Sign of ΔH for an exothermic reaction?

Negative: heat is given out and the surroundings get hotter.

Sign of ΔH for an endothermic reaction?

Positive: heat is taken in and the surroundings get colder.

What are standard conditions?

100 kPa and a stated temperature (usually 298 K), with substances in their standard states.

Define standard enthalpy of formation.

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.

Define standard enthalpy of combustion.

The enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.

Which equation is both a formation and a combustion?

C(s) + O₂(g) → CO₂(g), or H₂(g) + ½O₂(g) → H₂O(l).

Why is C(s) + ½O₂(g) → CO(g) not a combustion?

The carbon is not burned completely; CO₂ is not the product.

How do you scale an enthalpy value for a given mass?

Moles = mass ÷ M, then energy = moles × ΔH.

Enthalpy changes and standard conditionsAQA A-Level Chemistry: Flashcards

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What is an enthalpy change?

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What is an enthalpy change?: The heat energy change measured at constant pressure.
Sign of ΔH for an exothermic reaction?: Negative: heat is given out and the surroundings get hotter.
Sign of ΔH for an endothermic reaction?: Positive: heat is taken in and the surroundings get colder.
What are standard conditions?: 100 kPa and a stated temperature (usually 298 K), with substances in their standard states.
Define standard enthalpy of formation.: The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions.
Define standard enthalpy of combustion.: The enthalpy change when one mole of a substance burns completely in oxygen under standard conditions.
What is ΔfH of an element in its standard state?: Zero.
What state should water be in a combustion equation?: Liquid, H₂O(l).
Standard state of carbon?: Graphite, C(s).
Which equation is both a formation and a combustion?: C(s) + O₂(g) → CO₂(g), or H₂(g) + ½O₂(g) → H₂O(l).
Units of ΔH?: kJ mol⁻¹.
Why is C(s) + ½O₂(g) → CO(g) not a combustion?: The carbon is not burned completely; CO₂ is not the product.
How do you scale an enthalpy value for a given mass?: Moles = mass ÷ M, then energy = moles × ΔH.

Exam questions on Enthalpy changes and standard conditions

An instant cold pack used for sports injuries contains solid ammonium nitrate and a separate bag of water. When the bag is broken and the solid dissolves, the temperature of the mixture falls. An instant hand warmer works in the same way using calcium chloride, and the temperature rises. Both are measured at constant pressure.
Explain why the temperature of the cold pack falls when the ammonium nitrate dissolves.2 marks
Ethanol, C₂H₅OH, is used as a biofuel. Its standard enthalpy of combustion is −1367 kJ mol⁻¹. Standard enthalpy changes refer to 100 kPa and 298 K.
Calculate the enthalpy change when 2.00 mol of ethanol burn completely under standard conditions, and state what the sign of the value shows.2 marks
Hydrogen is a possible clean fuel for vehicles. Under standard conditions, H₂(g) + ½O₂(g) → H₂O(l) has ΔH = −286 kJ mol⁻¹, and the standard enthalpy of combustion of methane, the main component of natural gas, is −890 kJ mol⁻¹. The relative molecular masses are 2.0 for hydrogen and 16.0 for methane.
Define standard enthalpy of combustion, and explain why the value for hydrogen is also the standard enthalpy of formation of liquid water.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).