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Chem notesPhysicalChemical Equilibrium

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Chemical Equilibrium

Write the balanced equilibrium expression, compare Q with K, and solve one extent table—direction and composition stop being guesswork, including thermo–equilibrium joins.

Portion PhysicalFocus High-yield numericals — often paired with thermodynamics (ΔG°=−RT ln K) in Advanced multi-concept stemsTarget 45–75sPortion share 49.8%Trend Falling share
Equilibrium constants encode a particular balanced reaction at a particular temperature. Advanced stems exploit reversed/scaled equations, inert gas conditions, coupled equilibria, degree of dissociation and ΔG°/K links. Treat thermo and equilibrium as one language: Q, K and ΔG. Richer bank: treat every disturbance as a Q-vs-K audit — recompute activities after the change, leave K alone unless T changed, then decide direction. Coupled equilibria and α tables reward the same habit.

How Advanced asks this

  • K_c/K_p from equilibrium composition or degree of dissociation
  • Direction prediction using Q
  • Reverse, multiply or add reactions and transform K
  • Pressure, volume, inert-gas or temperature perturbation
  • Coupled equilibria and ΔG° ↔ K thermodynamic join

Traps that leak marks

  • Including pure solid concentration in K
  • Using initial rather than equilibrium concentration
  • Forgetting powers follow stoichiometric coefficients
  • Claiming pressure changes K
  • Using total moles before dissociation to calculate mole fractions
  • Including pure solid concentration in K
  • Claiming pressure or a catalyst changes K
  • Using initial moles for mole fractions after dissociation

01. K uses activities; in standard idealized problems pure solids and liquids have activity one and are omitted, while gaseous or dissolved species remain.

02. Q has the same form as K but uses current composition. Q<K drives forward and Q>K drives reverse until Q=K.

03. Changing concentration, pressure or adding inert gas may shift composition, but K changes only with temperature.

04. Thermodynamics supplies ΔG=ΔG°+RT ln Q and ΔG°=−RT ln K; equilibrium is the composition where ΔG reaches its minimum. A large K means products are favoured thermodynamically — not that the reaction is fast.

05. K is stamped to one balanced reaction at one temperature; reverse, scale or add equations and K transforms by exact algebra — never by intuition.

06. Q has the same algebraic form as K but uses current composition: Q < K drives forward, Q > K reverse, until Q = K.

07. Only temperature changes K. Catalysts, inert gas and pressure may change rates or composition, but they do not rewrite K at fixed T — and fixed-V inert gas does not even shift ideal-gas composition.

Official · 2026

Syllabus for this topic

Official syllabus points for Chemical Equilibrium only — not the full Physical portion map. Portion hub: Physical · Physical analysis.

Syllabus on this page is topic-scoped: Chemical Equilibrium. The parent portion (Physical) also covers other topics — use the portion hub for the full map.

Must cover (this topic)

  • Ionic equilibrium buffers/Ksp

Gaps / watch-outs

No topic-specific exclusions flagged — finish the unit points below for Chemical Equilibrium.

Source: https://jeeadv.ac.in/documents/jee-advanced-2026-syllabus.pdf

Unit 01

Chemical Equilibrium

high

Exam focusKp/Kc, reaction quotient, and Le Chatelier — not ionic/pH.

Syllabus points

  • Mass action; ΔG and ΔG°; Kp, Kc, Q; Le Chatelier

Full Chemistry syllabus →

Cross-topic stems

Mixed joins for this topic

Advanced rarely asks a chapter in isolation. These are tagged multi-idea Chemistry stems that touch Chemical Equilibrium — practise the knowledge bridge, not only the single-chapter drill.

Chemistry2023 · Paper 1 · Q5–Q6single-correct

Electrochemistry + chemical equilibrium SC pair

Electrochemistry+Chemical equilibrium+Thermodynamic links

Knowledge bridge

Step 1: relate E_cell / Nernst quantities carefully. Step 2: on equilibrium, write K in terms of activities and compare options. Step 3: note shared thermo language (ΔG, K, E) across both stems.

Why it feels hard

Conceptual overlap tempts overthinking simple algebra.

What to practise

Drill ΔG° = −nFE° = −RT ln K until the three forms feel identical.

2023 year review →Chemistry notes →Quick card →Syllabus →

All mixed joins →

Why methods matter

Common methods that change your score

Extent tables and Q-versus-K reasoning handle both numerical equilibrium and disturbances with one framework.

