Lighter, faster — answer key

1. SQ6 · Practice

Model answer. Ammonia has the lower M_r (17). Its rate of diffusion is higher: at the same temperature it spreads faster than hydrogen chloride. So in the same time the ammonia spreads further along the tube, and the white ring forms nearer the hydrogen chloride end.

A full answer includes

• Lower M_r: ammonia (17, against 36.5 for hydrogen chloride).
• Rate of diffusion: higher; ammonia diffuses faster.
• The ring forms nearer the hydrogen chloride end, because in the same time the ammonia spreads further along the tube.

Also accept

• ammonia; higher (it diffuses faster); hydrogen chloride
• NH₃; faster; the HCl end

Watch for

• “The ring forms nearer the ammonia end, because ammonia is faster and gets there first.” — The ring forms where the two gases meet. The faster gas covers more of the tube, so the meeting point is further from its own end: nearer the slower gas's end.

2. SQ7 · Practice

Options as numbered on the pupil sheet: 1 — The two gases meet nearer the hydrogen chloride end.; 2 — Ammonia particles are lighter (relative molecular mass 17, against 36.5), so they move faster on average.; 3 — There they react and make a white ring of ammonium chloride.; 4 — Both gases are at the same temperature, so their particles have the same average kinetic energy.; 5 — Both gases zigzag through the air, but in the same time the ammonia spreads further along the tube..

Model answer. Order: same temperature, so the same average kinetic energy; lighter ammonia particles move faster on average; in the same time the ammonia spreads further; the gases meet nearer the hydrogen chloride end; there they make the white ring. At the same temperature, the particles of both gases have the same average kinetic energy. Kinetic energy depends on mass as well as speed, so the lighter ammonia particles have that energy at a higher speed: they move faster on average. Both gases zigzag through the air, colliding all the time, but in the same time the ammonia spreads further. So the gases meet, and react to make the white ring of ammonium chloride, nearer the hydrogen chloride end.

A full answer includes

• Order: both gases are at the same temperature, so their particles have the same average kinetic energy; ammonia particles are lighter, so they move faster on average; in the same time the ammonia spreads further along the tube; the gases meet nearer the hydrogen chloride end; there they react and make the white ring of ammonium chloride.
• I explained that kinetic energy depends on mass as well as speed, so a lighter particle has the same kinetic energy at a higher speed.

Also accept

• Same average kinetic energy, lighter so faster, further in the same time, meet nearer the hydrogen chloride end, ring forms there; a lighter particle has the same kinetic energy at a higher speed.

Watch for

• “Lighter particles have more energy, so they move faster.” — At the same temperature, both gases have the same average kinetic energy. The lighter particles move faster because they have that energy at a higher speed.

3. SQ8 · Practice

Options as numbered on the pupil sheet: 1 — Nearer the left end; 2 — In the middle; 3 — Nearer the right end.

Model answer. Tube 1: nearer the left end. Ammonia (M_r 17) has a much lower M_r than hydrogen iodide (128), so it spreads further in the same time and the gases meet nearer the hydrogen iodide end. Tube 2: nearer the right end, because methylamine (31) diffuses faster than hydrogen bromide (81). Tube 3: in the middle, because gases with equal M_r diffuse at the same rate. Tube 4: nearer the right end. Hydrogen chloride (36.5) is lighter than trimethylamine (59), so at the same temperature its particles move faster on average, it spreads further in the same time, and the gases meet nearer the trimethylamine end. The ring forms nearer the heavier gas's end, whichever gas is the acid.

A full answer includes

• Tube 1: nearer the left end. Hydrogen iodide (128) is much heavier than ammonia (17), so it diffuses more slowly, and the ring forms nearer its end.
• Tube 2: nearer the right end, where the hydrogen bromide (81) started, because methylamine (31) diffuses faster.
• Tube 3: in the middle. The two gases have equal M_r, so they diffuse at the same rate.
• Tube 4: nearer the right end, where the trimethylamine (59) started. This time the acid gas, hydrogen chloride (36.5), is the lighter, faster one.
• My explanation for tube 4 said that hydrogen chloride has the lower M_r, so at the same temperature its particles move faster on average and it spreads further in the same time; the ring forms nearer the end of the slower, heavier gas, whichever gas is the acid.

Also accept

• Tube 1 nearer the left end; tube 2 nearer the right end; tube 3 in the middle; tube 4 nearer the right end.
• Tube 1 nearer the hydrogen iodide end; tube 2 nearer the hydrogen bromide end; tube 3 in the middle; tube 4 nearer the trimethylamine end.

Watch for

• “Tube 4: nearer the hydrogen chloride end, because the ring forms near the acid.” — Only when the acid gas is the heavier one. Trimethylamine (59) is heavier than hydrogen chloride (36.5), so it is slower, and the ring forms nearer its end.
• “Tube 1: in the middle, because both gases have the same time to spread.” — They have the same time, but not the same speed. In the same time the lighter ammonia covers more of the tube, so the meeting point is nearer the hydrogen iodide end.

