Exothermic and Endothermic Reactions: Chemistry 0620 (Cambridge O Level / IGCSE)

A gardener wants to stop the water in a small ornamental birdbath from freezing overnight, so tries stirring a scoop of anhydrous calcium chloride, $\text{CaCl}_2$, into the water while a thermometer sits in place.

Before the calcium chloride is added, the water temperature is $17\,^{\circ}\text{C}$. Within a minute of stirring the calcium chloride in, the thermometer reading rises to $34\,^{\circ}\text{C}$.

Which statement correctly classifies this change and explains it?

Rail maintenance engineers use the thermite reaction to weld two sections of steel track together on site. Powdered aluminium is mixed with iron(III) oxide and ignited; the reaction produces molten iron, which fills the gap between the rails, and solid aluminium oxide.

$2\text{Al}(s) + \text{Fe}_2\text{O}_3(s) \rightarrow 2\text{Fe}(l) + \text{Al}_2\text{O}_3(s)$

A reaction pathway diagram for this reaction has energy on the vertical axis and progress of reaction on the horizontal axis. The curve starts at a high point labelled "reactants: aluminium + iron(III) oxide", rises briefly to a peak, and then falls to a much lower point labelled "products: iron + aluminium oxide".

(a) Using the reaction pathway diagram described above, state whether the thermite reaction is exothermic or endothermic. Give a reason for your answer based on the relative energy levels of the reactants and products. [2]

(b) Describe what happens to the temperature of the surroundings during the thermite reaction, and explain why this happens. [2]

A metal-recycling company recovers copper metal from waste copper(II) sulfate solution collected during circuit-board processing. Excess iron filings are stirred into the solution, and the overall reaction is:

$\text{Fe}(s) + \text{CuSO}_4(aq) \rightarrow \text{FeSO}_4(aq) + \text{Cu}(s)$

Technicians record that the solution temperature rises noticeably as the reaction proceeds.

(a) Define activation energy, $E_a$. [1]

(b) State the term used for the thermal energy change of a reaction, together with its usual symbol. For this reaction, state whether its value is positive or negative. [2]

(c) A fully labelled reaction pathway diagram is to be drawn for this reaction. Describe how each of the following four features would appear on the diagram, including their relative positions: reactants, products, the enthalpy change of the reaction, and the activation energy. [4]

Early rocket engineers used concentrated hydrogen peroxide as a "monopropellant" for small emergency thrusters: passing it over a solid catalyst makes it decompose rapidly, producing a jet of hot steam and oxygen gas that provides thrust.

$2\text{H}_2\text{O}_2(l) \rightarrow 2\text{H}_2\text{O}(g) + \text{O}_2(g)$

This decomposition is strongly exothermic.

Which statement correctly explains this, in terms of bond breaking and bond making?

A student investigates the reaction between excess magnesium ribbon and dilute hydrochloric acid. She places a measured volume of the acid in an uncovered polystyrene cup, records its starting temperature, adds the magnesium ribbon, stirs with the thermometer, and records the temperature every 30 seconds.

(a) Use the table to determine the maximum temperature rise recorded during the experiment. [2]

(b) State, with a reason, whether this reaction is exothermic or endothermic. [1]

(c) The reaction is complete well before 120 s, yet the recorded temperature keeps rising until 120 s and then starts to fall. Suggest why the temperature falls after 120 s. [1]

(d) The student is asked to repeat the experiment to obtain a more accurate value for the maximum temperature rise. Suggest one change she could make to the apparatus, and explain how it would improve the accuracy of the result. [2]

A technician heats solid copper(II) carbonate strongly in a test tube. The green solid decomposes to form black copper(II) oxide and carbon dioxide gas. The Bunsen burner must be kept alight under the test tube for the whole time the reaction is happening; if it is removed, the decomposition stops almost immediately.

$\text{CuCO}_3(s) \rightarrow \text{CuO}(s) + \text{CO}_2(g)$

A reaction pathway diagram for this reaction has energy on the vertical axis and progress of reaction on the horizontal axis. The curve starts at a lower point labelled "reactant: copper(II) carbonate", rises steadily, passes through a peak, and settles at a higher point labelled "products: copper(II) oxide + carbon dioxide".

(a) Using the reaction pathway diagram described above, state whether this reaction is exothermic or endothermic. Give a reason for your answer based on the relative energy levels of the reactant and products. [2]

(b) On a copy of this diagram, describe where the activation energy, $E_a$, would be marked. [1]

(c) Describe what happens to the temperature of the copper(II) carbonate and its immediate surroundings if the Bunsen burner is removed partway through the reaction, and explain why this happens. [2]

A student carries out four separate reactions in the laboratory and records the temperature change of the surroundings in each case.

Which of these four reactions is endothermic?

Hydrogen gas reacts with chlorine gas to form hydrogen chloride gas:

$\text{H}_2(g) + \text{Cl}_2(g) \rightarrow 2\text{HCl}(g)$

The bond energies are:

• H–H bond: $436\,\text{kJ/mol}$
• Cl–Cl bond: $243\,\text{kJ/mol}$
• H–Cl bond: $431\,\text{kJ/mol}$

(a) Calculate the total energy absorbed in breaking all the bonds in the reactants. [2]

(b) Calculate the total energy released in forming all the bonds in the products. [2]

(c) Calculate the overall energy change, $\Delta H$, for this reaction, and state whether the reaction is exothermic or endothermic. [2]

Methane burns completely in oxygen according to the equation:

$\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g)$

The bond energies are:

• C–H bond: $413\,\text{kJ/mol}$
• O=O bond: $498\,\text{kJ/mol}$
• C=O bond: $805\,\text{kJ/mol}$
• O–H bond: $463\,\text{kJ/mol}$

(a) One molecule of $\text{CH}_4$ contains four C–H bonds, and each $\text{O}_2$ molecule contains one O=O bond. Determine the total number of each bond broken when one mole of $\text{CH}_4$ reacts completely, and calculate the total energy absorbed breaking these bonds. [3]

(b) One molecule of $\text{CO}_2$ contains two C=O bonds, and each $\text{H}_2\text{O}$ molecule contains two O–H bonds. Determine the total number of each bond formed, and calculate the total energy released forming these bonds. [3]

(c) Calculate the overall energy change, $\Delta H$, for the complete combustion of one mole of methane, and state whether the reaction is exothermic or endothermic. [2]

In the Haber process, nitrogen and hydrogen react to form ammonia:

$\text{N}_2(g) + 3\text{H}_2(g) \rightarrow 2\text{NH}_3(g)$

The bond energies are:

• N≡N bond: $944\,\text{kJ/mol}$
• H–H bond: $436\,\text{kJ/mol}$
• N–H bond: $391\,\text{kJ/mol}$

Using these bond energies, which of the following gives the correct value of $\Delta H$ for this reaction and correctly classifies it?