Ammonia and sulfuric acid — answer key

1. XQ15 · Practice

Model answer. (a) N₂(g) + 3H₂(g) ⇌ 2NH₃(g). (b) Nitrogen comes from the air; hydrogen from methane (natural gas) reacted with steam. (c) About 450 °C, 20 000 kPa (200 atm) and an iron catalyst.

A full answer includes

• (a) N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
• (b) Nitrogen from the air; hydrogen from methane.
• (c) 450 °C, 20 000 kPa (200 atm), an iron catalyst.

Also accept

• N₂ + 3H₂ ⇌ 2NH₃; air and methane; 450 °C, 200 atm, iron

Watch for

• “The hydrogen comes from water.” — In the Haber process the hydrogen comes from methane.

2. XQ16 · Practice

Options as numbered on the pupil sheet: 1 — 20 000 kPa (200 atm); 2 — burning sulfur or roasting sulfide ores; 3 — 450 °C; 4 — iron; 5 — 200 kPa (2 atm); 6 — vanadium(V) oxide.

Model answer. The Contact process, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), runs at about 450 °C and 200 kPa (2 atm) with a vanadium(V) oxide catalyst. Sulfur dioxide comes from burning sulfur or roasting sulfide ores; oxygen from the air. Iron and 200 atm belong to the Haber process.

A full answer includes

• Temperature: 450 °C; pressure: 200 kPa (2 atm); catalyst: vanadium(V) oxide; sulfur dioxide from: burning sulfur or roasting sulfide ores.

Also accept

• 450 °C; 2 atm; vanadium(V) oxide; burning sulfur or roasting sulfide ores

Watch for

• “Both processes use 200 atm.” — The Haber process uses 200 atm; the Contact process only 2 atm.

3. XQ17 · Practice

Model answer. (a) 28 %. (b) 36 − 16 = 20 percentage points. (c) There are 4 gas molecules on the left and 2 on the right, so a higher pressure moves the position of equilibrium to the right: more ammonia.

Checked automatically

• (a) Ammonia at 200 atm: 28 % (exact value)
• (b) Rise from 100 atm to 300 atm: 20 % (exact value)
  • Common wrong answer 36 : That is the value at 300 atm. Subtract the value at 100 atm.

A full answer includes

• (a) 28 %.
• (b) 20 percentage points.
• (c) A higher pressure moves the position to the side with fewer gas molecules (4 → 2): more ammonia.

Also accept

• 28 %; 20; fewer gas molecules on the right

Watch for

• “A higher pressure always moves the position to the right.” — A higher pressure moves it to the side with fewer gas molecules, which may be either side; with the same number on each side it makes no difference.

4. XQ18 · Practice

Model answer. (a) Cooling favours the exothermic direction, the forward reaction, so the yield would be higher. (b) At a lower temperature the reaction would be much slower, even with the iron catalyst, so less ammonia would be made each hour. 450 °C is a compromise between yield and rate.

A full answer includes

• (a) It would increase the yield (the exothermic forward reaction is favoured).
• (b) The rate would be too slow, even with the catalyst: 450 °C is a compromise.

Also accept

• Higher yield; too slow; compromise

Watch for

• “450 °C is used because it gives the highest yield.” — A lower temperature gives a higher yield, because the forward reaction is exothermic. 450 °C is a compromise: a lower yield, but made fast enough.

5. XQ19 · Practice

Model answer. N₂ + 3H₂ ⇌ 2NH₃ has 4 gas molecules on the left and 2 on the right, so a higher pressure moves the position to the right: more ammonia (it also makes the reaction faster). But very high pressures need thick, expensive pipes and vessels and a lot of energy for the pumps, and a leak or burst is dangerous. 200 atm is a compromise.

A full answer includes

• 4 gas molecules on the left, 2 on the right: higher pressure moves the position to the right.
• Higher pressure costs more (stronger equipment, more energy for compressing).
• It is more dangerous (leaks or bursts).

Also accept

• Fewer gas molecules on the right; cost and safety

Watch for

• “The pressure is limited because ammonia would turn to a liquid.” — The limits are cost and safety: very high pressures need expensive, strong equipment and are dangerous.

6. XQ20 · Practice

Model answer. There are 3 gas molecules on the left and 2 on the right, so a higher pressure would help a little. But at 2 atm the position is already far to the right: well over nine-tenths of the sulfur dioxide is converted. The small extra yield isn't worth the cost of stronger equipment and pumping, or the extra danger. (c) The forward reaction is exothermic, so a lower temperature would give a higher yield, but the reaction would be far too slow. 450 °C is a compromise between a good yield and a fast enough rate, and the vanadium(V) oxide catalyst makes the reaction fast enough there; it doesn't change the yield.

A full answer includes

• (a) The yield is already high at 2 atm (most of the sulfur dioxide is converted).
• (b) A higher pressure would add little product for a much greater cost and risk.
• (c) A lower temperature would give a higher yield (the forward reaction is exothermic) but too slow a rate; 450 °C is a compromise.
• (c) The vanadium(V) oxide catalyst makes the reaction fast enough at 450 °C without changing the yield.

Also accept

• Already a high yield; not worth the cost
• 450 °C: a compromise between yield and rate; the catalyst speeds it up

Watch for

• “A higher pressure isn't used because it would move the position to the left.” — There are 3 gas molecules on the left and 2 on the right, so a higher pressure moves it to the right, towards the fewer molecules. It isn't used because at 2 atm the yield is already high.
• “450 °C is used because it gives the highest yield.” — The forward reaction is exothermic, so a lower temperature would give a higher yield. 450 °C is used because the reaction is fast enough there.

7. XT3 · Apply

Model answer. (a) 1 + 1 = 2 gas molecules on the left and 1 on the right, so a high pressure moves the position to the right: more ethanol. (b) The forward reaction is exothermic, so a lower temperature would give a higher yield, but the rate would be too slow; 300 °C is a compromise. (c) The reaction is reversible, so only some ethene turns into ethanol each time; passing the rest through again means none is wasted.

A full answer includes

• (a) 2 gas molecules on the left, 1 on the right: higher pressure moves the position to the right.
• (b) A lower temperature would give a higher yield but too slow a rate.
• (c) Only some ethene reacts each pass; recycling avoids wasting it (saves money).

Also accept

• Fewer gas molecules on the right; yield against rate; not wasted

Watch for

• “300 °C is used because it gives the highest yield.” — A lower temperature gives a higher yield, because the forward reaction is exothermic. 300 °C is a compromise: a lower yield, but made fast enough.