Syllabus Edition

First teaching 2014

Last exams 2024

|

Standard Enthalpy Change (DP IB Chemistry: SL)

Revision Note

Test yourself
Stewart

Author

Stewart

Last updated

Standard Enthalpy Change

Standard conditions

  • To compare the changes in enthalpy between reactions, all thermodynamic measurements are made under standard conditions
  • These standard conditions are:
    • A pressure of 100 kPa
    • A concentration of 1 mol dm-3 for all solutions
    • Each substance involved in the reaction is in its standard state (solid, gas or liquid)

  • Temperature is not part of the definition of standard state, but a temperature of 298 K (25 oC) is usually given as the specified temperature
  • To show that a reaction has been carried out under standard conditions, the symbol is used
    • Eg. ΔH = the standard enthalpy change

Standard Enthalpies

  • There are a few Standard Enthalpy changes which are used commonly in energy calculations and they are summarised below:

    Chemical Energetics Standard Enthalpies (1), downloadable AS & A Level Chemistry revision notes

 

  • Practice your understanding of enthalpy changes on the following worked examples:

Worked example

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

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

Calculate ΔHr for the reaction below:

2H2 (g) + O2 (g)  2H2O (l)

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

Worked example

Calculate ΔHr for the reaction below

4Fe (s) +O2 (g)    2Fe2O3 (s)

given that ΔH[Fe2O3 (s)]  = - 824 kJ mol-1

Answer:

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

ΔHf =  2 mol x ( -824 kJ mol-1)

= - 1648 kJ

Worked example

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 (l)

 

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 of an acid and alkali

Examiner Tip

You need to learn well the Standard Enthalpy change definitions as they are frequently tested in exam papers

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?

Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.