4. SQ9 · Practice

Model answer. (a) The pupil's idea doesn't fit the evidence. If hydrogen chloride got further, the ring would form nearer the ammonia end, but it forms nearer the hydrogen chloride end. So in the same time the hydrogen chloride spread less far. (b) Both gases zigzag through the air, changing direction at every collision, heavy or light. What matters most is speed: at the same temperature, the heavier hydrogen chloride particles (M_r 36.5) move more slowly on average than the lighter ammonia particles (M_r 17). Hydrogen chloride has the lower rate of diffusion, and the ammonia spreads further before they meet.

A full answer includes

• (a) I said the ring forms nearer the hydrogen chloride end, so in the same time the hydrogen chloride got less far than the ammonia: the pupil's idea predicts the opposite of what happens.
• (b) I said that both gases zigzag, changing direction at every collision, and that at the same temperature heavier particles move more slowly on average, so hydrogen chloride diffuses more slowly.
• (b) I said that the lighter ammonia particles move faster on average, so ammonia spreads further in the same time.

Also accept

• (a) Wrong: the ring is nearer the hydrogen chloride end, so hydrogen chloride got less far. (b) Every particle keeps colliding and changing direction; heavier particles move more slowly on average at the same temperature, so hydrogen chloride diffuses more slowly than ammonia.
• (a) The ring position shows the opposite of the pupil's claim. (b) Lighter ammonia particles are faster on average, so ammonia covers more of the tube.

Watch for

• “Heavier particles barely change direction, so they travel in straighter lines.” — Every gas particle still changes direction at collisions and zigzags. A heavier particle may be knocked off course a little less, but the ring shows that the speed difference wins: heavier particles move more slowly on average, so they get less far.

5. SQ10 · Practice

Model answer. Three statements are correct: chlorine molecules move more slowly on average than air molecules; each one changes direction at every collision; and bromine would take longer to spread evenly. At the same temperature, chlorine molecules (M_r 71) move more slowly on average than nitrogen (28) and oxygen (32) molecules, and bromine molecules (160) more slowly still. The statement that none can spread up is wrong: the molecules move at random, upwards as well as downwards, and there are more chlorine molecules at the bottom, so there is a net movement upwards. The statement that they stop moving is wrong: the molecules keep moving; only the net movement stops. The statement about a draught is wrong: diffusion needs no draught; it happens in perfectly still air.

A full answer includes

• I chose: chlorine molecules are heavier than the nitrogen and oxygen molecules in air, so at the same temperature they move more slowly on average; each chlorine molecule changes direction every time it collides; bromine, heavier still, would take longer to spread evenly.
• I corrected one rejected statement: chlorine does spread up into the top jar, because its molecules move at random, up as well as down; the molecules keep moving once the colour is even; diffusion needs no draught or moving air.

Also accept

• Heavier than air molecules so slower on average; changes direction at every collision; bromine would take longer.
• Those three, with the reason that chlorine still spreads up: random motion takes molecules upwards as well as downwards.

Watch for

• “A gas that is denser than air stays at the bottom.” — It spreads upwards slowly, but it does spread, because its molecules move at random in every direction and there are more of them at the bottom.
• “Heavier molecules diffuse faster, because they have more energy.” — At the same temperature they have the same average kinetic energy, so heavier molecules move more slowly on average.

6. ST2 · Apply

Model answer. (a) Slightly nearer the left end, where the ethylamine started. (b) Hydrogen chloride has the lower M_r (36.5, against 45 for ethylamine). At the same temperature its particles move faster on average, so it diffuses faster and spreads further along the tube in the same time. The gases meet, and form the ring, nearer the ethylamine end. The two M_r values are fairly close, so the ring is only a little way from the middle. (c) Slightly nearer the right end, now the ethylamine end. The ring forms nearer the heavier gas's end, whichever side it starts from and whichever gas is the acid.

A full answer includes

• (a) Nearer the left end, where the ethylamine started (only a little way from the middle).
• (b) Hydrogen chloride has the lower M_r (36.5, against 45), so at the same temperature its particles move faster on average; it diffuses faster and spreads further in the same time, so the gases meet nearer the ethylamine end.
• (c) Nearer the right end: the ring forms nearer the ethylamine end, wherever the ethylamine starts, because it is the heavier, slower gas.

Also accept

• (a) Nearer the ethylamine end (left), only slightly. (b) Hydrogen chloride is lighter (36.5 against 45), so its particles are faster on average and it travels further in the same time. (c) Nearer the right end: still the ethylamine end.
• (a) “Nearer the left end” or “a little left of the middle” is fine; “slightly” is not required.

Watch for

• “The ring forms nearer the acid end, as it did with ammonia.” — That happened because hydrogen chloride was the heavier gas there. Here ethylamine is heavier, so the ring forms nearer its end.
• “The ethylamine moves towards the acid to react with it.” — No particle aims anywhere. Each gas spreads at random from where it started; the faster one simply covers more of the tube.