Standard enthalpy changes and level diagrams

Standard conditions

Standard conditions are 100 kPa pressure, a stated temperature (usually 298 K), solutions at 1.0 mol/dm³, and every substance in its standard state (its normal physical state under these conditions). The symbol is ΔH with a ⦵ or the word standard.

The definitions

• Enthalpy change of reaction, ΔrH: for the amounts shown in the equation.
• Formation, ΔfH: 1 mol of a compound formed from its elements in their standard states.
• Combustion, ΔcH: 1 mol of a substance completely burned in oxygen.
• Neutralisation, ΔneutH: 1 mol of water formed by the reaction of an acid with an alkali.
• Atomisation, ΔatH: 1 mol of gaseous atoms formed from the element in its standard state.

Enthalpy level diagrams

Draw reactants and products as horizontal lines on an enthalpy axis. Exothermic: products lower, ΔH negative, arrow pointing down. Endothermic: products higher, ΔH positive, arrow pointing up. The diagram shows the overall change, not the route.

Worked example

Write the equation for the standard enthalpy change of formation of ethanol.

1. 1 mol of C2H5OH(l) from elements in standard states.
2. Carbon is C(s, graphite), hydrogen H2(g), oxygen O2(g).

Answer: 2C(s) + 3H2(g) + 1/2 O2(g) -> C2H5OH(l)

Check yourself

1. Which equation represents the standard enthalpy change of formation of water?
   • 2H2(g) + O2(g) -> 2H2O(l)
   • H2(g) + 1/2 O2(g) -> H2O(g)
   • H2(g) + 1/2 O2(g) -> H2O(l)
   • H2O(l) -> H2(g) + 1/2 O2(g)
   Show answer

   H2(g) + 1/2 O2(g) -> H2O(l). 1 mol of water in its standard state (liquid at 298 K) from its elements.

2. Which equation represents the enthalpy change of atomisation of chlorine?
   • 1/2 Cl2(g) -> Cl(g)
   • Cl2(g) -> 2Cl(g)
   • Cl(g) -> 1/2 Cl2(g)
   • Cl2(l) -> 2Cl(g)
   Show answer

   1/2 Cl2(g) -> Cl(g). Atomisation forms 1 mol of gaseous atoms.

3. The enthalpy change of neutralisation is defined per mole of
   • acid used
   • alkali used
   • water formed
   • salt formed
   Show answer

   water formed. It is per mole of water.

4. On an enthalpy level diagram for an endothermic reaction, the products are
   • lower than the reactants
   • higher than the reactants
   • at the same level
   • below zero
   Show answer

   higher than the reactants. Energy is taken in, so the products have more enthalpy.

5. What is the standard state of bromine at 298 K?
   • Gas
   • Solid
   • Aqueous
   • Liquid
   Show answer

   Liquid. Bromine is a red-brown liquid at room temperature.

6. Which equation represents the standard enthalpy change of combustion of methane?
   • CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)
   • CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)
   • 2CH4(g) + 4O2(g) -> 2CO2(g) + 4H2O(l)
   • CH4(g) + 3/2 O2(g) -> CO(g) + 2H2O(l)
   Show answer

   CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l). 1 mol burned completely; water is a liquid under standard conditions.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.