Topic 2: Managing chemical processes

Name the change first, then say which way the system shifts to oppose it. Only a temperature change alters Kc; a catalyst changes the rate, not the position.

A lesson for this topic

Quick checks: four options

Question 1

A student adds marble chips to dilute hydrochloric acid in a conical flask at 22 °C and collects the carbon dioxide in a gas syringe. The graph shows the volume collected. What is the mean rate of production of carbon dioxide over the first 20 s?

Question 2

Marble chips react with hydrochloric acid in an open flask on a balance: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). In the first 2.00 min the mass of the flask falls by 1.32 g as carbon dioxide escapes. The molar mass of CO₂ is 44.01 g mol⁻¹. What is the mean rate at which hydrochloric acid is used up over this time?

Question 3

Excess marble chips are added to dilute hydrochloric acid. The rate at which gas is produced falls steadily as the reaction goes on. What is the main reason?

Question 4

Warming a reaction mixture from 20 °C to 30 °C roughly doubles the rate of a reaction. What is the main reason?

Question 5

In a school laboratory in Mount Gambier, 1.0 g of zinc powder reacts with dilute sulfuric acid much faster than 1.0 g of zinc granules does with the same volume of the same acid. Why?

Question 6

Nitrogen monoxide and oxygen react in a gas syringe: 2NO(g) + O₂(g) → 2NO₂(g). At constant temperature the plunger is pushed in to halve the volume of the mixture. What happens to the rate of reaction?

Question 7

Catalase in potato breaks down hydrogen peroxide. In an investigation, the rate rises from 20 °C to about 37 °C but falls sharply above 45 °C. Why does the rate fall at the higher temperatures?

Question 8

The energy profile shows a reaction with and without a catalyst. What is the activation energy of the catalysed forward reaction?

Question 9

For a reaction, the activation energy of the forward reaction is 130 kJ mol⁻¹ and ΔH = +50 kJ mol⁻¹. What is the activation energy of the reverse reaction?

Question 10

0.100 g of magnesium ribbon reacts with 50.0 mL of 1.00 mol L⁻¹ hydrochloric acid (an excess) at 25 °C, giving the original curve on the graph. The experiment is repeated with 0.050 g of magnesium powder and a fresh 50.0 mL of the same acid at 25 °C. Which curve shows the repeat?

Question 11

A student times how long a cross under a flask takes to disappear when sodium thiosulfate solution reacts with hydrochloric acid at 20, 30, 40 and 50 °C. Which is a controlled variable in this investigation?

Question 12

In the same thiosulfate investigation, different students stop the stopwatch at slightly different moments when they judge that the cross has disappeared. Which statement about this error is correct?

Question 13

Mean time for the cross to disappear at four thiosulfate concentrations, each the mean of three trials at 25 °C.
[S₂O₃²⁻] (mol L⁻¹)	Mean time (s)
0.050	84
0.100	41
0.150	28
0.200	21

Thiosulfate ions react with acid: S₂O₃²⁻(aq) + 2H⁺(aq) → S(s) + SO₂(g) + H₂O(l). A student varies the thiosulfate concentration at 25 °C, keeping everything else the same, and records the results in the table. Which conclusion is valid from these data?

Question 14

Limestone heated in an open kiln decomposes completely: CaCO₃(s) → CaO(s) + CO₂(g). Why does this reaction not reach equilibrium?

Question 15

Which statement describes a chemical system at equilibrium in a closed flask at constant temperature?

Question 16

Which is the equilibrium expression for N₂(g) + 3H₂(g) ⇌ 2NH₃(g)?

Question 17

A 2.00 L flask at a fixed temperature holds an equilibrium mixture of 0.40 mol SO₂, 0.20 mol O₂ and 1.20 mol SO₃: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). What is the value of Kc at this temperature?

Question 18

1.00 mol of N₂ and 3.00 mol of H₂ are sealed in a 2.00 L vessel and heated: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). At equilibrium the vessel contains 0.40 mol of NH₃. What is the value of Kc at this temperature?

