Chemistry 5070 · O Level
Reversible reactions and equilibrium
84 practice questions on Reversible reactions and equilibrium, with worked solutions and instant marking.
In which of these equilibria does an increase in pressure favour the forward reaction?
May/June 2015
The equation for the reaction that occurs during ammonia manufacture is shown. $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ $\Delta H = -92\,\text{kJ mol}^{-1}$ The reaction is performed at about $200$ atmospheres pressure, with an iron catalyst, and at temperatures from $350\,^{\circ}\text{C}$ to $450\,^{\circ}\text{C}$. Which statement is incorrect?
May/June 2015
Ozone molecules are constantly broken apart and rebuilt in the upper atmosphere.
May/June 2015
Hydrogen combines with halogens to make hydrogen halides.
May/June 2015
Ammonia may be manufactured on an industrial scale from nitrogen and hydrogen. $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$. The forward reaction is exothermic. Which change would leave the ammonia yield unchanged?
May/June 2016
Ammonia is produced in the Haber process by reacting hydrogen with nitrogen.
May/June 2016
In this reversible reaction, carbon reacts with steam. $\text{C(s)} + \text{H}_2\text{O(g)} \rightleftharpoons \text{H}_2\text{(g)} + \text{CO(g)} \quad \Delta H = +131\,\text{kJ mol}^{-1}$
May/June 2016
Sulfuric acid is produced by the contact process.
May/June 2016
This equation represents sulfur trioxide being formed in the contact process. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g) \quad \Delta H = -196\,\text{kJ mol}^{-1}$ What would lower the yield of sulfur trioxide?
May/June 2017
The equation below represents the formation of sulfur trioxide in the contact process. $2\text{SO}_2(\text{g}) + \text{O}_2(\text{g}) \rightleftharpoons 2\text{SO}_3(\text{g}) \qquad \Delta H = -196\,\text{kJ mol}^{-1}$ Which change would reduce the yield of sulfur trioxide?
May/June 2017
In what way can the reaction between nitrogen and hydrogen be described?
May/June 2017
Nitrogen dioxide, $\text{NO}_2$, combines with itself to form dinitrogen tetroxide, $\text{N}_2\text{O}_4$, in an exothermic reaction. This reaction is studied at $140^\circ\text{C}$ inside a sealed container. A dynamic equilibrium mixture is then obtained. $2\text{NO}_2(g) \rightleftharpoons \text{N}_2\text{O}_4(g)$ $\text{NO}_2(g)$ is a dark brown gas. $\text{N}_2\text{O}_4(g)$ is a colourless gas.
May/June 2017
Nitrogen combines with oxygen in an equilibrium reaction. $\text{N}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{NO}(g)$ \quad $\Delta H = +170\,\text{kJ mol}^{-1}$ At equilibrium for this reaction, which statement is correct?
May/June 2018
Nitrogen undergoes reaction with oxygen in an equilibrium reaction. $\text{N}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{NO}(g) \quad \Delta H = +170\,\text{kJ mol}^{-1}$ At equilibrium for this reaction, which statement is correct?
May/June 2018
The diagram illustrates three stages in the production of sulfuric acid. sulfur $\xrightarrow{\text{step P}}$ sulfur dioxide $\xrightarrow{\text{step Q}}$ sulfur trioxide $\xrightarrow{\text{step R}}$ sulfuric acid At which stages is a catalyst employed?
May/June 2018
A scientist examines how iron reacts with steam in a closed system. A dynamic equilibrium mixture is formed. $3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightleftharpoons \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)$
May/June 2018
A scientist studies the thermal decomposition of calcium carbonate in a closed vessel. A dynamic equilibrium mixture is formed. $\text{CaCO}_3(s) \rightleftharpoons \text{CaO}(s) + \text{CO}_2(g)$
May/June 2018
Ammonia is made when nitrogen reacts with hydrogen in the presence of a catalyst.
May/June 2018
The equations represent four reversible reactions. For which reaction would the position of equilibrium shift to the right when both the pressure and the temperature are increased?
