IGCSE Chemistry 0620: Reaction Pathway Diagram for Thermal Decomposition

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]

Part (a): Classifying the reaction from the diagram

On the described reaction pathway diagram, the curve starts low (labelled “reactant”) and, after passing through a peak, settles higher (labelled “products”).

Since the products end up at a higher energy level than the reactant, energy has been taken in overall during the reaction. The copper(II) carbonate particles have gained energy from their surroundings.

This means the thermal decomposition is endothermic.

Part (b): Marking the activation energy

The activation energy, $E_a$ , is the minimum energy the reactant particles need before they can react. On the diagram, it would be marked as a vertical arrow (or bracket) drawn from the reactant level up to the peak of the curve, not from the reactant level to the products level, which instead represents the overall enthalpy change.

Part (c): Effect of removing the heat source

Because this reaction is endothermic, it needs a continuous supply of thermal energy from the Bunsen burner to keep going. If the burner is removed, the copper(II) carbonate and its immediate surroundings stop receiving this energy, so their temperature falls, and the decomposition stops almost immediately. There is no longer enough energy being supplied for the reactant particles to keep reacting.

Final answers

(a) Endothermic. The products are at a higher energy level than the reactant on the diagram, so energy is taken in overall.
(b) $E_a$ is marked as an upward arrow from the reactant level to the peak of the curve.
(c) The temperature falls and the reaction stops, because without the burner supplying thermal energy the endothermic reaction has no energy source to keep taking energy in.