Question 19

Pure N₂O₄ is sealed in a flask at constant temperature and reacts: N₂O₄(g) ⇌ 2NO₂(g). The graph shows the concentrations of both gases. What is the value of Kc at this temperature?

Question 20

A solution contains the equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq), in which Fe³⁺ is pale yellow and FeSCN²⁺ is deep red. A few drops of concentrated potassium thiocyanate solution are added. What is observed once equilibrium is re-established?

Question 21

A red solution contains the equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq). Water is added to double its volume at constant temperature. Which statement describes the shift in the position of equilibrium?

Question 22

For which equilibrium does halving the volume of the container, at constant temperature, NOT change the position of equilibrium?

Question 23

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. An equilibrium mixture is cooled from 500 °C to 400 °C and allowed to reach equilibrium again. What happens to the amount of ammonia and to the value of Kc?

Question 24

A sealed tube holds the equilibrium 2NO₂(g) ⇌ N₂O₄(g); NO₂ is brown and N₂O₄ is colourless. When the tube is put in iced water the gas becomes paler. What does this show about the forward reaction?

Question 25

For N₂(g) + O₂(g) ⇌ 2NO(g), Kc = 4.5 × 10⁻³¹ at 25 °C. What does this value show about an equilibrium mixture at 25 °C?

Question 26

At 450 °C, Kc = 49.0 for H₂(g) + I₂(g) ⇌ 2HI(g). The equation is then written the other way round: 2HI(g) ⇌ H₂(g) + I₂(g). What is Kc for this reversed equation at 450 °C?

Question 27

A catalyst is added to a mixture of reactants before they start to react in a closed vessel. Compared with no catalyst, which statement is correct?

Question 28

For 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), Kc = 85.0 at a certain temperature. A 2.50 L vessel at this temperature holds an equilibrium mixture with [SO₂] = 0.0400 mol L⁻¹ and [O₂] = 0.0500 mol L⁻¹. What amount of SO₃ is in the vessel?

Question 29

The graph shows a sealed flask of fixed volume containing N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. One change was made at 10 min and another at 20 min. What were the two changes?

Question 30

The stages of a sulfuric acid plant, with what goes into and comes out of each stage.
Stage	Substances in	Substances out	Conditions
1 Sulfur burner	molten sulfur, dry air	SO₂, N₂, unused O₂	about 1000 °C
2 Converter	gases from stage 1, extra dry air	SO₃, N₂, a little unreacted SO₂ and O₂	V₂O₅ catalyst, 400 to 450 °C, about 200 kPa
3 Absorber	gases from stage 2, 98% H₂SO₄	H₂S₂O₇ (oleum); tail gas of N₂, O₂ and a little SO₂	below 100 °C
4 Dilution	H₂S₂O₇, water	98% H₂SO₄, part of it returned to stage 3	cooled

The table summarises a sulfuric acid plant. Which list gives only the raw materials that the plant takes in?

Question 31

The stages of a sulfuric acid plant, with what goes into and comes out of each stage.
Stage	Substances in	Substances out	Conditions
1 Sulfur burner	molten sulfur, dry air	SO₂, N₂, unused O₂	about 1000 °C
2 Converter	gases from stage 1, extra dry air	SO₃, N₂, a little unreacted SO₂ and O₂	V₂O₅ catalyst, 400 to 450 °C, about 200 kPa
3 Absorber	gases from stage 2, 98% H₂SO₄	H₂S₂O₇ (oleum); tail gas of N₂, O₂ and a little SO₂	below 100 °C
4 Dilution	H₂S₂O₇, water	98% H₂SO₄, part of it returned to stage 3	cooled

The table summarises a sulfuric acid plant. Which substance in the tail gas from stage 3 must be kept to a very low level to protect the environment?

Question 32

In the converter of a sulfuric acid plant, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). Why is air supplied in excess?

Question 33

In ammonia production, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. Plants run the converter at about 450 °C rather than 200 °C. Why?

Question 34

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), a higher pressure raises both the rate and the equilibrium yield. Why do many plants stop at about 20 MPa rather than going much higher?