May/June 2019
A student puts forward three statements about the Haber process and the Contact process. 1. Just one of the processes uses a raw material that is obtained by fractional distillation of air. 2. Just one of the processes makes use of a catalyst. 3. The product formed in each catalysed reaction has a formula of the type $XY_3$. Which statements are correct?
May/June 2019
Ammonia is produced by the Haber process. $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g) \quad \Delta H = -92\,\text{kJ mol}^{-1}$ For this reaction, which rows show a statement that is true and a reason that is correct?
May/June 2019
A scientist warms a sample of phosphorus(V) chloride in a sealed container. Dynamic equilibrium is reached. $\text{PCl}_5(g)\rightleftharpoons\text{PCl}_3(g)+\text{Cl}_2(g)$
May/June 2019
Nitrogen monoxide combines with oxygen to produce nitrogen dioxide. If this reaction is examined in a closed system, dynamic equilibrium is reached. $\text{2NO}(g) + \text{O}_2(g) \rightleftharpoons \text{2NO}_2(g)$
May/June 2019
The Haber process changes nitrogen and hydrogen into ammonia. $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ Which row is right?
May/June 2021
Ammonia is manufactured by the Haber process at a temperature of $400\,^{\circ}\text{C}$ and a pressure of $20\,000\,\text{kPa}$. The temperature is then raised to $500\,^{\circ}\text{C}$ while the pressure remains unchanged. What effects will this change have on ammonia production?
May/June 2021
Brown nitrogen dioxide combines to produce colourless dinitrogen tetroxide in a reversible reaction. The forward reaction is exothermic. $\text{2NO}_2(\text{g}) \rightleftharpoons \text{N}_2\text{O}_4(\text{g})$ Which changes would cause the equilibrium mixture to become darker in colour?
May/June 2021
Carbon dioxide and water vapour are both greenhouse gases present in air.
May/June 2021
For the forward direction of a reversible reaction, the enthalpy change of reaction, $\Delta H$, is $-50\,\text{kJ mol}^{-1}$ and the activation energy, $E_a$, is $+60\,\text{kJ mol}^{-1}$. What is the activation energy of the reverse reaction?
May/June 2022
Nitrogen dioxide, $\text{NO}_2$, is a dark brown gas which breaks down as shown in the equation. $2\text{NO}_2(\text{g}) \rightleftharpoons 2\text{NO}(\text{g}) + \text{O}_2(\text{g})$ dark brown colourless A glass flask contains a mixture of these three gases. The mixture appears pale brown. Additional oxygen is pushed into the flask. Which colour change is observed in the mixture?
May/June 2022
Nitrogen dioxide, $\text{NO}_2$, is a dark brown gas that breaks down according to the equation $2\text{NO}_2(g) \rightleftharpoons 2\text{NO}(g) + \text{O}_2(g)$. The diagram shows a glass flask holding a mixture of these three gases. The mixture is pale brown. More oxygen is pushed into the flask. Which colour change is observed in the mixture?
May/June 2022
Iron undergoes reaction with steam and with dilute sulfuric acid.
May/June 2022
Ethanol is produced by reacting ethene with steam. The conditions are $300\,^{\circ}\text{C}$, high pressure and a phosphoric acid catalyst.
May/June 2022
The reaction equation for the Contact process is given below. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g) \qquad \Delta H = -197\,\text{kJ mol}^{-1}$ The reaction is carried out at $450^\circ\text{C}$, $2$ atmospheres pressure and in the presence of a catalyst. What happens if the temperature is lowered to $250^\circ\text{C}$ and the catalyst is taken away?
May/June 2023
Hydrogen is produced on an industrial scale by the reaction below. $\text{CO}(g) + \text{H}_2\text{O}(g) \rightleftharpoons \text{CO}_2(g) + \text{H}_2(g) \qquad \Delta H = -41\,\text{kJ mol}^{-1}$ Several statements about this industrial process are listed. Which of them are correct?
