(a) Different gases diffuse at different speeds.
(i) What is meant by the term diffusion? [1]
(ii) What property of a gas molecule affects the speed at which it diffuses? [1]
(b) Helium is a gas used to fill balloons. It is present in the air in very small quantities. Diffusion can be used to separate it from the air.
(i) Air at 1000 °C is on one side of a porous barrier. The air which passes through the barrier has a larger amount of helium in it. Why does the air on the other side of the barrier contain more helium? [1]
(ii) Why is it an advantage to have the air at a high temperature? [1]
(c) Most helium is obtained from natural gas found in the USA. Natural gas contains methane and 7% helium. One possible way to obtain the helium would be to burn the methane.
(i) Write an equation for the complete combustion of methane. [1]
(ii) Suggest why this would not be a suitable method to obtain the helium. [1]
(iii) Suggest another method, other than diffusion, by which helium could be separated from the mixture of gases in natural gas. [1] [Total: 7]
▶️ Answer/Explanation
(a) (i) Diffusion is the movement of particles from an area of high concentration to low concentration until evenly distributed. (1)
(a) (ii) The mass or molecular weight (\( M_r \)) of the gas molecule affects its diffusion speed. (1)
(b) (i) Helium diffuses faster through the barrier because it is lighter than nitrogen (\( N_2 \)) and oxygen (\( O_2 \)) molecules in air. (1)
(b) (ii) At higher temperatures, gas molecules move faster, increasing the rate of diffusion. (1)
(c) (i) \( CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O \) (1)
(c) (ii) Burning methane produces carbon dioxide, which mixes with helium, making separation difficult. (1)
(c) (iii) Fractional distillation can separate helium from natural gas by exploiting differences in boiling points. (1)
A list of techniques used to separate mixtures is given below.
- filtration
- diffusion
- fractional distillation
- simple distillation
- crystallisation
- chromatography
From this list, choose the most suitable technique to separate the following mixtures.
A technique may be used once, more than once or not at all.
(a) butane from a mixture of propane and butane
(b) oxygen from liquid air
(c) water from aqueous magnesium sulfate
(d) potassium chloride from aqueous potassium chloride
(e) silver chloride from a mixture of silver chloride and water
(f) glucose from a mixture of glucose and maltose
▶️ Answer/Explanation
(a) Ans: diffusion or fractional distillation
Butane and propane are gases with different molecular masses, so diffusion can separate them based on their rates. Fractional distillation is also suitable due to their different boiling points.
(b) Ans: fractional distillation
Liquid air contains oxygen and nitrogen, which have different boiling points. Fractional distillation is ideal for separating components of air.
(c) Ans: simple distillation
Water can be evaporated from aqueous magnesium sulfate, leaving the solid behind. Simple distillation collects the pure water vapor.
(d) Ans: crystallisation
Potassium chloride is a soluble solid. Crystallization involves evaporating the water to form solid crystals of potassium chloride.
(e) Ans: filtration
Silver chloride is insoluble in water, so filtration separates the solid silver chloride from the liquid water.
(f) Ans: chromatography
Glucose and maltose are sugars with different solubilities. Chromatography separates them based on their movement through a medium.