Calculate Enthalpy Changes Using ΔHf⦵ (HL) (DP IB Chemistry): Revision Note
Calculate Enthalpy Changes Using ΔHf⦵
Standard Enthalpy of Formation is defined as
“The enthalpy change when one mole of a compound is formed from its elements under standard conditions”
We can use enthalpy of formation of substances to find an unknown enthalpy change using a Hess cycle
In this type of cycle the elements are always placed at the bottom of the diagram:
In this cycle the arrows will always be pointing upwards because the definition of the enthalpy of formation must go from elements to compounds
This means the Hess's Law calculation of ΔH will always be in the same arrangement
ΔHꝊr = ΣΔHfꝊ (products) - ΣΔHfꝊ (reactants)
Worked Example
Using the data in the table calculate the enthalpy change of the reaction, ΔHꝊr, for the following reaction:
4NH3 (g) + O2 (g) → 2N2 (g) + 6H2O (g)
Substance | NH3 (g) | H2O (g) | O2 (g) | 6H2O (g) |
|---|---|---|---|---|
∆Hf /kJmol-1 | -46 | -241 | 0 | 0 |
Answer:
Step 1: Find the sum of the enthalpies of formation of products
2 mol N2 (g) = 2 × 0 = 0
6 mol H2O (g) = 6 × (–241) = –1446 kJ
Step 2: Find the sum of the enthalpies of formation of reactants
4 mol NH3 (g) = 4 × (–46.0) = –184.0 kJ
3 mol O2 (g) = 3 × 0 = 0
Careful: There is no enthalpy of formation for elements as ΔHf of elements by definition is zero
Step 3: Calculate the enthalpy change
ΔHꝊr = (−1446)−(−184) = -1262 kJ mol-1
Examiner Tips and Tricks
Enthalpy of formation data are given to you in Section 13 of the Data Booklet
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