May/June 2023
When heated, ammonium chloride breaks down to form hydrogen chloride and ammonia. The reaction can proceed in both directions, and the forward change takes in thermal energy. $\text{NH}_4\text{Cl}(s) \rightleftharpoons \text{HCl}(g) + \text{NH}_3(g)$
May/June 2023
Carbon reacts with steam to form carbon monoxide and hydrogen. This reaction is reversible, and the forward reaction takes in thermal energy. $\text{C}(s) + \text{H}_2\text{O}(g) \rightleftharpoons \text{CO}(g) + \text{H}_2(g)$
May/June 2023
$157.75\ \text{g}$ of bismuth(III) chloride, $\text{BiCl}_3$, is taken to prepare $500\ \text{cm}^3$ of solution with distilled water. When in water, the aqueous bismuth(III) chloride gradually turns cloudy because it reacts with water and produces insoluble bismuth oxychloride. This reaction is reversible. $\text{BiCl}_3\text{(aq)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{BiOCl(s)} + 2\text{HCl(aq)}$ Which of the statements are correct? $[A_r:\ \text{Bi},\ 209;\ \text{Cl},\ 35.5]$
May/June 2024
The reaction between calcium oxide and carbon dioxide is reversible, and the forward reaction gives out heat. $\text{CaO}(s) + \text{CO}_2(g) \rightleftharpoons \text{CaCO}_3(s)$ Calcium oxide and carbon dioxide are put into a heated flask, and the reaction attains equilibrium. Which statement about this equilibrium is correct?
May/June 2024
Heating a sample of zinc sulfite in a sealed system produces an equilibrium mixture. $\text{ZnSO}_3(\text{s}) \rightleftharpoons \text{ZnO}(\text{s}) + \text{SO}_2(\text{g})$ The forward reaction is endothermic.
May/June 2024
If a sample of zinc carbonate is heated in a closed system, an equilibrium mixture results. $\text{ZnCO}_3(\text{s}) \rightleftharpoons \text{ZnO}(\text{s}) + \text{CO}_2(\text{g})$ The forward reaction takes in heat.
May/June 2024
Inside a sealed flask, gases Q and R come to a dynamic equilibrium. $\text{Q}(g) \rightleftharpoons 2\text{R}(g) \qquad \Delta H \text{ is positive}$ Which change would shift the equilibrium to the right?
May/June 2025
Which row gives the usual conditions employed to convert sulfur dioxide into sulfur trioxide in the Contact process?
May/June 2025
In industry, methanol is produced by reacting carbon monoxide with hydrogen. $\text{CO(g)} + 2\text{H}_2\text{(g)} \rightleftharpoons \text{CH}_3\text{OH(g)} \quad \Delta H = -90\ \text{kJ/mol}$ This process is normally conducted at a temperature of $400^\circ\text{C}$. Which row shows correctly how raising the temperature to above $400^\circ\text{C}$ affects both the equilibrium position and the reaction rate?
Oct/Nov 2015
Hydrogen and nitrogen combine to produce ammonia. $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$ Which of the following statements is correct?
Oct/Nov 2015
Two gases, X and Y, combine to produce a gas Z, as shown. $\text{X(g)} + 3\text{Y(g)} \rightleftharpoons 2\text{Z(g)} \quad \Delta H = -92\,\text{kJ mol}^{-1}$ Which alteration in the conditions would increase both the reaction rate and the equilibrium amount of Z?
Oct/Nov 2015
The reaction diagram for the Haber process is able to attain equilibrium. $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ Which row indicates the gases present at equilibrium?
Oct/Nov 2015
Methanol is made by letting carbon monoxide react with hydrogen. The forward reaction releases heat. $\text{CO(g)} + 2\text{H}_2\text{(g)} \rightleftharpoons \text{CH}_3\text{OH(g)} \qquad \Delta H = -91\,\text{kJ mol}^{-1}$
Oct/Nov 2015
At 200 atmospheres pressure and $200^\circ\text{C}$, phosphorus(V) chloride exists as an equilibrium mixture.
Oct/Nov 2015
During the Haber process, hydrogen and nitrogen combine to produce ammonia in the presence of a catalyst. Which of these two reactants is separated by fractional distillation, and what catalyst is used in the Haber process?
Oct/Nov 2016
In liquid water, a small amount of ionisation occurs. $\text{H}_2\text{O}(l) \rightleftharpoons \text{H}^+(aq) + \text{OH}^-(aq)$ The forward reaction absorbs heat. If the temperature of the water is raised, what change(s) occur?
