IGCSE Chemistry 0620: Enthalpy Change, Activation Energy and Pathway Diagrams

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]

Part (a): Defining activation energy

Activation energy, $E_a$ , is the minimum energy that colliding particles must have in order to react.

Part (b): Naming and signing the enthalpy change

The thermal energy change of a reaction is called the enthalpy change, symbol $\Delta H$ .

The technicians observe the solution temperature rising, so this reaction transfers thermal energy to the surroundings. It is exothermic. Since $\Delta H$ is negative for exothermic reactions, $\Delta H$ is negative for this reaction.

Part (c): Labelling the reaction pathway diagram

A fully labelled diagram for this exothermic reaction would show:

Reactants, drawn as a horizontal energy level at the start (left-hand side) of the diagram.
Products. Drawn as a horizontal energy level at the end (right-hand side) of the diagram, positioned lower than the reactants’ level, since energy is released overall.
Enthalpy change, $\Delta H$ , shown as a vertical arrow (or bracket) pointing downward, from the reactants’ level to the products’ level. This arrow represents the overall energy released, and here $\Delta H$ is negative.
Activation energy, $E_a$ , shown as a vertical arrow (or bracket) pointing upward, from the reactants’ level to the peak of the curve (the highest point reached during the reaction). This represents the minimum energy the colliding iron and copper(II) sulfate particles need before they can react.

Final answers

(a) $E_a$ = the minimum energy that colliding particles must have to react.
(b) Enthalpy change, $\Delta H$ ; negative, since the reaction is exothermic.
(c) Reactants high at the start, products lower at the end, $\Delta H$ as a downward arrow between them, $E_a$ as an upward arrow from the reactants to the peak of the curve.