Diffusion Worked ExamplesIGCSE Chemistry 0620

(a) The process is diffusion. (The crystal also dissolves first.)

(b) The purple particles and the water particles are all moving randomly. They collide with each other and gradually mix. There is a net movement of purple particles from where their concentration is high (near the crystal) to where it is low (the rest of the water).

(c) Diffusion continues until the purple particles are evenly spread, so the whole beaker becomes an even, paler purple colour.

Gas: the particles are far apart and move quickly in random directions, so they spread out quickly — diffusion is fastest.

Liquid: the particles are close together and move more slowly, sliding past each other. A diffusing particle collides with neighbouring particles very often, so it spreads out more slowly.

Solid: the particles only vibrate about fixed positions; they do not move from place to place, so diffusion is extremely slow.

At the same temperature, the gas with the highest relative molecular mass diffuses most slowly, so it is detected last. Work out each $M_r$ using $A_r$: C $= 12$, H $= 1$, N $= 14$, O $= 16$, S $= 32$, Cl $= 35.5$.

Calculate the relative molecular masses.

Chlorine has the largest $M_r$ ($71$), so its molecules move most slowly on average. Answer: C. D is tempting because sulfur dioxide is also heavy, but $64 < 71$. A is the gas detected first.

(a) Use $A_r$: H $= 1$, C $= 12$, O $= 16$, Cl $= 35.5$.

(b) The lower the $M_r$, the faster the diffusion: hydrogen ($2$) > methane ($16$) > carbon dioxide = propane ($44$) > chlorine ($71$).

(c) Carbon dioxide and propane both have $M_r = 44$, so at the same temperature their molecules move at the same average speed and they diffuse at the same rate — even though the molecules contain different atoms.

(d) At the same temperature, molecules with a lower relative molecular mass (lighter molecules) move faster on average, so they spread out more quickly. Heavier molecules move more slowly and diffuse more slowly.

Compare the relative molecular masses: ammonia $\text{NH}_3 = 14 + 3 = 17$; sulfur dioxide $\text{SO}_2 = 32 + 32 = 64$.

Sulfur dioxide has the higher $M_r$, so its molecules move more slowly and it takes longer than $25$ s to reach the paper. That rules out A and B.

Sulfur dioxide dissolves in the water on the damp paper to form an acidic solution, which turns blue litmus red. Answer: C. D has the right time but the paper is already blue — an acidic gas cannot turn it blue.

(a) The brown colour slowly moves upwards into the top jar. After some time both jars are the same, paler brown colour.

(b) Nitrogen dioxide molecules and air molecules move randomly and mix by diffusion. There is a net movement of nitrogen dioxide from high concentration (bottom) to low concentration (top) until it is evenly spread. The colour is paler because the same number of brown molecules now fills twice the volume.

(c) In experiment 2 the brown colour fills the top jar almost instantly (much faster). With no air molecules in the way, the nitrogen dioxide molecules do not collide with other particles, so they travel straight into the empty space.

(d) Nitrogen dioxide ($M_r = 14 + 32 = 46$) is denser than air, so it would sink if it were on top. Placing it at the bottom shows that the gas spreads upwards by diffusion, not because it falls.

(a) Ammonia must dissolve in water to form an alkaline solution before it can change the colour of the litmus.

(b) By diffusion: the ammonia molecules and the air molecules move randomly and collide, giving a net movement of ammonia from high concentration (near the cotton wool) to low concentration (further along the tube).

(c) The times would be shorter. At a higher temperature the molecules have more kinetic energy and move faster, so they diffuse more quickly.

(d) The times would be longer. Hydrogen chloride ($M_r = 1 + 35.5 = 36.5$) has a higher relative molecular mass than ammonia ($M_r = 14 + 3 = 17$), so its molecules move more slowly at the same temperature.

(e) To stop the ammonia escaping (it is toxic and has a strong smell) and to stop draughts moving the gas, so that the gas moves along the tube only by diffusion.

(a) Relative masses: helium, $A_r = 4$; air is mainly nitrogen, $\text{N}_2$ ($M_r = 28$), and oxygen, $\text{O}_2$ ($M_r = 32$).

Helium atoms have a much lower relative mass, so at the same temperature they move faster. They reach and pass through the tiny gaps in the rubber more often, so helium diffuses out faster and the balloon shrinks more quickly.

(b) Carbon dioxide, $\text{CO}_2$, has $M_r = 12 + 32 = 44$, which is higher than both nitrogen and oxygen.

Its molecules move more slowly, so it diffuses out more slowly: the carbon dioxide balloon shrinks more slowly than the air balloon.

(a) As the temperature increases, the time taken decreases (the curve gets less steep at higher temperatures). So the rate of diffusion increases with temperature.

(b) Go up from $45^{\circ}\text{C}$ to the curve and across to the time axis: about $175$ s (accept $170$–$180$ s).

(c) At a higher temperature the particles (the ions in the solution) have more kinetic energy and move faster, so they spread through the jelly more quickly and the colour reaches $5$ mm in less time.

(d) Any two: the volume of copper(II) sulfate solution; its concentration; the size of the hole; the same type / depth of agar jelly; the distance (5 mm) used to judge the end point.

(a) Hydrogen ($M_r = 2$) is much lighter than the air inside the pot (mainly $\text{N}_2$, $28$, and $\text{O}_2$, $32$). Hydrogen molecules therefore diffuse into the pot faster than air molecules diffuse out.

More gas particles build up inside the pot, so the pressure inside increases. This pushes the water down in arm X (and up in arm Y).

(b) Now the pot is full of hydrogen and surrounded by air. Hydrogen diffuses out faster than air diffuses in, so the pressure inside falls and the water level in X rises above its starting level (for a time).

(c) Carbon dioxide ($M_r = 44$) is heavier than the air molecules. Air diffuses out of the pot faster than carbon dioxide diffuses in, so the pressure inside falls and the water level in X rises.

(a) Ammonia, $\text{NH}_3$, has $M_r = 17$; hydrogen chloride, $\text{HCl}$, has $M_r = 36.5$. The lighter ammonia molecules move faster, so ammonia travels further ($59$ cm) than hydrogen chloride ($41$ cm) before they meet. The ring forms nearer the hydrogen chloride end.

(b) In experiment 2 the ring is only $31$ cm from the gas X end — even closer to that end than in experiment 1. So gas X travels a shorter distance than hydrogen chloride did, meaning X diffuses more slowly than hydrogen chloride and has a higher $M_r$.

Compare the relative molecular masses of the three possible gases.

Only hydrogen bromide has $M_r$ greater than $36.5$, so X is hydrogen bromide.

(c) Ammonia reacts with hydrogen bromide to form the white solid ammonium bromide. Check: N 1, H 4, Br 1 on each side.

Method: Place a strip of damp red litmus paper inside a long glass tube, a fixed distance (for example $30$ cm) from one end.

Stand the tube in a water bath (or trough of water) at a measured temperature and leave it to reach that temperature, checked with a thermometer.

Soak a piece of cotton wool in a fixed volume (for example $1\text{ cm}^3$, measured with a pipette or syringe) of the ammonia solution, push it into the end of the tube, close the end with a bung and start a stopwatch.

Measure the time taken for the litmus paper to turn blue. Repeat at several different temperatures (for example $20$, $30$, $40$, $50^{\circ}\text{C}$), using a fresh tube and litmus paper each time.

Keep the same: distance to the paper, volume and concentration of ammonia solution, size of tube. Conclusion: the shorter the time, the faster the diffusion — compare the times at each temperature.

Safety: concentrated ammonia gives off a toxic, irritating gas, so work in a fume cupboard.