Exothermic and endothermic reactions

The diagram illustrates the energy profile of a chemical reaction. Two energy changes are marked $X$ and $Y$. Which statement about the reaction is correct?

May/June 2015

The energy profile shows a chemical reaction taking place both in the presence of a catalyst and in its absence. What is the enthalpy change for the catalysed reaction?

May/June 2015

The diagram presents the energy profile for calcium hydroxide dissolving in water.

May/June 2016

The energy profile diagram for the forward direction of a reversible reaction is shown. For the reverse reaction, which row correctly gives the sign of the activation energy and the kind of enthalpy change?

May/June 2017

The conversion of hydrogen and oxygen into liquid water can happen in three steps. 1. $2\text{H}_2(g) + \text{O}_2(g) \rightarrow 4\text{H}(g) + 2\text{O}(g)$ 2. $4\text{H}(g) + 2\text{O}(g) \rightarrow 2\text{H}_2\text{O}(g)$ 3. $2\text{H}_2\text{O}(g) \rightarrow 2\text{H}_2\text{O}(l)$ Which of these steps are exothermic?

May/June 2017

A reversible reaction has its energy profile diagram displayed for the forward direction. For the reverse reaction, which row gives the correct sign of the activation energy and the kind of enthalpy change?

May/June 2017

The production of liquid water from hydrogen and oxygen can take place in three steps. $1\quad 2\text{H}_2(\text{g}) + \text{O}_2(\text{g}) \rightarrow 4\text{H}(\text{g}) + 2\text{O}(\text{g})$ $2\quad 4\text{H}(\text{g}) + 2\text{O}(\text{g}) \rightarrow 2\text{H}_2\text{O}(\text{g})$ $3\quad 2\text{H}_2\text{O}(\text{g}) \rightarrow 2\text{H}_2\text{O}(\ell)$ Which stage or stages would be exothermic?

May/June 2017

Photosynthesis is a chemical reaction that takes place in the leaves of green plants. Carbon dioxide and water are changed into glucose and oxygen. $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$ This reaction absorbs energy and is catalysed by enzymes.

May/June 2017

Respiration is the process that occurs in living cells to release energy. The overall reaction includes the oxidation of glucose. $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$ This reaction is exothermic and is catalysed by enzymes.

May/June 2017

The diagram illustrates the energy profile for a reaction. Which statements about this reaction are correct?

May/June 2018

A reaction is exothermic. Which diagram shows the correct energy profile for the reaction and the correct enthalpy change?

May/June 2018

The photosynthesis equation is shown below. $6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow 6\text{O}_2 + \text{C}_6\text{H}_{12}\text{O}_6$ Which of the following statements about photosynthesis is correct?

May/June 2018

A student carries out an investigation into two fuels, hexane and octane. The student uses the apparatus shown to burn the fuels. In each experiment, a fixed volume of water is heated for five minutes. The diagrams display sections of the thermometer stem showing the starting and ending temperatures.

May/June 2018

The diagram illustrates the energy profile for a reaction. Which statement concerning this reaction is correct?

May/June 2019

The diagram illustrates the energy profile for a chemical reaction. Which row is right?

May/June 2019

A student examines how concentration affects the energy change in a neutralisation reaction. The student: - measures out $25.0\,\text{cm}^3$ of aqueous sodium hydroxide, $\text{NaOH(aq)}$, into a beaker - takes the temperature of this solution - pours $25.0\,\text{cm}^3$ of $0.10\,\text{mol dm}^{-3}$ hydrochloric acid, $\text{HCl(aq)}$, into the beaker - stirs the mixture - records the highest temperature reached by the solution in the beaker - repeats the experiment using different concentrations of hydrochloric acid.

May/June 2021

Among the reactions shown, which one is exothermic?

May/June 2022

State the balanced equation and enthalpy change, $\Delta H$, for the complete combustion of butanol, $\text{C}_4\text{H}_9\text{OH}$.

