and explain the effect of an increase in temperature on the equilibrium yield of sulfur trioxide.
State and explain the effect of a decrease in pressure on the equilibrium yield of sulfur trioxide.
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Ammonia is manufactured from nitrogen and hydrogen in the Haber process.
N2 (g) + 3H2 (g) ⇌ 2NH3 (g)
State the effect of an iron catalyst on the position of equilibrium.
Write an expression for the equilibrium constant, Kc, for this reaction.
Kc =
The equilibrium concentrations of the gases at 450 °C are shown in Table 3.1.
Table 3.1
gas | equilibrium concentration / mol dm-3 |
|---|---|
N2 (g) | 17.4 |
H2 (g) | 0.850 |
NH3 (g) | 1.25 |
Calculate the value of Kc at 450 °C. Give your answer to three significant figures. Show your working.
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Dinitrogen pentoxide, N2O5 (g), decomposes to form oxygen and nitrogen dioxide. The equation for the decomposition is shown.
2N2O5 (g) ⇌ O2 (g) + 4NO2 (g)
Write an expression for the equilibrium constant, Kp, for this reaction.
Kp =
The equilibrium partial pressures of the gases at temperature T are shown in Table 4.1.
Table 4.1
gas | equilibrium partial pressure / Pa |
|---|---|
N2O5 (g) | 10 000 |
O2 (g) | 400 |
NO2 (g) | 600 |
Use your expression in (a) and the data in Table 4.1 to calculate the value of Kp at temperature T. State its units. Show your working.
Kp = .......... units = ..........
The reaction is repeated at a lower pressure. State the effect of a decrease in pressure on the value of Kp.
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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 that increasing the overall pressure would have on the equilibrium yield of ethanol.
Although a compromise must often be made when selecting the best conditions for a reaction, Le Chatelier's principle helps to identify the optimum conditions.
i) State and explain whether a high or low temperature should be used to produce the maximum equilibrium yield of ethanol in the reaction from part (a).
[2]
ii) Suggest one disadvantage of using this temperature.
[1]
Ethanol can be used as a reactant in another equilibrium reaction; the manufacture of ethyl ethanoate.
CH3CH2OH (l) + CH3COOH (l) ⇌ CH3COOCH2CH3 (l) + H2O (l)
Write an expression for the equilibrium constant, Kc, 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 1.1 shows the amount of each substance present in the equilibrium mixture.
Table 1.1
Substance | Amount / mol |
|---|---|
CH3CH2OH | 0.0375 |
CH3COOH | 0.0615 |
CH3COOCH2CH3 | 0.0776 |
H2O | 0.0834 |
Calculate Kc for this reaction to 2 decimal places, and state its units. Show your working.
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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 Kc for this equilibrium.
Kc =
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.
i) Calculate Kc at this temperature. Give your answer to three significant figures.
Show your working.
Kc = ......................
[5]
ii) State the units for Kc.
units = ...............
[1]
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 Kc would be greater, smaller or the same.
The chemist increases the pressure of the mixture at constant temperature.
Explain whether the value of Kc would be greater, smaller or the same.
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Hydrogen chloride undergoes a reversible reaction with oxygen at 400 °C in the presence of copper(II) chloride catalyst forming chlorine and water.
i) Construct the equation for this reaction. Include state symbols.
[2]
ii) The reaction exists in dynamic equilibrium. The reaction was repeated at the same temperature but at a lower pressure.
State and explain the effect on the composition of the equilibrium mixture of the change in pressure.
[2]
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.
i) Calculate the amounts, in mol, of HCl and O2 in the equilibrium mixture. Show your working.
HCl = .............................. mol
O2 = .............................. mol
[2]
ii) Calculate the equilibrium partial pressure of each species in the equilibrium mixture.
pHCl = .............................. Pa
pO2 = .............................. Pa
pCl2 = .............................. Pa
pH2O = .............................. Pa
[3]
iii) Write an expression for Kp for this reaction.
Kp =
[1]
iv) Calculate a value for Kp and state its units. Show your working.
Kp = .................................
units = .................................
[2]
The reaction is repeated without a catalyst. State the effect of this on the value of Kp.
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Ethanoic acid can be reacted with methanol to form methyl ethanoate, and an equilibrium mixture is formed.
