CCEA GCSE Chemistry
CCEA GCSE Chemistry 2.9 Gas Chemistry Exam Questions
Free CCEA GCSE Chemistry revision for 2.9 Gas Chemistry: what you need to know, common mistakes and exam-style questions with answers. Covers composition of air, nitrogen and ammonia, and the preparation, collection and tests of hydrogen, oxygen and carbon dioxide.
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About CCEA GCSE Chemistry 2.9 Gas Chemistry
Gas Chemistry is the final topic of Unit 2 in CCEA GCSE Chemistry. You need to know the composition of clean dry air and the properties and uses of nitrogen and ammonia, including why ammonia is so soluble. The prescribed practical covers preparing, collecting and testing hydrogen, oxygen and carbon dioxide and investigating their properties and reactions. You must choose and justify collection methods based on solubility and density. The topic draws on tests from chemical analysis, acids, rates (catalysts) and the Haber process. Examiners often ask for preparation equations, labelled apparatus, collection methods and the reactions of elements burning in oxygen.
What you need to know
- Composition of air Clean dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and about 0.04% carbon dioxide, with traces of other noble gases.
- Nitrogen and ammonia Nitrogen is very unreactive because of its strong triple bond, so it is used in food packaging. Ammonia is a colourless, pungent gas, less dense than air and very soluble in water (shown by the fountain experiment), forming an alkaline solution. It turns damp red litmus blue and forms white smoke of ammonium chloride with hydrogen chloride.
- Hydrogen Prepared from zinc and dilute hydrochloric acid: Zn + 2HCl → ZnCl2 + H2. Collected over water because it is insoluble. Test: a lit splint gives a squeaky pop. It burns to form water and is less dense than air.
- Oxygen Prepared by decomposing hydrogen peroxide with a manganese(IV) oxide catalyst: 2H2O2 → 2H2O + O2. Collected over water. Test: relights a glowing splint. Elements burn more brightly in oxygen to form oxides; magnesium gives a bright white flame and a white solid, and sulfur burns with a blue flame.
- Carbon dioxide Prepared from calcium carbonate and dilute hydrochloric acid: CaCO3 + 2HCl → CaCl2 + H2O + CO2. Collected by downward delivery as it is denser than air, or over water. Test: turns limewater milky. It does not support combustion and dissolves in water to form a weakly acidic solution.
- Collection methods Collection over water suits insoluble or slightly soluble gases. Upward delivery suits gases less dense than air; downward delivery suits gases denser than air. A gas syringe measures volume accurately.
Common mistakes in 2.9 Gas Chemistry
- Collecting ammonia over water. It is very soluble, so it is collected by upward delivery.
- Describing the oxygen test as relighting a lit splint. It relights a glowing splint.
- Calling manganese(IV) oxide a reactant. It is a catalyst and is not used up.
- Writing that limewater turns cloudy white for hydrogen. Limewater is the test for carbon dioxide only.
- Saying nitrogen is unreactive because it is a noble gas. It is unreactive because of the strong N≡N triple bond.
Try three 2.9 Gas Chemistry exam-style questions
1. Describe the test for oxygen and give the positive result. (2 marks)
Show the mark scheme answer
Insert a glowing splint; it relights.
2. Give two properties of ammonia shown by the fountain experiment. (2 marks)
Show the mark scheme answer
Ammonia is very soluble in water; the solution is alkaline (indicator changes colour, for example red litmus solution turns blue).
3. Calculate the volume of oxygen, at room temperature and pressure, produced when 3.4 g of hydrogen peroxide decomposes: 2H2O2 → 2H2O + O2. (Mr H2O2 = 34; molar gas volume 24 dm3) (3 marks)
Show the mark scheme answer
Moles H2O2 = 3.4 / 34 = 0.10; moles O2 = 0.050; volume = 0.050 x 24 = 1.2 dm3.
2.9 Gas Chemistry FAQs
What does CCEA GCSE Chemistry 2.9 Gas Chemistry cover?
It covers the composition of air, the properties and uses of nitrogen and ammonia, and the preparation, collection, properties and tests of hydrogen, oxygen and carbon dioxide.
Which unit is 2.9 Gas Chemistry examined in?
It is examined in the Unit 2 written paper, and the gas preparation practical can also be assessed in Unit 3 Practical Skills.
What should I learn for each gas?
For each gas learn the reactants and equation for its preparation, how it is collected and why, its test and result, and its key physical and chemical properties.
