What is meant by a standard enthalpy change, ΔH⦵?

A standard enthalpy change is the heat change at constant pressure measured under standard conditions. All substances must be in their standard states.

State the standard conditions used for enthalpy changes.

Standard conditions are a pressure of 100 kPa and a stated temperature, usually 298 K. Solutions have a concentration of 1 mol dm⁻³.

Write the equation, with state symbols, that represents the standard enthalpy change of combustion of methane.

The equation is CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l). One mole of methane burns completely, and water is a liquid under standard conditions.

CCEA A-Level Life and Health Sciences 0008

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Physical Chemistry in Industrial Processes

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Physical Chemistry in Industrial Processes

Enthalpy changes and calorimetry

3 things to know
AS 3

Overview

Exothermic and endothermic reactions, standard enthalpy changes and conditions, the enthalpy of neutralisation, Q = mcΔT, and measuring neutralisation and combustion with worked examples.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Why do chemical reactions usually involve a heat change?

Key term

Contrast exothermic and endothermic reactions.

Key term

Give one industrial example of an exothermic reaction and one of an endothermic reaction.

Questions from this subtopic

Each one with the model answer underneath.

1
Why do chemical reactions usually involve a heat change?
Model answer
Energy is taken in to break bonds in the reactants and given out when bonds form in the products. The two amounts are rarely equal, so heat is exchanged.
2
Contrast exothermic and endothermic reactions.
Model answer
An exothermic reaction releases heat to the surroundings, so the temperature rises and ΔH is negative. An endothermic reaction absorbs heat, so the temperature falls and ΔH is positive.
3
Give one industrial example of an exothermic reaction and one of an endothermic reaction.
Model answer
The Haber synthesis of ammonia is exothermic. The thermal decomposition of limestone to calcium oxide in a lime kiln is endothermic.
4
What is meant by a standard enthalpy change, ΔH⦵?
Model answer
A standard enthalpy change is the heat change at constant pressure measured under standard conditions. All substances must be in their standard states.
5
State the standard conditions used for enthalpy changes.
Model answer
Standard conditions are a pressure of 100 kPa and a stated temperature, usually 298 K. Solutions have a concentration of 1 mol dm⁻³.
6
Write the equation, with state symbols, that represents the standard enthalpy change of combustion of methane.
Model answer
The equation is CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l). One mole of methane burns completely, and water is a liquid under standard conditions.

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