01

Equation fingerprint

K is meaningless without coefficients; reversal and scaling then become automatic.

Do thisWrite the exact balanced target reaction above every K.

Why it works

K is meaningless without coefficients; reversal and scaling then become automatic.

See it once

If N₂+3H₂⇌2NH₃ has K, then NH₃⇌½N₂+3/2H₂ has K^(−1/2).

02

Q-first direction

Handles mixed simultaneous changes without qualitative ambiguity.

Do thisCalculate Q from the present state before invoking Le Chatelier.

Why it works

Handles mixed simultaneous changes without qualitative ambiguity.

See it once

After a volume change, recalculate partial pressures and Q; its comparison with unchanged K gives direction.

03

Extent with physical root

Quadratic equilibrium equations often contain one algebraic but impossible root.

Do thisUse n_i=n_i,0+ν_iξ and reject roots causing negative moles.

Why it works

Quadratic equilibrium equations often contain one algebraic but impossible root.

See it once

For A⇌B+C, ξ must lie between 0 and initial A when no products are initially present.

04

Inert-gas condition split

At constant V reacting-species partial pressures are unchanged; at constant P volume expands and Q may change.

Do thisSeparate constant V from constant P before predicting shift.

Why it works

At constant V reacting-species partial pressures are unchanged; at constant P volume expands and Q may change.

See it once

Adding inert gas at fixed V changes total pressure only, so ideal-gas equilibrium composition does not shift.

Processed examples

Sample questions · full working

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.

01

bankFamily · Kc, Kp and reaction manipulation

Sample question

For N₂O₄⇌2NO₂, Kp=4. Find Kp for NO₂⇌½N₂O₄.

This stem rewards early recognition of the kc, kp and reaction manipulation model.

Model chosenReverse then halve the reaction.

Processed solution

  1. Reversal changes K to 1/4.
  2. Halving raises it to power 1/2.
  3. K=(1/4)¹ᐟ²=1/2.

Answer0.5

Takeaway: Transform K with the equation.

02

standardFamily · ICE-table composition

Sample question

One mole PCl₅ is placed in a 1 L vessel. At equilibrium 40% dissociates. Find Kc for PCl₅⇌PCl₃+Cl₂.

This stem rewards early recognition of the ice-table composition model.

Model chosenUse α as the extent.

Processed solution

  1. Equilibrium concentrations are 0.6, 0.4 and 0.4 M.
  2. Kc=[PCl₃][Cl₂]/[PCl₅].
  3. Kc=0.16/0.6.
  4. Thus Kc=0.267 M.

Answer0.267 M

Takeaway: Degree of dissociation fills the ICE row directly.

03

twistFamily · Reaction quotient direction

Sample question

For N₂O₄(g)⇌2NO₂(g) at equilibrium, the volume is suddenly halved at constant temperature. Determine the immediate change in Qp and direction of shift.

This Advanced-level variation couples hidden constraints with the reaction quotient direction model.

Model chosenScale all partial pressures before any composition changes.

Processed solution

  1. Immediately after compression, each partial pressure doubles.
  2. Qp=p(NO₂)²/p(N₂O₄).
  3. Therefore Qp becomes (2²/2)Qp=2Kp.
  4. Since Qp>Kp, the reverse reaction is favored.
  5. The system shifts toward N₂O₄.

AnswerQp immediately doubles; equilibrium shifts left

Takeaway: Evaluate the instantaneous quotient before invoking Le Châtelier.

Reusable models

Common question families

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.

01

Kc, Kp and reaction manipulation

Looks likeReverse, multiply or add equilibrium reactions

Fast modelTransform K exactly as the chemical equation is transformed.

Same solve path

  1. Balance the target reaction.
  2. Reverse/power component reactions.
  3. Multiply/divide their constants.
  4. Convert Kc↔Kp if needed.

Rough skeleton

  • reaction operation
  • K operation
  • result

Why faster

Recognition fixes the model before arithmetic begins.

Refuse this

Do not start blind expansion or casework before identifying the governing model.

02

ICE-table composition

Looks likeInitial moles and equilibrium amount, pressure or dissociation

Fast modelUse one reaction extent in an ICE table.

