Reversible Reactions and Dynamic Equilibrium

1. Give the symbol for a reversible reaction.

Answer: The symbol is "⇋".


2. If the forward reaction of a reversible reaction is exothermic, what is the energy change in the backwards reaction?

Answer: endothermic


3. Describe what is meant by "dynamic equilibrium".

Answer: When a reversible reaction happens in a closed container and the forward and reverse reactions happen at the same rate.


4. Name the principle that can be used to predict the effect of changing conditions on the equilibrium position of a reaction.

Answer: Le Chatelier’s Principle


5. A reversible reaction has an exothermic forward reaction. Describe what happens to the yield of the products when the temperature is increased.

Answer: Yield decreases.


6. Describe what happens to the position of equilibrium when the pressure is reduced for an equilibrium system involving gases.

Answer: Moves to the side with the lowest number of moles of gas (lowest pressure).


7. Ammonium chloride can thermally decompose to make hydrogen chloride gas and ammonia. This is a reversible reaction.

a) Write the word equation for this reaction.

Answer: ammonium chloride  ⇋  hydrogen chloride + ammonia


b) Write down whether the forward reaction is exothermic or endothermic.

Answer: endothermic


c) Define the term "reversible reaction".

Answer: A chemical reaction where the products of the reaction can react to produce the original reactants.


8. Hydrated copper(II) sulfate can undergo a reversible reaction. The following word equation summarises this reaction: hydrated copper(II)  ⇋  sulfate anhydrous copper(II) sulfate + water

a) Write down the colour change you would observe in this reaction.

Answer: From blue to white.


b) Describe the type of bonding in anhydrous copper(II) sulfate. Write your answer in terms of forces.

Answer: Ionic bonding, because of electrostatic forces of attraction between oppositely charged ions.


c) Suggest how anhydrous copper(II) sulfate could be used as a test for water. Describe any changes you would observe.

Answer: White anhydrous copper sulfate could be put into a liquid. If there is a colour change from white to blue, this shows that water is present.


9. Hydrogen chloride gas (HCl) and ammonia gas (NH3 ) react in a reversible reaction.

a) Write a balanced symbol equation for this reaction.

Answer: HCl + NH3  ⇋  NH4Cl


b) The forward reaction has an energy change of -176 kJ/mol. Give the energy change for the reverse reaction.

Answer: +176 kJ/mol


c) Predict what would happen to the amount of reactants if the reaction was carried out in a sealed container and the temperature was increased.

Answer: The system would oppose the change. It would try to reduce the temperature, favouring the backwards reaction and increasing the amount of reactants.


10. In a chemical reaction, reactants A and B undergo a reversible reaction to make products C and D.

a) Write an equation to describe this reaction.

Answer: A + B  ⇋  C + D


b) Sketch a graph to show the change in the concentration of the chemicals from the start of the reaction to when the system reaches dynamic equilibrium.

Answer:   OR 


11. The Haber process is an important method for manufacturing fertiliser. Nitrogen from the air is reacted with hydrogen to make ammonia.

a) Write the symbol equation by balancing.

Answer: N2 + 3H2  ⇋  2NH3


b) The forward reaction is exothermic. When the system is at equilibrium, suggest the effect of increasing the temperature on the yield of ammonia.

Answer: The system will oppose the change and try to reduce the temperature. It will favour the endothermic (backwards reaction) and reduce the yield of ammonia.


c) Name the catalyst used in this reaction. Explain the effect of the catalyst on the position of equilibrium.

Answer: Iron (Fe). No effect on position of equilibrium or yield of ammonia, as the rate of both the forwards and backwards reactions are increased by the same amount.


12. The Haber process makes gaseous ammonia. The figure below shows how the yield changes when the temperature and pressure are changed.


a) One atmosphere pressure is 101000 Pa. Convert 200 atmospheres into Pa. Give your answer in standard form.

Answer: 101000 × 200 = 20200000 Pa = 2.02 × 107 Pa


b) Give the conclusions that can be drawn from the graphs in the figure above.

Answer: As temperature increases, percentage of ammonia decreases (or inverse). As pressure increases, percentage of ammonia increases (or inverse).


c) Use the figure above and your own knowledge to justify the use of a temperature of about 450°C and a pressure of about 200 atmospheres in the industrial Haber process.

