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Thermodynamics
Chemistry · Class 11NCERT pp. 136–1642 sets × 25 questions
Key facts and formulas for Thermodynamics, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.
Formula sheet for this chapter
| Idea | Rule or fact |
|---|
| First law | ΔU = q + w (IUPAC: heat absorbed and work done on system are positive) |
| Expansion work | w = −p_ex ΔV; free expansion into vacuum gives w = 0 |
| Isothermal reversible | w = −2.303 n R T log(V₂/V₁); ΔS = n R ln(V₂/V₁); ΔU = ΔH = 0 for ideal gas |
| ΔH and ΔU | ΔH = ΔU + Δn_g R T; C_p − C_V = R; ΔU = n C_V ΔT |
| Heat and calorimetry | q = m c ΔT = C ΔT; Kirchhoff: ΔH(T₂) = ΔH(T₁) + Δ_r C_p (T₂ − T₁) |
| Enthalpy from data | Δ_r H = Σ Δ_f H(products) − Σ Δ_f H(reactants); Hess: add steps, scale with the equation |
| Bond and lattice | Δ_r H = Σ bonds broken − Σ bonds formed; Δ_sol H = Δ_lattice H + Δ_hyd H |
| Entropy | ΔS_surr = −ΔH / T; solid < liquid < gas; at phase change ΔS = ΔH / T |
| Gibbs energy | ΔG = ΔH − TΔS; spontaneous if ΔG < 0; equilibrium ΔG = 0; maximum useful work = −ΔG |
| Gibbs and K | ΔG⊖ = −2.303 R T log K; ΔG = ΔG⊖ + 2.303 R T log Q |
Three sample questions
Q1. A well-insulated, sealed flask that allows no exchange of matter or energy is best described as
- an open system
- an isolated system
- a closed system
- an adiabatic wall
Hint and solution
Hint: Ask what crosses the boundary. If neither matter nor energy can cross, which type of system is it?
- Open: matter + energy; Closed: energy only; Isolated: neither — Systems are classified by what can cross the boundary.
- Sealed flask: no matter transfer; insulated: no heat transfer — Sealed stops matter, and insulation stops heat. The flask is also rigid, so no work either.
- Neither matter nor energy crosses, so the system is isolated — This matches the definition of an isolated system, so closed and open are ruled out.
Answer: B. With neither matter nor energy crossing the boundary, the system is isolated. A closed system still exchanges energy.
Q2. Consider the statements. I: Internal energy is a state function. II: Heat absorbed by a system is a state function. III: Work done depends on the path. Which are correct?
- I and II only
- II and III only
- I, II and III
- I and III only
Hint and solution
Hint: A state function depends only on initial and final states. Ask which of U, q and w pass this test.
- State function: depends only on initial and final state — U, H, S, G, p, V and T are state functions. Their change does not depend on the route.
- q and w are path functions — Heat and work depend on how the process is carried out, so statement II is false.
- I true, II false, III true — Statements I and III are correct. Only the option with exactly I and III fits.
- Answer: I and III only — So the correct choice is I and III only. Option D.
Answer: D. U is a state function. q and w depend on the path, so II is wrong.
Q3. A system absorbs 30 kJ of heat and 50 kJ of work is done on it. ΔU is
- +80 kJ
- +20 kJ
- −20 kJ
- −80 kJ
Hint and solution
Hint: Use ΔU = q + w. Heat absorbed and work done on the system are both positive in the IUPAC convention.
- ΔU = q + w — This is the first law in the IUPAC sign convention.
- q = +30 kJ (absorbed), w = +50 kJ (done on system) — Energy flowing into the system is positive for both.
- ΔU = 30 + 50 = +80 kJ — Add the two inputs. Internal energy rises by 80 kilojoules. Option A.
Answer: A. Both are positive in the IUPAC convention: ΔU = 30 + 50 = +80 kJ.
How to use this chapter
Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.