Chemistry · Thermodynamics · NEET
Δn_g is short for "change in moles of gas." Take the balanced equation. Add up the moles (the big numbers in front) of every GAS on the right side (products). Then add up the moles of every GAS on the left side (reactants). Subtract: Δn_g = (moles of gaseous products) − (moles of gaseous reactants). That single number is Δn_g. NEET uses it in the formula ΔH = ΔU + Δn_g RT, so getting Δn_g right is the first step in many enthalpy questions.
No. This is the most common mistake. Δn_g counts ONLY substances marked (g) for gas. You completely skip anything marked (s) for solid, (l) for liquid, or (aq) for aqueous (dissolved). Example: for 2NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(g), the solids are ignored. On the product side you have 2 moles of gas (CO₂ + H₂O), on the reactant side 0 moles of gas. So Δn_g = 2 − 0 = +2.
Step 1: gaseous products = 2 moles of D = 2. Step 2: gaseous reactants = 2 moles A + 1 mole B = 3. Step 3: Δn_g = products − reactants = 2 − 3 = −1. This is exactly the value used in NEET 2026: Δn_g = 2 − (2 + 1) = −1. A negative Δn_g means the number of gas molecules went down.
Yes, all three. Positive Δn_g: more gas is made than used, e.g. CaCO₃(s) → CaO(s) + CO₂(g) gives Δn_g = +1. Negative Δn_g: gas is consumed, e.g. 2Cl(g) → Cl₂(g) gives Δn_g = 2 − ... wait, 1 − 2 = −1. Zero Δn_g: gas moles are equal on both sides, e.g. H₂(g) + Cl₂(g) → 2HCl(g) gives 2 − 2 = 0. When Δn_g = 0, ΔH = ΔU.
Because ΔH and ΔU are linked by ΔH = ΔU + Δn_g RT. The whole difference between enthalpy change and internal-energy change for a gas reaction is that Δn_g RT term. If you count Δn_g wrong (for example by counting a solid), your ΔH will be wrong. NEET regularly asks you to convert ΔU to ΔH first, then find ΔG — and Δn_g is the very first number you need.
Δn_g is a pure number of moles (like +1, −1, +2). It has the unit "mol." When you multiply Δn_g × R × T, you get energy: mol × (J mol⁻¹ K⁻¹) × K = J. So Δn_g RT comes out in joules (or kilojoules), which is why it can be added to ΔU to give ΔH.
For the reaction 2A(g) + B(g) → 2D(g), ΔU° = −10 kJ mol⁻¹ and ΔS° = −44 J K⁻¹ at 298 K. Identify the correct ΔG° and the spontaneity at 298 K. (R = 8.31 J mol⁻¹ K⁻¹)
Which amongst the following options is the correct relation between change in enthalpy and change in internal energy?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Δn_g counts only GAS moles. Total moles would also include solids, liquids, and dissolved substances. For thermodynamics you use gas moles only, because only gases do meaningful pressure–volume work in these equations.
Gaseous products = 2 (2 HCl). Gaseous reactants = 1 + 1 = 2. Δn_g = 2 − 2 = 0. Because Δn_g = 0, ΔH = ΔU for this reaction.
No. Δn_g comes only from the balanced equation coefficients, so it is a fixed whole number. Temperature appears separately in the term Δn_g RT, but it does not change Δn_g itself.
A negative Δn_g means the reaction makes fewer gas molecules than it started with. Fewer gas molecules usually means less disorder, so such reactions often have a negative entropy change (ΔS < 0) too, as seen in 2Cl(g) → Cl₂(g).
Δn_g itself is just a number of moles. It becomes energy only after multiplying by R and T. If you use R = 8.314 J mol⁻¹ K⁻¹, the Δn_g RT term comes out in joules, so divide by 1000 to add it to a ΔU given in kJ.
