Chemistry · Thermodynamics · NEET
Δng means the change in the number of moles of GAS. You count only gaseous species. Δng = (moles of gaseous products) − (moles of gaseous reactants). Solids (s), liquids (l), and aqueous ions (aq) are ignored because their volume change is tiny. Example: for N2(g) + 3H2(g) → 2NH3(g), gas moles go from 1 + 3 = 4 to 2, so Δng = 2 − 4 = −2.
It depends on the SIGN of Δng. If Δng is positive (gas moles increase), ΔH is greater than ΔU. If Δng is negative (gas moles decrease), ΔH is less than ΔU. If Δng = 0, then ΔH = ΔU exactly. The extra term ΔngRT is the work linked to the volume change of the gas at constant pressure.
ΔH = ΔU whenever Δng = 0. This happens when the number of gas moles on both sides is the same, like H2(g) + Cl2(g) → 2HCl(g) (2 gas moles both sides). It also holds for reactions with only solids and liquids (no gas at all), because then Δng = 0. This is a very common NEET point.
Step 1: Write the balanced equation. Step 2: Add up the coefficients of only the GASEOUS products. Step 3: Add up the coefficients of only the GASEOUS reactants. Step 4: Subtract: Δng = gaseous products − gaseous reactants. Do NOT count anything that is not a gas.
Use R = 8.314 J K⁻¹ mol⁻¹ when your energy is in joules. Keep temperature in kelvin. If ΔU is given in kJ, either convert ΔngRT to kJ (divide by 1000) or convert ΔU to J first, so both terms have the same unit before adding. Mixing kJ and J is the most common mistake.
Enthalpy is defined as H = U + pV. So ΔH = ΔU + Δ(pV). For ideal gases, pV = ngRT. At constant temperature and pressure, Δ(pV) = ΔngRT. Substituting gives ΔH = ΔU + ΔngRT. This is why the formula only uses gas moles — the pV term comes from the gas.
Which amongst the following options is the correct relation between change in enthalpy and change in internal energy?
For 2A(g) + B(g) → 2D(g), ΔU° = −10 kJ mol⁻¹ and ΔS° = −44 J K⁻¹ at 298 K. Find ΔG° and the spontaneity. (R = 8.31 J mol⁻¹ K⁻¹)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is products minus reactants: Δng = (moles of gaseous products) − (moles of gaseous reactants). Always use this order to get the correct sign.
Yes. If the total gas moles are the same on both sides, Δng = 0 and ΔH = ΔU. Example: H2(g) + I2(g) → 2HI(g) has 2 gas moles on each side.
Yes, the relation ΔH = ΔU + ΔngRT is used at a single fixed temperature T in kelvin. Always convert Celsius to kelvin by adding 273.
No. ΔH is greater only when Δng is positive. When Δng is negative, ΔH is smaller than ΔU. When Δng = 0, they are equal.
NEET regularly asks you to convert between ΔH and ΔU, or to use ΔU in a Gibbs energy problem. If you skip the ΔngRT step or count non-gas moles, you get the wrong answer, as seen in the 2026 PYQ above.