Oct/Nov 2016
Within the contact process, sulfur trioxide is formed by catalytic oxidation of sulfur dioxide. In a sealed vessel, the following equilibrium is established. 2SO$_2(g)$ + O$_2(g)$ $\rightleftharpoons$ 2SO$_3(g)$ The reaction is exothermic.
Oct/Nov 2016
Sulfur trioxide is formed in the reaction shown below. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g) \qquad \Delta H = -195\ \text{kJ}$ Which change in conditions would result in a larger amount of $\text{SO}_3$ at equilibrium?
Oct/Nov 2017
The table lists some symbols and what they represent. Which symbols should be included in the equation for the Haber process?
Oct/Nov 2017
Sulfur trioxide is formed by the reaction shown here. $2\text{SO}_2\text{(g)} + \text{O}_2\text{(g)} \rightleftharpoons 2\text{SO}_3\text{(g)} \quad \Delta H = -195\,\text{kJ}$ Which change in conditions would give a larger amount of $\text{SO}_3$ at equilibrium?
Oct/Nov 2017
When the temperature is high, hydrogen combines with iodine to produce hydrogen iodide. $\text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g) \qquad \Delta H = +53.0\ \text{kJ mol}^{-1}$
Oct/Nov 2017
On adding bismuth(III) chloride, $\text{BiCl}_3$, to water, a reaction takes place and a white precipitate of BiOCl is produced. $\text{BiCl}_3(aq) + \text{H}_2\text{O}(l) \rightleftharpoons \text{BiOCl}(s) + 2\text{HCl}(aq)$ Which of these changes would increase the mass of white precipitate produced? 1. adding extra water 2. adding aqueous sodium hydroxide 3. adding dilute hydrochloric acid
Oct/Nov 2018
Solid ammonium chloride is heated. Ammonia and hydrogen chloride gases are produced. This is reaction 1. Ammonia gas is combined with hydrogen chloride gas. Solid ammonium chloride is produced. This is reaction 2. Which statement is correct?
Oct/Nov 2018
Inside a sealed flask, gases Q and R establish a dynamic equilibrium. $\text{Q}(\text{g}) \rightleftharpoons 2\text{R}(\text{g}) \qquad \Delta H \text{ is positive}$ Which alteration will shift the equilibrium towards the right-hand side?
Oct/Nov 2018
Sulfuric acid is produced by the contact process. The equations for the reactions in the process are given. reaction 1: $\text{2SO}_2(g) + \text{O}_2(g) \rightleftharpoons \text{2SO}_3(g) \quad \Delta H = -198\,\text{kJ mol}^{-1}$ reaction 2: $\text{SO}_3(g) + \text{H}_2\text{O}(l) \rightarrow \text{H}_2\text{SO}_4(aq)$ Which of the statements below are true?
Oct/Nov 2019
Hydrogen may be formed when steam and carbon monoxide are passed over a catalyst. $\text{H}_2\text{O(g)} + \text{CO(g)} \rightleftharpoons \text{CO}_2\text{(g)} + \text{H}_2\text{(g)} \quad \Delta H = -41\,\text{kJ mol}^{-1}$
Oct/Nov 2019
The Contact process is used to produce sulfuric acid. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g) \quad \Delta H = -197\,\text{kJ mol}^{-1}$
Oct/Nov 2019
In liquid water, a small amount of ionisation occurs. $\text{H}_2\text{O}(l) \rightleftharpoons \text{H}^+(aq) + \text{OH}^-(aq)$ The forward reaction absorbs heat. If the temperature of the water is raised, what changes occur?
Oct/Nov 2020
The Haber process is used to produce ammonia. Which catalyst is used in the Haber process?
Oct/Nov 2020
Liquid water ionises only slightly. $\text{H}_2\text{O(l)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{OH}^-\text{(aq)}$. The forward reaction absorbs heat. If the temperature of the water is raised, which changes occur?
Oct/Nov 2020
Ammonia is made by means of the Haber process. $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$ The forward reaction gives out heat. Which one of the following statements about the Haber process is correct?
Oct/Nov 2020
Calcium is a metal belonging to Group II of the Periodic Table.
