Enthalpy Change - Definitions (Edexcel International A Level Chemistry)

Revision Note

Richard

Author

Richard

Last updated

Enthalpy Change - Definitions

  • To be able to compare the changes in enthalpy between reactions, all thermodynamic measurements are carried out under standard conditions
  • These standard conditions are:
    • A pressure of 100 kPa (you may see some older exam questions that use a figure of 101 kPa; the exact figure is 101 325 Pa, but it has been simplified in the current syllabus for problem-solving purposes)
    • A temperature of 298 K (25 oC)
    • Each substance involved in the reaction is in its standard physical state (solid, liquid or gas)

  • To show that a reaction has been carried out under standard conditions, the symbol ‘Ꝋ’ is used
    • ΔH = the standard enthalpy change

  • There are a number of standard definitions relating to enthalpy changes that you are expected to know

Enthalpy Definitions Table

Chemical Energetics Enthalpy Definition Table, downloadable AS & A Level Chemistry revision notes

  • A further definition that is expected knowledge, although less used at AS level, is:
    • Standard enthalpy of atomisation - the enthalpy change when one mole of gaseous atoms is formed from an element in its standard state, under standard conditions

Examiner Tip

You will see various enthalpy change symbols used with subtle changes, e.g.

ΔHc or ΔcH for enthalpy of combustion

Whichever symbol you use must have the following basic points:

  • Δ to represent change
  • H to represent enthlalpy
  • Ꝋ to respresent standard conditions
  • A symbol to represent the type of enthalpy change occurring, e.g.
    • c for combustion
    • f for formation
    • neut for neutralisation
    • r for reaction

Worked example

Calculating the enthalpy change of reaction

One mole water is formed from hydrogen and oxygen, releasing 286 kJ of energy

H2 (g) + ½O2 (g) H2O (I)      ΔHr = -286 kJ mol-1

Calculate ΔHrfor the reaction below:

2H2 (g) + O2 (g) 2H2O (I)

Answer

    • 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

Calculate ΔHf for the reaction below, given that ΔHf [Fe2O3(s)] = -824.2 kJ mol-1

4Fe(s) + 3 O2(g) → 2 Fe2O3(s)

Answer

    • 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

Identify each of the following as ΔHr, ΔHf, ΔHc or ΔHneut

  1. MgCO3 (s) MgO (s) + CO2 (g)
  2. C (graphite) + O2 (g) → CO2 (g)
  3. HCl (aq) + NaOH (aq) NaCl (aq) + H2O (I)

Answer

Answer 1: ΔHr

Answer 2: ΔHf 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 between an acid and an alkali

Examiner Tip

The ΔHfof an element in its standard state is zero.

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

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

Did this page help you?

Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.