Question 35

In an ammonia plant only about 15% of the nitrogen and hydrogen is converted in each pass through the converter, yet about 97% is converted overall. How?

Question 36

Why does the iron catalyst in an ammonia plant lower the plant's energy costs?

Question 37

Percentage of methanol in an equilibrium mixture made from CO and H₂ at three temperatures and two pressures.
Temperature (°C)	Methanol at 5 MPa (%)	Methanol at 10 MPa (%)
200	52	71
250	31	50
300	13	27

Methanol is made by CO(g) + 2H₂(g) ⇌ CH₃OH(g). The table shows results of an investigation of the equilibrium mixture. What do the data show about ΔH for the forward reaction?

Question 38

In a sulfuric acid plant, 98.0% of the SO₂ entering the converter becomes SO₃, and every other stage converts fully, so each mole of SO₃ formed gives one mole of H₂SO₄. What mass of sulfuric acid is made from 1.00 tonne of sulfur? Use S 32.06 g mol⁻¹ and H₂SO₄ 98.08 g mol⁻¹ (1 tonne = 10⁶ g).

Question 39

The converter reaction in a sulfuric acid plant, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), is exothermic. Many plants pass the hot gases through heat exchangers that turn water into steam. What is the main benefit?

Question 40

0.800 mol of NO₂ is sealed in a 2.50 L flask at a fixed temperature: 2NO₂(g) ⇌ N₂O₄(g). At equilibrium the flask holds 0.250 mol of N₂O₄. What is the value of Kc at this temperature?

Question 41

1.00 mol of SO₃ is sealed in a 4.00 L vessel and heated: 2SO₃(g) ⇌ 2SO₂(g) + O₂(g). At equilibrium the vessel contains 0.200 mol of O₂. What is the value of Kc at this temperature?

Question 42

0.500 mol of H₂ and 0.500 mol of I₂ are sealed in a 2.00 L flask at 450 °C: H₂(g) + I₂(g) ⇌ 2HI(g), Kc = 64.0 at 450 °C. What amount of HI is in the flask at equilibrium?

Question 43

Hydrogen peroxide decomposes over a catalyst and the oxygen is collected in a gas syringe. The graph shows the volume collected, with a tangent drawn at 40 s. What is the rate of production of oxygen at 40 s?

Question 44

The graph shows a sealed vessel of fixed volume containing 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −197 kJ mol⁻¹. One change was made at 10 min and another at 20 min. What were the two changes, in order?

Question 45

The graph shows a sealed vessel containing N₂(g) + 3H₂(g) ⇌ 2NH₃(g) at constant temperature. One change was made at 10 min and another at 20 min. What were the two changes?

Question 46

Four sets of trial conditions for a methanol converter: the percentage of the gas converted to methanol in one pass at equilibrium, and the time each pass takes to reach equilibrium.
Conditions	Methanol per pass (%)	Time per pass (min)
200 °C and 10 MPa	71	60
250 °C and 10 MPa	50	10
300 °C and 10 MPa	27	4
250 °C and 5 MPa	31	10

A methanol plant runs CO(g) + 2H₂(g) ⇌ CH₃OH(g). Unreacted gas is recycled, so the plant's output per hour is proportional to the percentage converted per pass divided by the time per pass. The copper catalyst is damaged above 270 °C. Using the table, which conditions should the plant choose?

Written questions

Marble chip size and the rate of reactionWritten question · 8 marks

A student investigated how the size of marble chips affects the rate of CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). She put 5.00 g of small chips and 50.0 mL of 1.00 mol L⁻¹ hydrochloric acid in a conical flask on a balance, plugged the neck with cotton wool, and recorded the mass lost every 30 s. She repeated the experiment with 5.00 g of large chips. The table shows her results. The molar mass of CO₂ is 44.01 g mol⁻¹.

