Hess' Law (OCR A Level Chemistry)

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Hess’ Law & Enthalpy Cycles

Calculating ΔHfrom ΔHf using Hess’s Law energy cycles

  • The products can be directly formed from the elements = ΔH2

   OR

  • The products can be indirectly formed from the elements = ΔH1 + ΔHrChemical Energetics Direct and Indirect Routes, downloadable AS & A Level Chemistry revision notes

The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route)

  • Equation

ΔH2 = ΔH1 + ΔHr

   Therefore,

ΔHr = ΔH2 – ΔH1

 

Worked example

Calculating the enthalpy change of reaction

Calculate the ΔHf for the following reaction:

   2NaHCO3 (s)   Na2CO3 (s) + CO2 (g) + H2O (I)

The table below shows the standard enthalpy of formations (ΔHf) relevant to this reaction:

Answer

Step 1: Write the balanced equation at the topChemical Energetics Step 1 - Calculating the enthalpy change of formation, downloadable AS & A Level Chemistry revision notes

Step 2: Draw the cycle with the elements at the bottomChemical Energetics Step 2 - Calculating the enthalpy change of formation, downloadable AS & A Level Chemistry revision notes

Step 3: Draw in all arrows, making sure they go in the correct directions. Write the standard enthalpy of formations

Chemical Energetics Step 3 - Calculating the enthalpy change of formation_1, downloadable AS & A Level Chemistry revision notes

Step 4: Apply Hess’s Law

    • ΔHr = ΔH2 - ΔH1 
      • ΔH2 = ΔHf [Na2CO3 (s)] + ΔHf [CO2 (g)] + ΔHf [H2O (l)]
      • ΔH1 = 2 x ΔHf [NaHCO3 (s)]
    • ΔHr = ((-1130.7) + (-393.5) + (-285.8)) - (2 x (-950.8))
    • ΔHr = +91.6 kJ mol-1 

  • You must make sure that you can apply Hess' Law effectively and calculate enthalpy changes in different situations
  • Remember - it is the data that is important
  • Check whether the data you have been given is formation data or combustion data, and then complete the cycle or calculation according to that

Worked example

Calculating the enthalpy change of formation of ethane

Calculate ΔH[ethane]. The relevant change in standard enthalpy of combustion (ΔHc) values are shown in the table below:

Answer

Step 1: Write the equation for enthalpy change of formation at the top and add oxygen on both sides

Chemical Energetics Step 1 - Calculating the enthalpy change of formation of ethane, downloadable AS & A Level Chemistry revision notes

Step 2: Draw the cycle with the combustion products at the bottomChemical Energetics Step 2 - Calculating the enthalpy change of formation of ethane, downloadable AS & A Level Chemistry revision notes

Step 3: Draw all arrows in the correct direction

Chemical Energetics Step 3 - Calculating the enthalpy change of formation of ethane, downloadable AS & A Level Chemistry revision notes

Step 4: Apply Hess’s Law

    • ΔHf = ΔH1 - ΔH2 
      • ΔH2 = ΔHc [C2H6 (g)] 
      • ΔH1 = 2 x ΔHc [Cgraphite (s)] + 3 x ΔHc [H2 (g)]
    • ΔHr = (2 x (-393.5) + 3 x (-285.8)) - (-1559.7)
    • ΔHr = -84.7 kJ mol-1 

Examiner Tip

There are two rules that you can use to help with your calculations using enthalpy changes of formation and combustion

  • Using enthalpy changes of formation, ΔHf
    • ΔrH = ∑ΔHf(products) - ∑ΔHf(reactants)
  • Using enthalpy changes of combustion, ΔHc
    • ΔrH = ∑ΔHc(reactants) - ∑ΔHc(products)

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.