An enthalpy change depends on temperature, pressure and how much of everything you used. If two chemists are going to compare results, they have to agree on all of that first. That agreement is what “standard” means — and the definitions that follow are among the most reliably examined lines in the syllabus.
📚 What you need to know
Standard conditions: pressure 100 kPa, solution concentration 1 mol dm–3, every substance in its standard state. Temperature is stated separately, usually 298 K.
Standard values are written ΔH∅, with units of kJ mol–1.
ΔH∅r — enthalpy of reaction, for the equation as written. Can be either sign.
ΔH∅f — enthalpy of formation: one mole of a compound from its elements. Either sign.
ΔH∅c — enthalpy of combustion: one mole of a substance burnt in excess oxygen. Always exothermic.
ΔH∅neut — enthalpy of neutralisation: one mole of water formed from acid and alkali. Always exothermic.
Standard conditions
Fix these four things and any two chemists measuring the same reaction should get the same number.
A standard state just means the physical state an element or compound is actually in under those conditions. Water’s standard state is liquid; oxygen’s is O2 gas; carbon’s is graphite. Getting the state symbols right is part of getting the value right — forming H2O(l) releases more energy than forming H2O(g).
A detail worth knowing: temperature is not part of the definition of a standard state. That is why values are usually quoted “at 298 K” rather than the temperature being assumed.
The four definitions
Every one of these contains the phrase one mole — but of different things. That is the whole difficulty, and the whole examinable point.
Name
Symbol
One mole of what?
Sign
Reaction
ΔH∅r
The amounts in the equation as written
Either
Formation
ΔH∅f
One mole of the compound made from its elements
Either
Combustion
ΔH∅c
One mole of the substance burnt, in excess oxygen
Always negative
Neutralisation
ΔH∅neut
One mole of water formed
Always negative
Both definitions say one mole, but of different substances. Carbon dioxide is the case where the two happen to coincide.
Notice what the carbon dioxide example shows. The same equation can be two different standard enthalpy changes at once, depending on which substance you’re counting the mole of. When a question asks you to identify a type, ask yourself: one mole of what is involved here?
A useful special case
The enthalpy of formation of an element in its standard state is zero. Forming oxygen from oxygen involves no change at all, so ΔH∅f[O2(g)] = 0. This becomes very handy in calculations later.
Note the “1” in front of CH4. A formation equation must produce exactly one mole of the compound, which often forces you to use fractions on the left-hand side.
Scaling an enthalpy change
Because ΔH values are quoted per mole, doubling the equation doubles the energy. This is the most common calculation on this page.
The quoted value is for ONE mole of Al₂O₃The equation given makes 2 moles, so scale by 2.ΔH = 2 × (−1676)ΔH = −3352 kJUnits are kJ, not kJ mol , because this is for the equation as written.
WORKED EXAMPLE
Identify each of these as ΔH∅r, ΔH∅f, ΔH∅c or ΔH∅neut.
(a) ½N₂(g) + ₃⁄₂H₂(g) → NH₃(g)One mole of a compound made from its elements — the fractions are there to force exactly 1 mol NH₃.ΔH f(b) C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)One mole of ethanol burnt completely in excess oxygen.ΔH c(c) HNO₃(aq) + KOH(aq) → KNO₃(aq) + H₂O(l)Acid + alkali making one mole of water.ΔH neut(d) CaCO₃(s) → CaO(s) + CO₂(g)Not a formation (two products), not a combustion, not a neutralisation.ΔH r
WORKED EXAMPLE
Write the equation representing the standard enthalpy of combustion of ethane, C2H6.
Exactly ONE mole of ethane must be burntSo C₂H₆ gets a coefficient of 1, and oxygen takes the fraction instead.C: 1 → 2CO₂ H: 6 → 3H₂O O needed = 4 + 3 = 7C₂H₆(g) + 3½O₂(g) → 2CO₂(g) + 3H₂O(l)Water must be liquid — that is its standard state.
💡 Exam tip
These definitions are learned, not worked out. Every one needs “one mole of…”, “under standard conditions”, and for combustion, “in excess oxygen”.
Use fractions without hesitation. A formation or combustion equation must have a coefficient of exactly 1 on the key substance.
Include state symbols. H2O(l) and H2O(g) give different values.
When you scale an equation, the answer is in kJ; a per-mole value is in kJ mol–1.
⚠️ Common mix-up
Formation counts one mole of the product; combustion counts one mole of the fuel. Same phrase, different substance.
Neutralisation counts one mole of water, not one mole of acid.
ΔHf of an element is zero, not undefined.
Combustion and neutralisation are always exothermic, so a positive answer means an error.
Standard conditions are 100 kPa, and 298 K is quoted separately rather than being part of the standard state.
Up next: Calorimetry — actually measuring these values in the lab, and understanding why your answer always comes out a bit too small.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
