AQA GCSE Chemistry (8462), Higher tier · The rate and extent of chemical change › Reversible reactions and dynamic equilibrium
Practise Reversible reactions. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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When ammonium chloride is heated in a test tube, it decomposes into ammonia and hydrogen chloride. A white solid forms near the cool top of the test tube. ammonium chloride ⇌ ammonia + hydrogen chloride What does the symbol ⇌ show about this reaction?
The reaction is reversible.
Write a balanced symbol equation, with state symbols, for the reversible reaction.
NH4Cl(s) ⇌ NH3(g) + HCl(g)
Complete the word equation for the reversible reaction. anhydrous cobalt chloride (blue) + water ⇌ ______________
hydrated cobalt chloride (pink)
What does the symbol ⇌ mean when it is used in an equation?
The reaction can go in both directions (it is reversible).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
When ammonium chloride is heated in a test tube, it decomposes into ammonia and hydrogen chloride. A white solid forms near the cool top of the test tube. ammonium chloride ⇌ ammonia + hydrogen chloride
(a) What does the symbol ⇌ show about this reaction?[1]
(b) Name the white solid that forms near the top of the test tube.[1]
(c) Explain why the white solid forms near the top of the test tube.[2]
(d) How can the direction of a reversible reaction be changed?[1]
Show the answer and mark scheme
(a)Answer: The reaction is reversible.
the reaction is reversible
(b)Answer: Ammonium chloride
ammonium chloride
(c)Answer: The top of the tube is cooler, so ammonia and hydrogen chloride react to re-form ammonium chloride.
the top of the tube is cooler
so the reverse reaction happens / ammonia and hydrogen chloride react to form ammonium chloride
(d)Answer: By changing the conditions, such as the temperature.
by changing the conditions (e.g. the temperature)
Question 2Medium6 marks
A student heated solid ammonium chloride, NH4Cl, in a test tube. The solid disappeared from the bottom of the tube and a white solid formed near the top of the tube.
(a) Write a balanced symbol equation, with state symbols, for the reversible reaction.[2]
(b) A student says: 'The ammonium chloride has just turned into a gas and then back into a solid, so this is a physical change.' Explain why the student is wrong.[2]
(c) The student held a piece of damp red litmus paper at the mouth of the tube. The litmus paper turned blue. Name the gas that turned the litmus paper blue.[1]
(d) Suggest why the ammonium chloride must be heated for the forward reaction to happen.[1]
Show the answer and mark scheme
(a)Answer: NH4Cl(s) ⇌ NH3(g) + HCl(g)
NH4Cl ⇌ NH3 + HCl
state symbols: (s) on the left and (g), (g) on the right
(b)Answer: New substances (ammonia and hydrogen chloride) are formed, so it is a chemical change.
new substances (ammonia and hydrogen chloride) are formed
so it is a chemical change / the solid decomposes rather than just changing state
(c)Answer: Ammonia
ammonia
(d)Answer: The forward reaction is endothermic, so it needs energy.
the forward reaction is endothermic / needs energy to break bonds
Question 3Hard6 marks
When calcium carbonate is heated strongly it decomposes. The reaction is reversible. CaCO3(s) ⇌ CaO(s) + CO2(g)
(a) A student heated 25.0 g of calcium carbonate strongly in an open crucible until the mass stopped changing. Explain why all of the calcium carbonate could decompose, even though the reaction is reversible.[2]
(b) Calculate the mass of calcium oxide left in the crucible. Relative formula masses (Mr): CaCO3 = 100 CaO = 56[2]
(c) The student left the calcium oxide in an open dish for several weeks. The mass of the solid increased. Suggest why.[2]
Show the answer and mark scheme
(a)Answer: The carbon dioxide escapes from the open crucible, so the reverse reaction cannot happen.
the carbon dioxide escapes (the system is not closed)
so the reverse reaction cannot happen / there is no carbon dioxide to react with the calcium oxide
(b)Answer: 14.0 g
25.0 ÷ 100 = 0.250 (mol)
0.250 × 56 = 14.0 (g)
(c)Answer: Calcium oxide reacts with carbon dioxide (and water vapour) from the air, so the mass of solid increases.
calcium oxide reacts with carbon dioxide from the air (the reverse reaction) / with water vapour from the air
so substances from the air are added to the solid / calcium carbonate (or calcium hydroxide) forms