IB DP Chemistry: SL复习笔记5.1.2 Standard Enthalpy Change

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:
    • pressure of 100 kPa
    • 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:1.5-Chemical-Energetics-Standard-Enthalpies-1
  • 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)   H2O (l)


    • 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 ΔHꝊ [Fe2O3 (s)]  = - 824 kJ mol-1


    • 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. MgCO(s)  MgO (s) + CO2 (g)
  2. C (graphite) + O2 (g)  CO2 (g)
  3. HCl (aq) + NaOH (aq)   NaCl (aq) + H2O (l)


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

Exam Tip

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