Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- The standard enthalpy of formation, ΔH°f, is the enthalpy change when 1 mol of a substance forms from its elements in their standard states, under standard conditions (1 bar, data usually at 298 K).
- The standard state of an element is its most stable form: O₂(g), N₂(g), H₂(g), C(graphite), Br₂(l), Fe(s) and so on. For these, ΔH°f = 0.
- Other forms of an element are not zero: O₃(g) and C(diamond) have positive values.
- A table of ΔH°f values lets you calculate ΔH° for any reaction or physical change between listed substances.
Key relationships
| Idea | Rule |
|---|---|
| Reaction enthalpy | ΔH°rxn = ΣnΔH°f(products) − ΣnΔH°f(reactants) |
| Coefficients | multiply each ΔH°f by its coefficient n |
| Element in standard state | ΔH°f = 0 |
| Formation equation | exactly 1 mol of product, so fractions such as ½N₂ are allowed |
| Physical change | same formula, e.g. H₂O(l) → H₂O(g): +44.0 kJ mol⁻¹ |
| Unknown ΔH°f | rearrange the formula and solve for the missing value |
Assumptions behind the numbers
- All values refer to the same standard conditions, so differences between them are meaningful.
- ΔH° is per mole of the reaction as written; double the equation and ΔH° doubles.
- Each substance is in the state shown in the equation, and the table value for that state is used.
Mistakes to avoid
- Reactants − products. Wrong sign.
- Ignoring coefficients. 6H₂O means 6 × ΔH°f.
- Wrong state. H₂O(l) −285.8, H₂O(g) −241.8 kJ mol⁻¹.
- Non-zero value for O₂(g), H₂(g) or C(graphite), or zero for O₃.
- Losing a minus sign when subtracting a negative total. Use brackets.
- Confusing with bond enthalpies, where it is broken − formed and the reactants come first.
Quick self-check
- What is ΔH°f of N₂(g)? (0, because it is an element in its standard state.)
- Using NO(g) +90.3 and NO₂(g) +33.2 kJ mol⁻¹, find ΔH° for 2NO(g) + O₂(g) → 2NO₂(g). (66.4 − 180.6 = −114.2 kJ mol⁻¹.)
- Write the equation whose ΔH° is ΔH°f of CO₂(g). (C(graphite) + O₂(g) → CO₂(g).)
Next: practice questions.