CH3COOH (l) + CH3OH (l) ⇌ CH3COOCH3 (l) + H2O (l)
The reaction is usually carried out in the presence of an acid catalyst.
Write an expression for the equilibrium constant, Kc, for this reaction, clearly stating the units.
Kc =
In an experiment to determine Kc 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.
i) Construct a balanced equation for the reaction between ethanoic acid and potassium hydroxide.
[1]
ii) Suggest a suitable choice of indicator for the titration.
[1]
The student carried out the following calculations.
i) Calculate the amount, in moles, of KOH used in the titration.
[1]
ii) State the amount, in moles, of KOH that reacted with the hydrogen chloride.
[1]
iii) Hence calculate the amount, in moles, of KOH that reacted with the ethanoic acid.
[1]
iv) Use your results from parts (i), (ii) and (iii) to calculate the amount, in moles, of ethanoic acid present at equilibrium.
Hence complete the table below.
[2]
CH3COOH | CH3OH | CH3COOCH3 | H2O | |
|---|---|---|---|---|
Initial amount / mol | 0.20 | 0.20 | 0 | 0 |
Equilibrium amount / mol |
v) Use your results to calculate a value for Kc for this reaction. State its units.
Kc = ........................
units = .......................
[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.
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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 Kc for the reaction was 255 mol-3 dm9 when the equilibrium mixture contained 3.34 mol of A and 4.28 mol of C.
i) Explain what is meant by the term dynamic equilibrium.
[2]
ii) Write an expression for the equilibrium constant, Kc, for this reaction.
[1]
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. Show your working.
State and explain the effect of an increase in temperature on the equilibrium concentration of B.
Calculate the value of the equilibrium constant for the following reaction at temperature T. Show your working.
2C (g) ⇌ 3A (g) + 2B (g)
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The graph in Fig. 2.1 shows the effect of pressure and temperature on the equilibrium yield of a gaseous product.
Fig. 2.1
Use Fig. 2.1 to deduce whether the forward reaction is exothermic or endothermic. Explain your answer.
Use Fig. 2.1 to deduce whether the forward reaction produces an increase or a decrease in the number of moles of gas. Explain your answer.
Fig. 2.2 shows the relationship between temperature and Kc for a different dynamic equilibrium.
Fig. 2.2
Use the information in Fig. 2.2 to deduce whether the forward reaction is exothermic or endothermic. Explain your answer.
The equilibrium in Fig. 2.2 is used to manufacture a gaseous product industrially.
Suggest one reason why the industrial temperature may be higher than the temperature that gives the maximum equilibrium yield.
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At high temperatures, gaseous phosphorus(V) chloride dissociates according to the following equation:
PCl5 (g) ⇌ PCl3 (g) + Cl2 (g)
At 550 K, the equilibrium amount of each gas is shown in Table 3.1.
Table 3.1
Chemical | PCl5 (g) | PCl3 (g) | Cl2 (g) |
|---|---|---|---|
Equilibrium moles | 0.192 | 0.822 | 0.822 |
Calculate the total pressure of the reaction, if Kp = 526 kPa. Give your answer to an appropriate number of significant figures. 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
Chemical | NO2 (g) | NO (g) | O2 (g) |
|---|---|---|---|
Equilibrium moles | 0.920 | 0.0600 | 0.0400 |
Calculate the total pressure of the reaction, if Kp = 9.8 x 10-3 kPa. Show your working.
P = ................... kPa
Hydrogen iodide can undergo thermal decomposition according to the following equation:
2HI (g) ⇌ H2 (g) + I2 (g)
The value of Kp 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 an appropriate number of significant figures. Show your working.
P = ......................... kPa
Dinitrogen tetroxide decomposes according to the following equilibrium:
N2O4 (g) ⇌ 2NO2 (g)
The value of Kp for this reaction at 300 K is 385 kPa.
The equilibrium mixture contained 2.40 mol of nitrogen dioxide after an initial 3.00 mol of dinitrogen tetroxide were heated at 300 K.
Calculate the partial pressures of N2O4 (g) and NO2 (g). Give your answer to an appropriate number of significant figures. Show your working.
pN2O4 = ................... kPa
pNO2 = ................... kPa
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