Edexcel A-Level Chemistry AS 6.1 The Enthalpy Change H Is the Heat Energy Change Measured At Constant Pressure and That Standard Conditions Are 100 Kpa Questions

Check temperature and pressure against the standard values before deciding whether the measured energy change can be reported as a standard molar enthalpy.

Exam points

State standard enthalpy conditions as 298 K and approximately 100 kPa pressure.
Explain why non-standard temperature data cannot give a standard molar enthalpy.

[Maximum number: 1]

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.

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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 $-5461 \mathrm{~kJ} \mathrm{~mol}^{-1}$ .

State the two standard conditions for this enthalpy change.

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Edexcel A-Level Chemistry AS 6.1 The Enthalpy Change H Is the Heat Energy Change Measured At Constant Pressure and That Standard Conditions Are 100 Kpa Questions question 1