AQA GCSE Combined Science (8464), Higher tier · Chemistry › 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.
Some reactions can go both ways: the products can react to re-form the original reactants. You need to recognise and use the ⇌ symbol, describe what happens when ammonium chloride is heated and cooled, and know that changing the conditions can change the direction of a reversible reaction.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Recognise the reversible reaction symbol ⇌The ⇌ symbol shows that a reaction can go in both directions.
4
Describe what a reversible reaction isThe products of the reaction can react to produce the original reactants.
5
Describe heating and cooling ammonium chlorideHeating decomposes it into ammonia and hydrogen chloride; cooling makes these gases re-form the white solid.
5
Write equations for reversible reactionsUse ⇌ in word and balanced symbol equations, e.g. NH4Cl ⇌ NH3 + HCl.
6
Explain how conditions change the directionChanging a condition such as temperature can make the forward or the reverse reaction happen.
6
Use observations as evidence of reversibilityE.g. a white solid re-forming at the cool top of a heated tube shows the reverse reaction happening.
Notes
What is a reversible reaction?
In a reversible reaction, the products of the reaction can react to produce the original reactants.
The symbol ⇌ shows this: A + B ⇌ C + D.
The forward reaction goes from left to right (A + B → C + D). The reverse reaction goes from right to left (C + D → A + B).
Reactions that cannot easily be reversed, such as burning a fuel, are shown with a single arrow →.
The direction of a reversible reaction can be changed by changing the conditions, for example the temperature.
Example: ammonium chloride
ammonium chloride ⇌ ammonia + hydrogen chloride
NH4Cl(s) ⇌ NH3(g) + HCl(g)
Heating: white solid ammonium chloride decomposes into two colourless gases, ammonia and hydrogen chloride (the forward reaction).
Cooling: the two gases react to form white solid ammonium chloride again (the reverse reaction).
In the lab: heat a little ammonium chloride at the bottom of a test tube. The solid at the bottom disappears, and a white solid forms again near the cooler top of the tube.
Cheatsheet
⇌ = reversible reaction
Reversible reaction: the products can react to re-form the original reactants
Forward reaction: left to right; reverse reaction: right to left
NH4Cl(s) ⇌ NH3(g) + HCl(g)
Heat ammonium chloride → ammonia + hydrogen chloride; cool the gases → ammonium chloride again
Changing the conditions can change the direction of a reversible reaction
How to answer each type of question
Recall: the ⇌ symbol and the ammonium chloride example
1 to 3 marks3
Say what ⇌ means in plain words: the reaction can go both ways.
Learn the names of both products of decomposing ammonium chloride.
Example. (a) What does the symbol ⇌ show about a reaction? (b) Complete the word equation for the reversible reaction that happens when ammonium chloride is heated. ammonium chloride ⇌ ________ + ________
Show the model answer
(a) The reaction is reversible / can go in both directions (1) (b) ammonia (1) + hydrogen chloride (1)
Explain observations in a reversible reaction
3 marks5
Describe what happens at each stage (colour, state).
Say which reaction happens: forward on heating, reverse on cooling.
Name the substances formed.
Example. A student heated a small amount of white ammonium chloride at the bottom of a test tube. The solid at the bottom disappeared and a white solid appeared near the top of the tube. Explain these observations.
Show the model answer
Heating decomposes the ammonium chloride into ammonia and hydrogen chloride (1). These are gases, so the solid at the bottom disappears (1). Near the cool top of the tube the gases react to form solid ammonium chloride again, which is the reverse reaction (1).
Write a balanced equation for a reversible reaction
2 marks5
Write the correct formula for each substance.
Balance the equation.
Use ⇌ instead of →, and add state symbols if the question asks for them.
Example. Hydrogen gas reacts with iodine gas to form hydrogen iodide gas, HI, in a reversible reaction. Write a balanced symbol equation for this reaction.
Show the model answer
H2 + I2 ⇌ 2HI correct formulae with the ⇌ symbol (1); correctly balanced (1)
Shortcuts and memory tricks
Two half-arrows = two directions: the top one points forward, the bottom one points back.
Ammonium chloride: heat splits it apart, cooling puts it back together.
If a question mentions 'the reverse reaction', read the equation from right to left.
Where marks are lost
Using → instead of ⇌ in the equation for a reversible reaction.
Saying ammonium chloride melts or evaporates when heated. It decomposes into two different gases.
Writing 'ammonium' or 'chlorine' instead of ammonia and hydrogen chloride.
Mixing up which change is the forward reaction and which is the reverse: forward is left to right as the equation is written.
Exam technique
Learn the ammonium chloride example fully: the names, the states and which way heating and cooling push the reaction.
When describing a reversible reaction, use the terms 'forward reaction' and 'reverse reaction'.
In observation questions, describe what you would see (white solid disappears, white solid reappears), not only the chemistry.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
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