May/June 2022

The student pours $25\text{ cm}^3$ of $1.0\text{ mol dm}^{-3}$ hydrochloric acid into a glass beaker, takes the temperature, then adds $30\text{ cm}^3$ of aqueous potassium hydroxide in excess and notes the highest temperature reached.

May/June 2022

Which statements concerning the energy changes in a chemical reaction are correct?

May/June 2023

Hydrogen may be obtained by reacting methane with steam. $\text{CH}_4 + 2\text{H}_2\text{O} \rightarrow \text{CO}_2 + 4\text{H}_2$ With the bond energies shown in the table, what is the enthalpy change of reaction, $\Delta H$?

May/June 2023

This question concerns the energy changes that occur during chemical reactions.

May/June 2023

This question concerns the energy transfers that occur in chemical reactions.

May/June 2023

Ammonium nitrate is dissolved in water. $\text{NH}_4\text{NO}_3\text{(s)} \xrightarrow{\text{H}_2\text{O}} \text{NH}_4\text{NO}_3\text{(aq)} \quad \Delta H = +25\ \text{kJ mol}^{-1}$ Which statements are correct?

May/June 2024

Hydrogen combines with oxygen to form water. $\text{H} - \text{H} + \text{O} = \text{O} \rightarrow \text{H} - \text{O} - \text{H}$ A table shows some bond energies. Use the data in the table to determine the enthalpy change of reaction.

May/June 2024

Which statements about endothermic reactions are correct?

May/June 2024

Hydrogen and chlorine combine to produce hydrogen chloride. $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$ Bond energy data are provided. What is the enthalpy change, $\Delta H$, for this reaction?

May/June 2024

A student examines how the temperature changes when a solid dissolves completely in water. The student: • measures $25\,\text{cm}^3$ of distilled water and pours it into a beaker • uses a thermometer to measure the water’s starting temperature in the beaker • writes this temperature in Table 2.1 at time $0\,\text{s}$ • adds a sample of the solid to the beaker and begins a stop-watch • stirs the mixture and notes the temperature and time every $60\,\text{s}$ for a total duration of $300\,\text{s}$. Some of the results are shown in Table 2.1.

May/June 2024

Which row is correct for a chemical reaction where $\Delta H$ is negative?

May/June 2025

The diagram below is a reaction pathway diagram. Which equation is represented by this reaction pathway diagram?

May/June 2025

Which one of the following changes is endothermic?

Oct/Nov 2015

The enthalpy changes for the complete combustion of methane, butane and octane in oxygen are given below. methane, $\text{CH}_4$: $-890\ \text{kJ/mol}$ butane, $\text{C}_4\text{H}_{10}$: $-2877\ \text{kJ/mol}$ octane, $\text{C}_8\text{H}_{18}$: $-5512\ \text{kJ/mol}$ What are the enthalpy changes when propane and pentane are burned completely in oxygen?

Oct/Nov 2015

A reaction’s energy profile diagram is shown. Which statement is correct?

Oct/Nov 2015

An energy profile diagram for a reaction is displayed. Which statement is correct?

Oct/Nov 2015

Which change absorbs heat from its surroundings?

Oct/Nov 2015

If 1 mole of sodium hydroxide reacts with excess hydrochloric acid, 57.1 kJ of energy is given out. $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$

Oct/Nov 2015

A reaction that absorbs heat has an activation energy of $x$. Which energy profile diagram shows this reaction correctly?

Oct/Nov 2016

When magnesium powder is introduced into excess hydrochloric acid, the temperature increases.

Oct/Nov 2016

Liquid water can be produced from hydrogen and oxygen in three stages. Which stages absorb heat?

Oct/Nov 2017

Compound Y undergoes a reaction with oxygen. This reaction shows a positive enthalpy change of reaction, $\Delta H$. What can be inferred about Y and its reaction with oxygen?

Oct/Nov 2017

The conversion of hydrogen and oxygen into liquid water can take place in three stages.

