Edexcel IAL Chemistry 6.3 Enthalpy level diagrams

Place species on labelled horizontal levels, draw the enthalpy-change arrow in the correct direction with sign and units and identify any activation-energy arrow separately.

Exam points

Draw reactant and product levels correctly for exothermic or endothermic changes.
Label the enthalpy-change arrow with direction, value, sign, units and species.
Distinguish overall reaction enthalpy from activation energy on diagrams with intermediates.

[Maximum number: 2]

Fuels

Fuels burn in oxygen to release a lot of energy.
Many hydrocarbons and alcohols are used as fuels. During complete combustion, they produce carbon dioxide and water.

Petrol contains 2,2,4-trimethylpentane, an isomer of octane, that promotes smooth combustion.

Figure for Question 6.3—Construct and interpret enthalpy level diagrams, showing exothermic and endothermic enthalpy changes question 1 — Edexcel A-Level Chemistry AS

2,2,4-trimethylpentane

Alcohols, such as methanol and ethanol, can be used as fuels either on their own or as additives in petrol.

The standard enthalpy change of combustion, $\Delta_{\mathrm{c}} H^{\ominus}$ , of 2,2,4-trimethylpentane is $-5461 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

Draw a labelled enthalpy level diagram for the complete combustion of 2,2,4-trimethylpentane.

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6.3—Construct and interpret enthalpy level diagrams, showing exothermic and endothermic enthalpy changes question 1