1.5.3 Standard Enthalpy Change

Enthalpy Changes at Standard Conditions

• To fairly compare the changes in enthalpy between reactions, all reactions should be carried out under standard conditions
• These standard conditions are:
• A pressure of 101 kPa
• A temperature of 298 K (25 oC)
• Each substance involved in the reaction is in its normal physical state (solid, gas or liquid)
• To show that a reaction has been carried out under standard conditions, the symbol Ꝋ is used
• ΔH = the standard enthalpy change
• These are a number of key definitions for common language relating to enthalpy change that all chemists need to know

Enthalpy definitions table

Worked example: Calculating the enthalpy change of reaction of water

Since two moles of water molecules are formed in the question above, the energy released is simply:

ΔHr = 2 mol x (-286 kJ mol-1)

= -572 kJ mol-1

Worked example: Calculating the enthalpy change of formation

Since two moles of Fe2O3 (s) are formed the total change in enthalpy for the reaction above is:

ΔHf =  2 x ( -824.2 kJ mol-1)

= – 1648 kJ

Worked example: Calculating enthalpy changes

Answer 2: ΔHr as one mole of CO2 is formed from its elements in standard state and ΔHc as one mole of carbon is burnt in oxygen

Answer 3: ΔHneut as one mole of water is formed from the reaction of an acid and alkali

Exam Tip

The ΔHf of an element in its standard state is zero.

For example, ΔHf of O2(g) is 0 kJ mol-1

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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