Thermodynamics basics

System and surroundings

System = the part you study. Surroundings = everything else that can exchange energy or matter with it.

Open: matter and energy can cross. Closed: energy can cross, matter cannot. Isolated: neither.

Extensive (depends on amount): mass, V, U, H, S. Intensive (does not): T, P, density.

State function: depends only on start and end (U, H, S, G). Path function: depends on the path (q, w).

Process labels

Isothermal: T constant. Isobaric: P constant. Isochoric: V constant. Adiabatic: no heat exchange (q = 0).

Reversible (ideal textbook path): system always nearly at equilibrium; driving and opposing forces differ by a tiny amount. Real lab changes are irreversible.

First law

ΔU = q + w. Sign convention in most Indian Class 11 books: heat absorbed by the system is +, work done on the system is +. Confirm the sign rule your board uses for expansion work.

Common expansion work form: w = −P_ext ΔV (when that is the convention in your text).

Internal energy U is a state function. For an ideal gas, U depends only on temperature.

Enthalpy and heat capacity

H = U + PV. At constant pressure, q_p = ΔH.

For ideal gases: ΔH = ΔU + Δn_g RT (Δn_g = change in moles of gas).

C_v = dU/dT, C_p = dH/dT. For an ideal gas: C_p − C_v = R.

Hess’s law: if a reaction is written as steps, ΔH_overall = sum of ΔH of steps. Same start and end → same ΔH, whatever path.

Entropy and Gibbs energy

Entropy S measures dispersal / disorder. For the same substance, gas > liquid > solid is the usual order students quote.

Second law (exam line): entropy of the universe increases for a spontaneous process.

ΔG = ΔH − TΔS. Spontaneous (at constant T, P) when ΔG < 0. Equilibrium when ΔG = 0.

Exothermic helps (ΔH negative), but a big negative ΔS can still block spontaneity at high T — and the reverse pattern can make endothermic processes go when TΔS wins.

What trips people in papers

Mixing up ΔU and ΔH, or forgetting Δn_g for gas reactions.

Treating q and w as state functions.

Saying “exothermic ⇒ always spontaneous” without looking at ΔS and T.

Hess’s law questions: flip equations and scale ΔH the same way you scale the equation.