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CBSE Class 11 · Chemistry

Equilibrium

Official NCERT chapter from Chemistry Part I–II (book code kech1). ExamMaster notes are original teaching at CBSE Class 11 depth.

This lesson follows the official chapter “Equilibrium” in Chemistry Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 11
  • Medium level
  • 12 concepts

1Equilibrium in Physical Processes

Physical equilibrium: a change of look (ice–water, liquid–vapour) that sits with both sides present at a taught T, P. Dynamic means bits still move both ways, net zero. A sealed water–vapour pair at a fixed T is the school case. Physical is not a reaction-equation.

An open puddle that dries away is not this sealed equilibrium.

Figure. In a closed flask, liquid water and its vapour sit together. Evaporation and condensation run at the same rate, so the amounts stop changing while molecules keep crossing. That is physical equilibrium, not a stopped flask.

How it works

  1. Name the two looks (ice/water or liquid/vapour)The pair.
  2. Say both remain, bits still swapDynamic physical.
  3. Keep it a look-change, not a new formulaPhysical.

2Equilibrium in Chemical Processes – Dynamic Equilibrium

Chemical equilibrium is dynamic: forward and reverse rates match, concentrations sit. A double arrow is the write. “Stopped” is a miss — the rates are equal, not zero. One closed reaction-vessel is enough.

A burnt match that cannot unburn is not this heading.

Figure. Chemical equilibrium is dynamic: H₂ and I₂ still make HI, and HI still splits, but the two rates have become equal. The arrows are the same length on purpose. Flat amounts do not mean the reaction has stopped.

How it works

  1. Write a reversible reaction as taughtThe process.
  2. Say forward rate = reverse rateDynamic chemical.
  3. Keep amounts sitting, not “empty of reactant” unless K is hugeHonest.

3Law of Chemical Equilibrium and Equilibrium Constant

The equilibrium constant Kc (or Kp as taught) is a number from the law: products over reactants, each raised to its coefficient, at equilibrium. K is a ratio of concentrations (or pressures), not a mood. Solids and pure liquids drop out as taught.

Writing K as a sum of coefficients is a miss.

Figure. For H₂ + I₂ ⇌ 2HI the equilibrium constant is the product concentrations over the reactant concentrations, each raised to its balancing number. Kc is one number at one temperature. It is a ratio of amounts, not a rate.

How it works

  1. Write the balanced reversible equationThe given.
  2. Form [products]^coeff / [reactants]^coeffThe law.
  3. Drop pure solids/liquids if taughtThe caution.

4Homogeneous Equilibria

Homogeneous: every species in the same look (all gas, or all solution). K then uses those concentrations or partial pressures as taught. Homogeneous is a same-phase test, not “same formula”.

A gas plus a solid as “all one” is heterogeneous.

Figure. A homogeneous equilibrium keeps every species in one phase. N₂, H₂ and NH₃ are all gases in one vessel, so every concentration enters Kc. There is no leftover solid to drop from the expression.

How it works

  1. Check every formula’s lookSame → homogeneous.
  2. Write K with those speciesThe constant.
  3. Keep the phase-word in the sentenceThe heading.

5Heterogeneous Equilibria

Heterogeneous: at least two looks (solid + gas, solid + solution). K drops the pure solid as taught; CaCO3(s) ⇌ CaO(s) + CO2(g) leaves Kp = p_CO2 as the school case. Heterogeneous is not “messy”.

Putting [CaCO3] in K because it is written is a common miss.

Figure. CaCO₃(s) ⇌ CaO(s) + CO₂(g) is heterogeneous. Pure solids stay at activity 1, so they drop out of K. The constant collapses to Kp = p_CO₂. The solid heap can grow or shrink; only the gas pressure is in the expression.

How it works

  1. Spot the extra look (solid or liquid pure)Heterogeneous.
  2. Drop that pure from K as taughtThe write.
  3. Keep the gas or solution termWhat K actually watches.

6Applications of Equilibrium Constants

Applications of K: a large K means products sit favoured; small K means reactants. Comparing Qc to K tells the shift: Qc < K → forward as taught. Application is a yes/no or a direction, not a second biography.

A large K is not “fast”.

Figure. Compare the reaction quotient Q with Kc on one axis. The scale is linear from 0 to 8, so Q = 2 sits at one quarter and Kc = 4 sits at the midpoint. From Q = 2 the mixture must run forward until Q reaches 4. From the far side it would run reverse. Kc itself does not move.

How it works

  1. Read K as favoured-side, not speedThe meaning.
  2. Form Qc the same way as K, then compareThe application.
  3. Shift toward products if Qc < KThe use.

7Relationship between Equilibrium Constant K, Qc and Gibbs Energy

K, Qc and Gibbs: ΔG = ΔG° + RT ln Q as taught; at equilibrium Q = K and ΔG = 0, so ΔG° = −RT ln K. A huge K is a largely negative ΔG°. The three names are one energy-and-ratio story.

Setting K = ΔG is a unit-swap.

Figure. ΔG and Q share one test. Left of K, Q < K so ΔG is negative and the forward reaction is spontaneous. At Q = K, ΔG is zero and the mixture is at equilibrium. Right of K, Q > K so ΔG is positive and the reverse reaction is the spontaneous way home.

How it works

  1. Write ΔG° = −RT ln K as taughtThe link.
  2. Keep Q as the now-ratio, K as the eq-ratioQc versus K.
  3. Read a large K as product-favouredGibbs-talk.