Same solve path

  1. Write initial amounts.
  2. Change each by νᵢξ.
  3. Express equilibrium concentrations/pressures.
  4. Insert into K and select the physical root.

Rough skeleton

  • ICE table
  • K equation
  • physical root

Why faster

Recognition fixes the model before arithmetic begins.

Refuse this

Do not start blind expansion or casework before identifying the governing model.

03

Reaction quotient direction

Looks likeMixture suddenly changed in pressure, volume or composition

Fast modelCompare the instantaneous Q with K.

Same solve path

  1. Calculate Q immediately after disturbance.
  2. Compare Q and K.
  3. State forward or reverse shift.
  4. Update only if final composition is asked.

Rough skeleton

  • Q
  • Q versus K
  • direction

Why faster

Recognition fixes the model before arithmetic begins.

Refuse this

Do not start blind expansion or casework before identifying the governing model.

04

Coupled equilibria and degree of dissociation

Looks likeSuccessive reactions or gaseous dissociation with total pressure

Fast modelExpress all partial pressures through extent and total moles.

Same solve path

  1. Choose extents.
  2. Write equilibrium mole numbers.
  3. Convert to partial pressures.
  4. Solve coupled K equations.

Rough skeleton

  • extent table
  • partial pressures
  • K system

Why faster

Recognition fixes the model before arithmetic begins.

Refuse this

Do not start blind expansion or casework before identifying the governing model.

Hall timing

45–75s process

  1. 0–12s
    Balance reaction and mark phases; identify current versus equilibrium data.

    Underline data, units, phase/standard-state labels and the exact quantity asked. Reject any formula whose assumptions are already violated.

  2. 12–25s
    Write Q/K expression, omit only pure solids/liquids, and set an extent or dissociation variable.

    Write the governing relation symbolically before substitution; define one basis, reference state, or extent variable if the stem mixes quantities.

  3. 25–60s
    Compare Q with K for direction, then solve the equilibrium table with physical-root constraints.

    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.

  4. 60–75s
    Check K transformation, Δn_g, non-negative composition, temperature dependence and limiting α.

    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

Quick formula bank

Equilibrium constant

K_c=Π[c_i]^{ν_i}; K_p=Π(p_i/p°)^{ν_i} (dimensionless activity form)

Balanced reaction with products positive ν

K_p–K_c

K_p=K_c(RT)^{Δn_g}

Ideal gases; Δn_g=gas product coefficients−gas reactant coefficients

Reaction quotient

Q<K forward; Q>K reverse; Q=K equilibrium

Predict spontaneous reaction direction

Reaction transformations

reverse: K⁻¹; multiply equation by n: Kⁿ; add equations: K_total=ΠK_i

Constructed/coupled reactions

Free energy

ΔG=RT ln(Q/K); ΔG°=−RT ln K

Direction and equilibrium thermodynamics

van 't Hoff

ln(K₂/K₁)=−ΔH°/R(1/T₂−1/T₁)

Approximate K change if ΔH° constant

Dissociation example

A⇌nB, starting 1 mol: n_A=1−α, n_B=nα, n_total=1+(n−1)α

Degree-of-dissociation pressure expressions

Mental bank

  • K belongs to a written reaction and temperature
  • Pure solids/liquids omitted
  • Q<K forward; Q>K reverse
  • Only temperature changes K
  • Catalyst changes neither K nor equilibrium composition
  • At fixed V inert gas does not alter reacting-gas partial pressures
  • Large K means products favoured, not necessarily fast
  • Omit pure solids/liquids from K and Q
  • K_p = K_c (RT)^{Δn_g} for ideal gases
  • Catalyst changes neither K nor equilibrium composition
  • ΔG° = −RT ln K; large K ≠ fast

Make it automatic

Practice · checklist

Daily

  • Two Q-direction decisions and two K transformation drills.
  • One α equilibrium table with a physical-root check.

Weekly

  • One coupled-equilibrium and one inert-gas/pressure statement set.
  • Timed 10-question K_c/K_p mixed sheet.

100% checklist

  • Reaction balanced with phases
  • K expression tied to coefficients
  • Pure phases handled correctly
  • Q compared before direction claim
  • Extent root chemically physical
  • K_p/K_c uses gaseous Δn only
  • Temperature distinguished from other disturbances
  • Exact balanced reaction written above K
  • Q compared before any Le Chatelier claim
  • Extent root chemically physical (non-negative moles)
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