Answer: High temperatures favour the endothermic reaction and would reduce the yield of ammonia. So, for the highest yield, low temperatures should be used. However, low temperatures cause a slow rate of reaction. So, a compromise temperature is used so that an economical amount of ammonia is made in a reasonable amount of time. High pressures favour the product side as there are the fewest moles of gas. So, for the highest yield, high pressure should be used. However, high pressures are expensive to maintain and so a compromise pressure is used.


13. The Haber process is used to manufacture ammonia (NH3 ), which can be used to produce nitrogen-based fertilisers. In a reversible reaction, nitrogen and hydrogen are reacted to make the ammonia. The ammonia gas is then condensed and run off.

a) Calculate the atom economy of this reaction.

Answer: The atom economy of this reaction is 100%.


b) Write a balanced symbol equation to show ammonia gas condensing.

Answer: NH3(g) →  NH3(l)


c) At equilibrium, suggest what would happen to the yield of ammonia if more hydrogen gas was added to the system.

Answer: The system will oppose change and try to use up the reactants, favouring the forward reaction. This would increase the yield of ammonia.


14. The Contact process is used to make sulfuric acid. Sulfur dioxide (SO2 ) is combusted to make sulfur trioxide (SO3 ) in a reversible reaction.

a) Write a balanced symbol equation for this reaction. You do not need to include state symbols.

Answer: 2SO2 + O2  ⇋  2SO3


b) Write down whether the forward reaction is exothermic or endothermic.

Answer: exothermic


c) When the system is in equilibrium, suggest the effect of decreasing the temperature on the yield of sulfur dioxide.

Answer: The system will oppose the change and try to increase the temperature. It will favour the exothermic (forward reaction) and increase the yield of sulfur trioxide.


15. Ammonium chloride is a white solid. It breaks down when heated, forming ammonia and hydrogen chloride gases: NH4Cl(s)  ⇋  NH3(g) + HCl(g)

a) Describe what you would observe when ammonium chloride is heated in a test tube.

Answer: White solid at the bottom of the tube "disappearing", white solid appearing further up the test-tube.


b) When the system is at equilibrium, suggest the effect of increasing the pressure of the system.

Answer: The system will oppose the changemove to the left. This is the side with the fewest number of moles of gas.


c) Suggest a safety precaution when carrying out this experiment. Explain why this is necessary.

Answer: Carry out in a fume cupboard or well ventilated room. Hydrogen chloride gas and ammonia gas are irritants.


16. Hydrogen is a fuel and can be made by an endothermic reaction between carbon and steam.

a) Write the equation for this reaction.

Answer: C(s) + H2O(g)  ⇋  H2(g) + CO(g)


b) When the system is in equilibrium, suggest the effect of increasing the temperature on the amount of hydrogen.

Answer: The system will oppose the change and try to decrease the temperature. It will favour the endothermic (forward reaction) and increase the yield of hydrogen.


17. Iodine and hydrogen react in a reversible reaction to make hydrogen iodide. The figure below is a sketch graph to show how the concentration of the chemicals change during the reaction.


a) Suggest a suitable unit in which to measure concentration.

Answer: g/dm3 or mol/dm3


b) On a copy of the graph in the figure below, show the time at which equilibrium was reached.

Answer: Students should draw a line vertically at the point where both lines level out.


c) Write a balanced symbol equation for the reaction between iodine and hydrogen.

Answer: H2 + I2  ⇋  2HI


18. Phosphorus pentachloride can thermally decompose into chlorine and phosphorus trichloride: PCl5(g)  ⇋  PCl3(g) + Cl2(g)

a) When the system is in equilibrium, suggest the effect of decreasing the temperature on the yield of chlorine.

Answer: The system will oppose the change and try to increase the temperature. It will favour the exothermic (backwards reaction) and decrease the yield of chlorine.


b) When the system is in equilibrium, suggest the effect of decreasing the pressure of the system.

Answer: The system will oppose the change and try to increase the pressure. It will favour the products side as there are more moles of gas.