8Factors Affecting Equilibria

Factors (Le Chatelier as taught): add a species, change T, change P (gases). The system shifts to partly undo the change. A catalyst speeds both ways and does not move K. Factor is a shift-direction, not a new K-formula each time (except T).

Adding a solid to “shift” a heterogeneous K that dropped the solid is a miss.

Figure. N₂ + 3H₂ ⇌ 2NH₃ is exothermic and has fewer gas moles on the right. Adding H₂ or squeezing the volume both push toward NH₃. Heating treats heat as a product, so the mixture shifts back toward N₂ and H₂. K changes only for the temperature change.

How it works

  1. Name the change (add, T, P)The factor.
  2. Say which way undoes itThe shift.
  3. Keep a catalyst off the K-moveRates, not K.

9Ionic Equilibrium in Solution

Ionic equilibrium: a weak electrolyte sits partly split; a strong one sits almost fully split as taught. Water’s Kw = [H+][OH−] = 10⁻¹⁴ at 25 °C as framed. Ionic is ions-in-water, not a gas-Kp dump.

Treating every acid as fully split is a steal from the strong-only drawer.

Figure. A weak acid in water is an ionic equilibrium: HA ⇌ H⁺ + A⁻. Molecules and ions are both present, and the two rates match. Kw for water is the same idea with H₂O as the molecule: [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C.

How it works

  1. Name weak versus strong as the split-storyIonic eq.
  2. Write Kw if water was the caseThe water-constant.
  3. Keep concentrations of ions, not a mole-count of bottlesHonest.

10Acids, Bases and Salts

Acids, bases, salts (school): Arrhenius / Brønsted as taught — H+ giver, H+ taker. A salt is the ionic leftover of a neutralisation. Acid/base is a role, not a sour-taste quiz. Conjugate pairs if the lesson named them.

Calling NaCl an acid because it is “chemical” is a miss.

Figure. At this class an acid donates H⁺ in water and a base donates OH⁻ (or accepts H⁺). The leftover ions after that meeting are a salt. The figure is the transfer, not a colour word or an indicator.

How it works

  1. Name H+ give or take as taughtAcid or base.
  2. Name a salt as the ionic leftoverThe third word.
  3. Keep taste-talk out of the testA role-story.

11Ionization of Acids and Bases

Ionization of acids and bases: Ka, Kb as the split-constants. A small Ka is a weak acid. pH = −log[H+] as taught; [H+] = 10⁻³ → pH 3. Ionization is the split-number, not a colour.

A pH of 3 as “3 moles of acid” is a miss.

Figure. Percent ionized for 0.10 M solutions, drawn to scale. HCl is taken as fully ionized (100). For CH₃COOH, Ka = 1.8×10⁻⁵ so α = √(Ka/C) = √0.00018 ≈ 0.013, about 1.3%. The weak-acid bar is supposed to look tiny.

How it works

  1. Write HA ⇌ H+ + A− and KaThe ionization.
  2. Read small Ka as weakly splitThe meaning.
  3. Form pH from [H+] if askedThe scale.

pH from [H+]

[H+] = 1.0 × 10⁻⁴ M. Find pH.

  • [H+]1.0×10⁻⁴
  • pH=−log[H+]4
  • Readacidic as taught

Pro tip. pH is −log of the hydronium number, not the bottle-moles.

12Buffer Solutions

A buffer resists a small acid/base add: a weak acid plus its salt (or weak base plus its salt) as taught. Buffer is a resist-story, not “any mixture”. One HA / A− pair is enough. pH sits near pKa if the lesson used Henderson.

Mixing two strong acids is not a buffer.

Figure. An equimolar acetic buffer is two reservoirs of similar size: CH₃COOH and CH₃COO⁻. Added H⁺ is taken up by the acetate; added OH⁻ is taken up by the acid. The reservoirs shrink or grow a little; the pH barely moves.

How it works

  1. Name weak acid + salt (or weak base + salt)The pair.
  2. Say a small add is soaked by the pairThe resist.
  3. Keep it school-size, not a blood-dumpThis class.
A large K means
  1. Products sit favoured — not “fast”
  2. The reaction is always explosive
  3. Q is a sum

K is a sit-ratio.

Notes

  • Mapped to the official NCERT chapter “Equilibrium”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • Kc = [C]^c[D]^d / [A]^a[B]^b
  • ΔG° = −RT ln K
  • Kw = [H+][OH−]
  • pH = −log[H+]

Recap

Hold these pegs from the official chapter “Equilibrium”. The wording is ExamMaster’s teaching, not a textbook recap.

Equilibrium in Physical Processes
Physical equilibrium: a change of look (ice–water, liquid–vapour) that sits with both sides present at a taught T, P.
Equilibrium in Chemical Processes – Dynamic Equilibrium
Chemical equilibrium is dynamic: forward and reverse rates match, concentrations sit.
Law of Chemical Equilibrium and Equilibrium Constant
The equilibrium constant Kc (or Kp as taught) is a number from the law: products over reactants, each raised to its coefficient, at equilibrium.
Homogeneous Equilibria
Homogeneous: every species in the same look (all gas, or all solution).
Heterogeneous Equilibria
Heterogeneous: at least two looks (solid + gas, solid + solution).
Applications of Equilibrium Constants
Applications of K: a large K means products sit favoured; small K means reactants.

Practise Equilibrium

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