Total mass lost from the flask against time for 5.00 g of small marble chips and 5.00 g of large marble chips, each with 50.0 mL of 1.00 mol L⁻¹ hydrochloric acid.
Time (s)	Mass lost, small chips (g)	Mass lost, large chips (g)
0	0.00	0.00
30	0.42	0.18
60	0.74	0.34
90	0.96	0.48
120	1.06	0.60
180	1.10	0.78
240	1.10	0.90

(a) Calculate the mean rate of production of carbon dioxide, in mol s⁻¹, for the small chips over the first 60 s. [2]

mol s⁻¹

(b) Identify the independent variable and one controlled variable in this investigation. [2]

0 words · a sentence or two

(d) The cotton wool lets gas out but stops acid spray escaping. Explain how leaving it out would affect the rates the student calculates. [2]

0 words · a sentence or two

Making methanol in a sealed vesselWritten question · 7 marks

A chemist seals 0.400 mol of CO and 0.800 mol of H₂ in a 2.00 L vessel at 250 °C: CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH = −91 kJ mol⁻¹. When equilibrium is reached the vessel contains 0.120 mol of CH₃OH.

(a) Calculate the amount of hydrogen in the vessel at equilibrium. [2]

mol

(b) Calculate the value of Kc at 250 °C. [3]

(c) The vessel is then heated to 300 °C and left to reach equilibrium. Predict the effect on the value of Kc, and explain your prediction. [2]

0 words · a sentence or two

Choosing conditions for an ammonia plantWritten question · 6 marks

Ammonia is made by N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹, over an iron catalyst. The table shows the percentage of ammonia at equilibrium under different conditions. A plant in the north of the state runs its converter at 450 °C and 20 MPa.

Percentage of ammonia in an equilibrium mixture of N₂, H₂ and NH₃ at three temperatures and three pressures.
Temperature (°C)	NH₃ at 10 MPa (%)	NH₃ at 20 MPa (%)	NH₃ at 30 MPa (%)
350	32	46	55
450	16	27	35
550	7	13	19

(a) Explain why the plant runs at 450 °C rather than 350 °C, even though 350 °C gives a higher percentage of ammonia at equilibrium. [4]

0 words · about 80 to 120 words

(b) Describe the trend in the table as the pressure rises at 450 °C, and account for it. [2]

0 words · a sentence or two

Manganese dioxide and hydrogen peroxideWritten question · 5 marks

Hydrogen peroxide decomposes slowly at room temperature: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). A pinch of manganese dioxide makes oxygen bubble out quickly, and the MnO₂ can be filtered off unchanged afterwards. The energy profile shows both pathways for 2 mol of H₂O₂.

(a) Use the energy profile to determine ΔH for the reaction as written, in kJ mol⁻¹. [1]

kJ mol⁻¹

(b) Use the energy profile to determine the activation energy of the reaction with manganese dioxide, in kJ mol⁻¹. [1]

kJ mol⁻¹

(c) Explain, using collision theory and the energy profile, how manganese dioxide increases the rate of this reaction. [3]

0 words · about 50 to 80 words

A red equilibrium in a test tubeWritten question · 4 marks

A test tube holds the equilibrium Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq). Fe³⁺ is pale yellow, SCN⁻ is colourless and FeSCN²⁺ is deep red. The tube starts at 25 °C.

(a) The tube is placed in an ice bath and the solution turns a deeper red. Deduce whether the forward reaction is exothermic or endothermic, and explain your answer. [2]

0 words · a sentence or two

(b) The tube is returned to 25 °C and a few drops of sodium hydroxide solution are added. Fe³⁺ ions are removed as a solid: Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s). The solid settles to the bottom of the tube. Predict and explain the change in the red colour of the solution above it. [2]

0 words · a sentence or two

Air or pure oxygen in the converterWritten question · 7 marks

In a test of the converter reaction, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −197 kJ mol⁻¹, 0.500 mol of SO₂ and 0.300 mol of O₂ were sealed in a 10.0 L vessel at 450 °C. At equilibrium the vessel held 0.400 mol of SO₃. In the plant itself, air is fed to the converter and about 97% of the SO₂ is converted after several passes over the catalyst.

(a) Calculate the value of Kc at 450 °C. [3]

(b) An engineer suggests feeding pure oxygen to the converter instead of air. Evaluate this suggestion. [4]

0 words · about 80 to 120 words