ThermodynamicsNEET Chemistry · Class 11 · NCERT Chapter 5

ΔU is the change in internal energy at constant volume. ΔH is the change in enthalpy at constant pressure (which is the heat exchanged in most lab reactions). They are related by ΔH = ΔU + Δn_g RT, where Δn_g is the change in moles of gas. For reactions with no gaseous species or no change in moles of gas, ΔH ≈ ΔU.

Write the target reaction at the top. Look at the given reactions and adjust them (reverse, multiply, or divide) so that when you add them together, you get the target. For each operation: reversing changes the sign of ΔH; multiplying by n multiplies ΔH by n. The ΔH of the target reaction is the algebraic sum of the ΔH values of the adjusted reactions.

A reaction is spontaneous when ΔG < 0 (where ΔG = ΔH − TΔS). If ΔH < 0 and ΔS > 0: always spontaneous. If ΔH > 0 and ΔS < 0: never spontaneous. If ΔH < 0 and ΔS < 0: spontaneous at low temperature (ΔH term dominates). If ΔH > 0 and ΔS > 0: spontaneous at high temperature (TΔS term dominates). The crossover temperature is T = ΔH/ΔS.

For a strong acid + strong base, the net ionic reaction is just H⁺(aq) + OH⁻(aq) → H₂O(l), releasing about −57.1 kJ/mol. When a weak acid is used (e.g., acetic acid), some energy must first be used to ionise the weak acid (ionisation is endothermic). This reduces the net heat released, making ΔH_neutralisation less negative (say −55.2 kJ/mol for acetic acid + NaOH).

ΔG° = −RT ln K. If ΔG° is negative (large negative value), K is greater than 1, meaning products are favoured at equilibrium. If ΔG° is positive, K is less than 1, reactants are favoured. At equilibrium, ΔG = 0 (not ΔG°). ΔG° is the standard Gibbs energy change (all species at 1 bar, 298 K); ΔG = 0 describes the equilibrium condition under actual concentrations.

A positive ΔS means the products are more disordered (have more randomness) than the reactants. This happens when: the number of moles of gas increases in the reaction; a solid dissolves into ions; a liquid evaporates; a solid melts. ΔS > 0 makes ΔG more negative, favouring spontaneity, especially at high temperatures.