Chemistry · Thermodynamics · NEET
It is the enthalpy change when ONE mole of a substance burns completely in oxygen, and all reactants and products are in their standard states (pure form, 1 bar pressure, stated temperature, usually 298 K). Example from NCERT: C4H10(g) + 13/2 O2(g) -> 4CO2(g) + 5H2O(l), ΔcH° = -2658 kJ/mol. The value is per one mole of the fuel (butane here), not per mole of O2 or CO2.
Burning always RELEASES heat to the surroundings. Any reaction that gives out heat is exothermic, and exothermic means ΔH is negative. So ΔcH° is always a negative number. NCERT states plainly: 'Combustion reactions are exothermic in nature.' If you ever write a positive combustion enthalpy, it is wrong.
For STANDARD enthalpy of combustion, water is taken as LIQUID, H2O(l), because liquid water is the standard state at 298 K and 1 bar. This is a common NEET trap. Look at NCERT: butane gives 5H2O(l), glucose gives 6H2O(l) - both liquid. If water were gas, extra heat of vaporization would be missing, so the number would be less negative.
Enthalpy of FORMATION is for making 1 mole of a compound FROM its elements in their reference states. Enthalpy of COMBUSTION is for BURNING 1 mole of a substance in oxygen to give oxides (CO2 and H2O). They are different reactions. For example, forming CH4 from C and H2 is formation; burning CH4 in O2 is combustion. NEET often gives you combustion data and asks you to find formation using Hess's law.
Yes, it matters a lot. Standard enthalpy of combustion needs COMPLETE combustion, so carbon must become CO2 (not CO) and hydrogen must become H2O(l). If the product is CO (incomplete burning) or soot (C), it is NOT the standard enthalpy of combustion. Always balance to give CO2(g) and H2O(l).
It is per mole of the SUBSTANCE that is burning (the fuel), not per mole of O2. So for glucose C6H12O6, ΔcH° = -2802 kJ/mol means 2802 kJ is released when 1 mole of glucose burns, even though 6 moles of O2 are used. Always link the value to '1 mole of the fuel'.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The symbol is ΔcH° (delta-c-H with a standard-state circle). The small 'c' stands for combustion and the circle (⊖) means standard state at 1 bar.
kilojoules per mole (kJ/mol or kJ mol⁻¹), because it is defined per one mole of the burning substance.
From NCERT: butane ΔcH° = -2658 kJ/mol and glucose ΔcH° = -2802 kJ/mol. Both are negative because combustion releases heat.
No. Combustion always releases heat, so ΔcH° is always negative. A positive value would mean the reaction absorbs heat, which combustion never does.
It lets you use Hess's law to calculate hard-to-measure quantities, like the enthalpy of formation of a fuel, from easily measured combustion heats. NCERT's benzene problem does exactly this.
