Chem notesPhysicalThermodynamics
Independent topic page
Declare system, sign convention and path/state-function status first; then energy, entropy and spontaneity questions become controlled balances.
01. Using chemistry convention, ΔU=q+w and pressure–volume work on the system is w=−∫P_ext dV; expansion gives negative work.
02. At constant volume q_v=ΔU; at constant pressure with only PV work q_p=ΔH. Neither equality is universal without its conditions.
03. Hess's law works because enthalpy is a state function. Formation, combustion, bond-enthalpy and lattice cycles are alternative paths to one ΔH.
04. Spontaneity at constant T,P is ΔG<0, but rate is unrelated. Entropy accounting requires system plus surroundings or the equivalent Gibbs criterion.
05. Chemistry sign convention: ΔU = q + w with expansion work negative; stamp the convention once and never flip mid-problem.
06. q and w are path functions; U, H, S and G are state functions — cycles and Hess routes exist to exploit that difference.
07. Spontaneous (ΔG < 0 at constant T,P) is not fast; equilibrium is ΔG = 0 for the process as written, while ΔG° = −RT ln K links standard free energy to K.
Official · 2026
Official syllabus points for Chemical Thermodynamics only — not the full Physical portion map. Portion hub: Physical · Physical analysis.
No topic-specific exclusions flagged — finish the unit points below for Thermodynamics.
Source: https://jeeadv.ac.in/documents/jee-advanced-2026-syllabus.pdf
Exam focusHess, ΔG criteria, entropy, lattice enthalpy.
Syllabus points
Do / check
2020–2026 paper weight
Topic match inside parent portion Physical (49.8% of Chemistry tags). Stats below are portion backdrop; matched subtopics show where Chemical Thermodynamics sits.
Portion-wide type/difficulty mix lives on the Physical analysis — not repeated here so this page stays on Thermodynamics.
01. Dominant subtopics: Mixed (36), Mole/Gaseous (24), Electrochemistry (14), Thermodynamics (11).
Cross-topic stems
Advanced rarely asks a chapter in isolation. These are tagged multi-idea Chemistry stems that touch Chemical Thermodynamics — practise the knowledge bridge, not only the single-chapter drill.
Step 1: resolve the Organic linked numerical using reaction stoichiometry. Step 2: switch to thermo/mixed Physical NUMs with fresh variables. Step 3: keep metallurgy facts (if present) as recall, not calculation, items.
Branch switching inside one NUM section.
Label each Chem NUM as Org/Phy/Inorg in the margin before solving.
Why methods matter
State functions, signs and process constraints must be fixed before numbers are substituted.
Fixes the physical sign before algebra obscures it.
Do thisWrite expansion/compression and heat in/out before numbers.
Fixes the physical sign before algebra obscures it.
Expansion against positive P_ext: ΔV>0, so w=−P_extΔV<0; heat absorbed means q>0.
Prevents treating heat and work as state functions.
Do thisCompute ΔU/ΔH from endpoints, then derive q or w using the specified path.
Prevents treating heat and work as state functions.
For an isothermal ideal gas ΔU=0 on every path, but reversible and free-expansion work differ.
Avoids hidden sign and stoichiometric multipliers.
Do thisReverse/scale equations visibly and apply the same operations to ΔH.
Avoids hidden sign and stoichiometric multipliers.
Reversing combustion changes the sign of ΔH; halving the equation halves ΔH.
Determines all-T, no-T, high-T or low-T behaviour immediately.
Do thisClassify signs of ΔH and ΔS before solving ΔG=0.
Determines all-T, no-T, high-T or low-T behaviour immediately.
ΔH>0, ΔS>0 is spontaneous only at T>ΔH/ΔS, with compatible units.
Processed examples
Read the impact line first. Then notice which family/model was chosen. The solution is written the way your rough sheet should look in the hall.
For an ideal-gas reaction at 300 K, ΔU=10.0 kJ mol⁻¹ and Δn_g=2. Find ΔH.
This stem rewards early recognition of the state-function differentials model.
Model chosenUse ΔH=ΔU+Δn_gRT.
Answer15.0 kJ mol⁻¹
Takeaway: Keep Δn_g restricted to gases.
A reaction has ΔH=40 kJ mol⁻¹ and ΔS=100 J K⁻¹ mol⁻¹. When is it spontaneous?
This stem rewards early recognition of the entropy and spontaneity model.
Model chosenSolve ΔH−TΔS<0.
AnswerT>400 K
Takeaway: Convert entropy units before comparing.
100 g ice at 0 °C is mixed adiabatically with 100 g water at 80 °C. Use cwater=4.2 J g⁻¹ K⁻¹ and ΔHfus=336 J g⁻¹. Find final temperature.
This Advanced-level variation couples hidden constraints with the calorimetry and phase change model.
Model chosenTest whether hot-water cooling can melt all ice.
Answer0 °C, with all ice just melted
Takeaway: Check phase feasibility before solving for temperature.
Reusable models
Advanced stems cluster. Name the family in 10–15s, write the fast model, then run the same path. Do not invent a new plan each time.
Looks likeUnknown reaction enthalpy from several equations
Fast modelTreat reactions as algebraic vectors.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likeMixing, heating or latent heat in an insulated vessel
Fast modelSet total heat exchanged to zero with piecewise q.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likeTemperature range or feasibility from ΔH and ΔS
Fast modelUse ΔG=ΔH−TΔS with consistent units.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Looks likeRelations among ΔU, ΔH, q and w
Fast modelFix the chemistry sign convention before substitution.
Recognition fixes the model before arithmetic begins.
Do not start blind expansion or casework before identifying the governing model.
Hall timing
Underline data, units, phase/standard-state labels and the exact quantity asked. Reject any formula whose assumptions are already violated.
Write the governing relation symbolically before substitution; define one basis, reference state, or extent variable if the stem mixes quantities.
Keep ratios and logarithms exact as long as possible. Carry signs, powers of ten and units on the rough sheet instead of repairing them later.
Check dimensions, limiting cases and chemical sense. For multi-correct, test every option independently rather than forcing one common conclusion.
After the model is locked
ΔU=q+w; w_PV=−∫P_ext dV
Closed-system energy accounting, chemistry convention
w=−nRT ln(V₂/V₁)=−nRT ln(P₁/P₂)
Reversible isothermal ideal-gas path
ΔH=ΔU+Δ(PV); ideal-gas reaction: ΔH=ΔU+Δn_g RT
Relating reaction U and H at same T
q=mcΔT; q_cal=C_calΔT; q_system=−q_surroundings
Heat capacities and calorimeters
ΔH°_rxn=ΣνΔH°_f(products)−ΣνΔH°_f(reactants)
Standard formation data
ΔH≈ΣD(bonds broken)−ΣD(bonds formed)
Gas-phase average bond enthalpies
ΔG=ΔH−TΔS; ΔS_univ=ΔS_sys+ΔS_surr
Spontaneity at fixed T,P or universe criterion
ΔG=ΔG°+RT ln Q; ΔG°=−RT ln K
Nonstandard direction and equilibrium
Make it automatic