1.6.5 Hess' Law

Using Hess' Law

Calculating ΔHr from 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 + ΔHr

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) + O(I)

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

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

Calculating average bond energies using Hess’s cycles

• Bond energies cannot be found directly so enthalpy cycles are used to find the average bond energy
• This can be done using enthalpy changes of atomisation and combustion or formation
• The enthalpy change of atomisation (ΔHat ) is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.
• Eg. ΔHat [H2] relates to the equation:

½ H2(g) → H(g)

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