Oct/Nov 2020
Lead is a metal belonging to Group IV of the Periodic Table.
Oct/Nov 2020
The elements $X$ and $Y$ undergo a reversible reaction to produce $XY_2$. $X + 2Y \rightleftharpoons XY_2$ $1.0\,\text{mol}$ of $X$ is combined with $1.0\,\text{mol}$ of $Y$ and the mixture is allowed to react until equilibrium is established. Which of the following statements about this reaction are correct?
Oct/Nov 2021
Which statement is true for every reversible reaction that has reached dynamic equilibrium?
Oct/Nov 2021
Ammonia is produced through a reversible reaction. $\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)$\qquad $\Delta H = -92\,\text{kJ mol}^{-1}$ A chemist studies how the percentage of ammonia present at equilibrium varies as pressure changes. The investigation is done at both $250^\circ\text{C}$ and at $350^\circ\text{C}$. Which graph represents the chemist’s results?
Oct/Nov 2021
Sodium is a Group I metal in the Periodic Table. Diamond (carbon) is a non-metal and it conducts heat well.
Oct/Nov 2021
The equation represents a reaction in the Contact process. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g) \quad \Delta H = -196\,\text{kJ mol}^{-1}$ Which alteration would shift the equilibrium position to the left?
Oct/Nov 2022
During the Contact process, sulfur is changed into sulfuric acid. A catalyst is included in the reaction mixture represented by the equation. $2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3$ What is the purpose of the catalyst?
Oct/Nov 2022
The equation represents a reaction in the Contact process. $2\text{SO}_2(g) + \text{O}_2(g) \rightleftharpoons 2\text{SO}_3(g) \quad \Delta H = -196\,\text{kJ mol}^{-1}$ Which change would shift the equilibrium position to the left?
Oct/Nov 2022
This question concerns compounds formed from nitrogen.
Oct/Nov 2022
This question concerns nitrogen and oxides of nitrogen.
Oct/Nov 2022
Aqueous bromine exists as an equilibrium mixture. $\text{Br}_2(\text{aq}) + \text{H}_2\text{O}(\text{l}) \rightleftharpoons \text{OBr}^-(\text{aq}) + \text{Br}^-(\text{aq}) + 2\text{H}^+(\text{aq})$ Aqueous bromine is orange in colour. The species on the right side of the equation are colourless. Three separate portions of the equilibrium mixture are then each given a small amount of one substance. Which row indicates how the colour of the mixture changes when each substance is added?
Oct/Nov 2023
The equation for the Haber process that is used to produce ammonia is given below. $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$
Oct/Nov 2023
Methanol is used to manufacture ethanoic acid. $\text{CH}_3\text{OH(g)} + \text{CO(g)} \rightleftharpoons \text{CH}_3\text{COOH(g)}$ The forward reaction releases heat.
Oct/Nov 2023
The equation below shows how carbon monoxide reacts with hydrogen at a high temperature in a sealed container. $\text{CO}(\text{g}) + 2\text{H}_2(\text{g}) \rightleftharpoons \text{CH}_3\text{OH}(\text{g})$ (methanol) In the forward direction, the reaction is exothermic.
Oct/Nov 2023
When calcium carbonate is heated in a sealed container, it breaks down to give calcium oxide and carbon dioxide. Calcium oxide and carbon dioxide can then join together again to produce calcium carbonate. After a while, a point is reached at which calcium carbonate is decomposing, and calcium oxide and carbon dioxide are recombining at the same rate. What is this point called?
Oct/Nov 2024
Samples of nitrogen and hydrogen are mixed and left until equilibrium is reached. The equation is given as: $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3 \qquad \Delta H = -92\,\text{kJ mol}^{-1}$. The temperature is raised and a new equilibrium is then established. Which statement about this new equilibrium is correct?
Oct/Nov 2024
In the diagram, R shows one of the reactions in the Contact process. Which statement is correct?
Oct/Nov 2024
This equation represents the reversible reaction in which nitrogen and hydrogen combine to produce ammonia. $\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$. The forward reaction gives out heat. Which statement is correct when the temperature is increased?
Oct/Nov 2025