19. Ethanol (C2H5OH) is an important fuel that can be made by reacting ethene (C2H4 ) with steam. The reaction is exothermic and reversible.

a) Write a balanced symbol equation for this reaction.

Answer: C2H4 + H2O  ⇋  C2H5OH


b) Explain why the atom economy is 100%.

Answer: There is only one product so all the atoms from the reactants are in the product.


c) When the system is in equilibrium, suggest the effect of increasing the temperature on the yield of ethanol.

Answer: The system will oppose the change and try to decrease the temperature. It will favour the endothermic (backward reaction) and decrease the yield of ethanol.


d) When the system is in equilibrium, suggest the effect of increasing the temperature on the yield of ethanol.

Answer: The system will oppose the change and try to decrease the pressure. It will favour the product side as there are fewer moles of gas.


20. Hydrogen and chlorine gases react together to make hydrogen chloride gas. This is a reversible reaction.

a) Write a balanced symbol equation for this reaction.

Answer: H2(g) + Cl2(g)  ⇋  2HCl(g)


b) Explain why changing pressure has no effect on the yield of hydrogen chloride.

Answer: The system will oppose any change but as there are the same number of moles of gas on each side of the equation the equilibrium position will not change.


21. Methanol (CH3OH) is a fuel and a solvent. It can be made by reacting carbon monoxide with hydrogen. The reaction vessel is kept at about 50 atmospheres pressure and 200°C. This is an exothermic reaction.

a) Write a balanced symbol equation for this reaction.

Answer: CO(g) + 2H2(g)  ⇋  CH3OH(g)


b) Describe the conditions needed to make the maximum yield of methanol.

Answer: The reaction is in equilibrium. The forward reaction is exothermic, therefore high temperatures favour the forward reaction and the reaction mixture contains a higher proportion of product molecules. Since the forward reaction produces fewer molecules low pressures favour it and the proportion of product molecules in the reaction mixture increases.


c) Justify the conditions used in industry for the production of methanol. Explain why they do not give the highest possible yield.

Answer: Although low temperatures give the highest yield the rate of reaction is slower. It takes a longer time to make the methanol. A compromise is reached in which the equilibrium mixture contains an acceptably high proportion of methanol molecules (yield) in a suitable time. High pressures increases the rate of both forward and back reactions. However, high pressures reduce the proportion of methanol in the reaction mixture and are expensive and difficult to maintain. A compromise pressure is used to give a good yield in a reasonable time.


22. In the Contact process, sulfur dioxide is converted to sulfur trioxide by a reversible reaction with oxygen. The figure below shows how the rate of conversion is affected by temperature. Use this graph and your own knowledge to determine whether the reaction is exothermic or endothermic.


Answer: As temperature increases, the percentage of SO2 decreases. A reversible reaction at equilibrium, will oppose change. So, the reaction must be exothermic in the forward direction.


23. The concentrations of each chemical in the Haber process can be monitored. The figure below shows a graph of these results.


a) Use your own knowledge and the figure above to describe what is happening at point B.

Answer: Equilibrium has been reached and the concentration of each chemical is constant. The rate of the forward reaction is the same as the rate of the reverse reaction.


b) When this reaction is carried out in industry, the ammonia is constantly being removed from the reaction chamber. Suggest a reason for this. Justify your answer in terms of yield.

Answer: A system at equilibrium will oppose change by removing the ammonia. The forward reaction will be favoured, so greater yield of ammonia will be made.


24. The Haber process can be described by the following balanced symbol equation: N2 + 3H2  ⇋  2NH3 . The concentrations of chemicals in the Haber process were monitored. The figure below shows the results.


a) Use the graph and your own knowledge to suggest which line is the product and which line is hydrogen.

Answer: The product line is C, as there is 0 concentration at the start. The hydrogen line is A, because there is a greater amount of hydrogen in the balanced symbol equation.


b) Use the graph to estimate the equilibrium concentrations of each chemical.

Answer: A = 2.75 mol/dm3  ;  B = 2 mol/dm3  ;  C = 1.5 mol/dm3


c) Suggest how the graph would change if a catalyst is used in the reaction.

Answer: The concentration of each chemical at equilibrium would be the same. Equilibrium would be reached quicker initial gradients of the lines would be steeper as the rate of reaction would be quicker.