Worked example: finding an unknown enthalpy change of formationHess's law can be used backwards. If ΔrHθ is known, the usual expression can be rearranged to find one missing ΔfHθ.The first stage in making nitric acid is:4NH3(g) + 5O2(g) → 4NO(g) + 6H2O(g) ΔrHθ = -905.2 kJ mol-1 Data: ΔfHθ[NH3(g)] = -46.1, ΔfHθ[H2O(g)] = -241.8 kJ mol-1. Find ΔfHθ[NO(g)].Step 1. Write the usual expression, with x for the unknown:ΔrHθ = [4x + 6(-241.8)] - [4(-46.1) + 5(0)] Step 2. Work out the known sums: 6(-241.8) = -1450.8 and 4(-46.1) = -184.4.-905.2 = 4x - 1450.8 + 184.4 Step 3. Rearrange:4x = -905.2 + 1450.8 - 184.4 = 361.2
x = +90.3 kJ mol-1 Nitrogen monoxide has a positive enthalpy change of formation, so it is energetically less stable than its elements.Notice that the water here is H2O(g), not H2O(l). Its ΔfHθ is less negative than that of liquid water (-285.8 kJ mol-1), so always use the value for the state shown in the equation.
Hess's Law Example 10
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Specification: Edexcel Chemistry (8CH0), AS-level, Topic 8: Energetics I. Topic: Enthalpy.
Key terms in this lesson
- enthalpy change of formation
- The enthalpy change when one mole of a compound is formed from its elements in their standard states, under standard conditions.
- hess's law
- The principle that the total enthalpy change for a reaction is the same regardless of the route taken, provided the starting and finishing conditions are the same.
More in Enthalpy
- Hess's Law Example 2
- Hess's Law Example 3
- Hess's Law Example 4
- Hess's Law Example 5
- Hess's Law Example 6
- Hess's Law Example 7
- Hess's Law Example 8
- Hess's Law Example 9
All 21 lessons in Enthalpy · All Edexcel AS-level Chemistry topics