Oct/Nov 2017

When alcohols burn in air, heat energy is released to the surroundings. A student is investigating how much heat energy is produced when alcohols burn.

Oct/Nov 2017

Which of the following statements about endothermic reactions are correct? 1. Energy is taken in from the surroundings. 2. Energy is given out to the surroundings. 3. The temperature of the reaction mixture decreases. 4. The temperature of the reaction mixture increases.

Oct/Nov 2018

The equation shows the reaction of the two gases $\text{X}_2$ and $\text{Y}_2$ to produce the compound XY. $\text{X}_2(g) + \text{Y}_2(g) \rightarrow 2\text{XY}(g)$ The energy profile diagram for this reaction is given. Which of the following statements about this reaction is correct?

Oct/Nov 2018

Which statements regarding endothermic reactions are correct? 1 Energy is taken in from the surroundings. 2 Energy is given out to the surroundings. 3 The temperature of the reaction mixture decreases. 4 The temperature of the reaction mixture increases.

Oct/Nov 2018

A fuel undergoes complete combustion in air. Carbon dioxide, water and heat are formed. Which energy profile diagram is correct for the combustion of a fuel?

Oct/Nov 2018

The diagram shows an energy profile. What does arrow R on the diagram indicate?

Oct/Nov 2019

Which statement is correct about exothermic and endothermic reactions?

Oct/Nov 2019

An industrial process is represented by the equation below. $\text{C}(s) + \text{H}_2\text{O}(g) \rightarrow \text{CO}(g) + \text{H}_2(g) \qquad \Delta H = +131\,\text{kJ mol}^{-1}$ Which row is the correct one?

Oct/Nov 2019

Two energy profile diagrams are displayed. Both diagrams use the same scale on the y-axis. Which statement is correct?

Oct/Nov 2019

Ammonium nitrate is dissolved in water. $\text{NH}_4\text{NO}_3(s) \xrightarrow{\text{H}_2\text{O}} \text{NH}_4\text{NO}_3(aq)$ $\Delta H = +25\,\text{kJ mol}^{-1}$ Which statements are correct?

Oct/Nov 2019

Which of the following statements about photosynthesis is correct?

Oct/Nov 2019

When zinc is placed into aqueous iron(II) sulfate, the temperature of the mixture rises. $\text{Zn}(s) + \text{FeSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Fe}(s)$

Oct/Nov 2019

One unlabelled bottle contains solid sodium carbonate, $\text{Na}_2\text{CO}_3$. A different unlabelled bottle contains solid sodium hydrogencarbonate, $\text{NaHCO}_3$. The reaction of sodium carbonate with dilute hydrochloric acid is exothermic. The reaction of sodium hydrogencarbonate with dilute hydrochloric acid is endothermic. Plan experiments based on the reaction of each solid with dilute hydrochloric acid: • to work out which solid is which • to decide which reaction gives the greater energy change per gram of solid. You may use any apparatus normally available in a chemistry laboratory, but no other chemicals. You should include every measurement you would take. Chemical equations are not needed.

Oct/Nov 2019

A student is investigating how much heat is given out when ethanol burns. The diagram shows the apparatus the student uses.

Oct/Nov 2020

A student measures how the temperature increase changes when different masses of magnesium are added to dilute hydrochloric acid.

Oct/Nov 2020

Which process involves an endothermic change?

Oct/Nov 2021

The reaction between hydrogen and chlorine that produces gaseous hydrogen chloride gives out heat. Which statement is correct?

Oct/Nov 2021

The energy profile diagram for a chemical reaction is shown. Which statement is correct?

Oct/Nov 2021

Phosphorus reacts with hydrogen to make phosphine, $\text{PH}_3$. $2\text{P} + 3\text{H}_2 \rightarrow 2\text{PH}_3$ It is endothermic.

Oct/Nov 2021

Which pair of processes are both endothermic?

