Question
(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.
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.
(i) 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
Ans:
3 (a) (i) (particles) spread to fill total available volume/ move from high concentration to low concentration/moves down a concentration gradient (1)
(ii) mass or Mr (1)
(b) (i) helium atoms / molecules are lighter than molecules in air or N2 and O2
or helium is less dense than air or N2 and O2.
or helium diffuses (through the porous barrier) faster than air or N2 and O2. (1)
(ii) faster rate of diffusion / molecules move faster (at high temperatures). (1) [1]
(c) (i) CH4 + 2O2 → CO2 + 2H2O (1)
(ii) would get a mixture of helium and carbon dioxide
or would get a mixture of gases
or waste of methane/natural gas / fossil fuel (1)
(iii) fractional distillation (1)
Question
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
Answer:
(a) diffusion or fractional distillation;
(b) fractional distillation;
(c) simple distillation;
(d) crystallisation;
(e) filtration;
(f) chromatography;