Chemistry · Thermodynamics · NEET
It is a balanced chemical equation written together with the value of its enthalpy change, ΔrH°. Example: C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l), ΔrH° = −1367 kJ mol⁻¹. The chemical equation alone tells you what reacts; adding ΔrH° tells you how much heat is released or absorbed. For NEET, if the ΔrH value is missing, it is NOT a thermochemical equation.
Because the heat change depends on the state of each substance. Water as a liquid and water as a gas do not have the same energy. For example, forming H2O(l) releases more heat than forming H2O(g), because turning liquid into vapour needs extra energy. If you write the wrong state, the ΔrH value becomes wrong. NCERT tells you to also mention the allotropic form, like C(graphite) instead of just C.
It means the number of MOLES, never the number of molecules. So 2H2 means 2 moles of hydrogen gas, not 2 molecules. This matters because the ΔrH value is tied to exactly those mole amounts. If the equation says 2 moles react and releases 480 kJ, then 1 mole would release 240 kJ.
The numerical ΔrH° value refers to the exact number of moles shown in that balanced equation, and the unit is written as kJ mol⁻¹. So for 2H2(g) + O2(g) → 2H2O(g), ΔrH° = −483.64 kJ means that amount of heat is released when 2 moles of water form (not 1 mole). Always read the equation carefully to know how many moles the ΔrH belongs to.
A negative sign (−) means the reaction is exothermic: it releases heat to the surroundings. A positive sign (+) means the reaction is endothermic: it absorbs heat. For example ΔrH° = −1367 kJ mol⁻¹ for ethanol combustion means heat is given out. Getting the sign wrong is one of the most common NEET mistakes.
A normal chemical equation only shows reactants → products, balanced by atoms. A thermochemical equation adds three extra things: the ΔrH value, the physical state of every substance, and the meaning that coefficients are moles tied to that heat value. So every thermochemical equation is a chemical equation, but not every chemical equation is thermochemical.
Consider the following reaction: 2H2(g) + O2(g) → 2H2O(g), ΔrH° = −483.64 kJ. What is the enthalpy change for the decomposition of one mole of water?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Moles, never molecules. NCERT states this clearly. 2H2 means 2 moles of hydrogen, and the ΔrH value is tied to those exact mole amounts.
kJ mol⁻¹ (kilojoule per mole). The value corresponds to the number of moles shown in the balanced equation.
Because vapour has extra energy stored to keep it as a gas. Condensing that vapour to liquid releases more energy, so H2O(l) formation has a more negative ΔrH than H2O(g). This is why states must be written.
No. Fractions are allowed in thermochemical equations because coefficients are moles, and half a mole is a real amount. For example ½H2 + ½Cl2 → HCl is valid.
The magnitude stays the same but the sign flips. If forward is exothermic (−), the reverse is endothermic (+). This is covered fully in reversing and scaling thermochemical equations.