Oct/Nov 2022

Nitrogen oxides can be produced in the atmosphere when lightning occurs. $\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO}$ This reaction is endothermic. Which energy profile diagram is the correct one for this reaction?

Oct/Nov 2022

Which pair of processes are both endothermic?

Oct/Nov 2022

When the conditions are suitable, nitrogen reacts with oxygen to produce $\text{N}_2\text{O}$. $2\text{N}_2(\text{g}) + \text{O}_2(\text{g}) \rightleftharpoons 2\text{N}_2\text{O}(\text{g})$ A reaction pathway diagram is given. What is the activation energy for the reverse reaction?

Oct/Nov 2023

Hydrogen and bromine combine to make hydrogen bromide. $\text{H}_2 + \text{Br}_2 \rightarrow 2\text{HBr}$ The table provides bond energy data. What is the enthalpy change, $\Delta H$, for this reaction?

Oct/Nov 2023

Hydrogen combines with chlorine to make hydrogen chloride. $\text{H}_2(\text{g}) + \text{Cl}_2(\text{g}) \rightarrow 2\text{HCl}(\text{g})$ The enthalpy change, $\Delta H$, for this reaction may be worked out by using bond energies. Bond energies: $\text{H--H} = 436\,\text{kJ mol}^{-1}$, $\text{Cl--Cl} = 242\,\text{kJ mol}^{-1}$, $\text{H--Cl} = 431\,\text{kJ mol}^{-1}$. What is the value of $\Delta H$ for this reaction?

Oct/Nov 2023

The reaction between nitrogen and oxygen is shown below. $\text{N}_2(\text{g}) + 2\text{O}_2(\text{g}) \rightarrow 2\text{NO}_2(\text{g})$ For the reaction shown, the enthalpy change is $+66\,\text{kJ}$. If two moles of nitrogen and two moles of oxygen are used, what is the enthalpy change?

Oct/Nov 2023

Steam and carbon react to make carbon monoxide and hydrogen. $\text{steam} + \text{carbon} \rightleftharpoons \text{carbon monoxide} + \text{hydrogen}$ The forward reaction absorbs energy.

Oct/Nov 2023

Which of the following statements about exothermic and endothermic reactions is correct?

Oct/Nov 2024

When chemical reaction X occurs, thermal energy is released. Which row shows the correct information for this reaction?

Oct/Nov 2024

A table provides some bond energy data. Hydrazine, $\text{N}_2\text{H}_4$, reacts with oxygen, as shown. $\text{N}_2\text{H}_4 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{N}_2$. The enthalpy change, $\Delta H$, for this reaction is found by choosing and applying data from the table. What is the value of $\Delta H$?

Oct/Nov 2025

When sodium hydrogencarbonate is heated, it breaks down. $2\mathrm{NaHCO_3}(s) \rightarrow \mathrm{Na_2CO_3}(s)+\mathrm{H_2O}(g)+\mathrm{CO_2}(g)$

Oct/Nov 2025

The balanced equation for the reaction of aluminium with dilute hydrochloric acid is given here. $2Al(s)+6HCl(aq)\rightarrow2AlCl_3(aq)+3H_2(g)$. Fig. 3.1 shows a reaction pathway diagram. Reactants: $2Al(s)+6HCl(aq)$. Products: $2AlCl_3(aq)+3H_2(g)$. The energy changes are marked A and B.

Oct/Nov 2025

The reaction of a metal with dilute sulfuric acid is exothermic. Devise an experiment to find out whether magnesium or zinc gives out more thermal energy per gram of metal when it reacts with excess dilute sulfuric acid. You are given magnesium powder, zinc powder, dilute sulfuric acid and ordinary laboratory apparatus. Do not use any other chemicals. Your plan should state: the apparatus required; the method to follow and the measurements to make; how the measurements are used to decide which metal releases more thermal energy per gram. A diagram may be included if it helps answer the question.

Oct/Nov 2025