What is the standard molar enthalpy of combustion?

The standard molar enthalpy of combustion is the enthalpy change when one mole burns completely in oxygen. Standard conditions apply, and the value is always negative.

How is the molar enthalpy change calculated from calorimetry results?

The heat transferred, q, is divided by the moles of fuel burned. A negative sign is added because the reaction releases heat.

What is a mean bond enthalpy?

A mean bond enthalpy is the average energy needed to break one mole of a given bond, taken over many compounds. Bond breaking absorbs energy and bond making releases it.

How is an enthalpy change calculated from bond enthalpies?

The energy of all bonds broken is added up, and the energy of all bonds formed is subtracted. A negative answer shows an exothermic reaction.

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Analytical Chemistry Techniques

Molar enthalpy and bond energy calculations

3 things to know
A2 9

Overview

Standard enthalpy of combustion, worked molar enthalpies for ethanol and methanol, mean bond enthalpies and worked calculations for methane and ethanol, and explaining discrepancies.

What you need to know

Answer each one before you reveal it.

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Numbers

Burning 0.46 g of ethanol heats 100 g of water by 30.0 °C. Calculate the molar enthalpy of combustion.

Numbers

Burning 0.64 g of methanol heats 250 g of water by 20.0 °C. Calculate the molar enthalpy of combustion.

Numbers

Using C–H 413, O=O 498, C=O 805 and O–H 464 kJ mol⁻¹, calculate the enthalpy of combustion of methane.

Questions from this subtopic

Each one with the model answer underneath.

1
What is the standard molar enthalpy of combustion?
Model answer
The standard molar enthalpy of combustion is the enthalpy change when one mole burns completely in oxygen. Standard conditions apply, and the value is always negative.
2
How is the molar enthalpy change calculated from calorimetry results?
Model answer
The heat transferred, q, is divided by the moles of fuel burned. A negative sign is added because the reaction releases heat.
3
Burning 0.46 g of ethanol heats 100 g of water by 30.0 °C. Calculate the molar enthalpy of combustion.
Model answer
The molar enthalpy of combustion is about −1254 kJ mol⁻¹. The working is 100 × 4.18 × 30.0 = 12,540 J, and 0.46 ÷ 46 = 0.0100 mol, so −12.54 ÷ 0.0100 = −1254 kJ mol⁻¹.
4
Burning 0.64 g of methanol heats 250 g of water by 20.0 °C. Calculate the molar enthalpy of combustion.
Model answer
The molar enthalpy of combustion is about −1045 kJ mol⁻¹. The working is 250 × 4.18 × 20.0 = 20,900 J, and 0.64 ÷ 32.0 = 0.0200 mol, so −20.9 ÷ 0.0200 = −1045 kJ mol⁻¹.
5
What is a mean bond enthalpy?
Model answer
A mean bond enthalpy is the average energy needed to break one mole of a given bond, taken over many compounds. Bond breaking absorbs energy and bond making releases it.
6
How is an enthalpy change calculated from bond enthalpies?
Model answer
The energy of all bonds broken is added up, and the energy of all bonds formed is subtracted. A negative answer shows an exothermic reaction.

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AThe energy of all bonds broken is added up.

BThe energy of all bonds formed is subtracted.

CA negative answer shows an exothermic reaction.

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