Hess' Law (OCR AS Chemistry A) : Revision Note

Calculating the enthalpy change of reaction

Calculate the ΔH_f for the following reaction:

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

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

Answer

Step 1: Write the balanced equation at the top

Step 2: Draw the cycle with the elements at the bottom

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

Step 4: Apply Hess’s Law

• ΔH_r = ΔH₂ - ΔH₁ 

  • ΔH₂ = ΔH_f [Na₂CO₃ (s)] + ΔH_f [CO₂ (g)] + ΔH_f [H₂O (l)]

  • ΔH₁ = 2 x ΔH_f [NaHCO₃ (s)]

  • ΔH_r = ((-1130.7) + (-393.5) + (-285.8)) - (2 x (-950.8))

  • ΔH_r = +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

Calculating the enthalpy change of formation of ethane

Calculate ΔH_f [ethane]. The relevant change in standard enthalpy of combustion (ΔH_c) 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

Step 2: Draw the cycle with the combustion products at the bottom

Step 3: Draw all arrows in the correct direction

Step 4: Apply Hess’s Law

• ΔH_f = ΔH₁ - ΔH₂ 

  • ΔH₂ = ΔH_c [C₂H₆ (g)] 

  • ΔH₁ = 2 x ΔH_c [C_graphite (s)] + 3 x ΔH_c [H₂ (g)]

  • ΔH_r = (2 x (-393.5) + 3 x (-285.8)) - (-1559.7)

  • ΔH_r = -84.7 kJ mol^-1 

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, ΔH_f

  • Δ_rH = ∑ΔH_f(products) - ∑ΔH_f(reactants)

• Using enthalpy changes of combustion, ΔH_c

  • Δ_rH = ∑ΔH_c(reactants) - ∑ΔH_c(products)