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Ionic Equilibrium revision notes

Every idea in the Ionic Equilibrium chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 20 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Acid–base concepts and water

Arrhenius and Brønsted–Lowry
2× asked, last 2023
HA+HX2O⇌HX3OX++AX−(acid1+base2⇌acid2+base1)\ce{HA + H2O <=> H3O+ + A-}\quad(\text{acid}_1 + \text{base}_2 \rightleftharpoons \text{acid}_2 + \text{base}_1)
Arrhenius: gives HX+\ce{H+}/OHX−\ce{OH-} in water. Brønsted: HX+\ce{H+} donor/acceptor. Stronger acid, weaker conjugate base.
HA++HX2O\ce{H2O}⇌\rightleftharpoonsHX3OX+\ce{H3O+}++AX−\ce{A-}acid₁base₁base₂acid₂conjugate pair: differ by one HX+\ce{H+}conjugate pair
The acid on each side has a conjugate base on the other; the stronger acid has the weaker conjugate base
  • Acid strength of H−A\ce{H-A}: down a group, bond strength decides: HF<HCl<HBr<HI\ce{HF}<\ce{HCl}<\ce{HBr}<\ce{HI} (weakest bond, strongest acid). Across a period, electronegativity of A decides: CHX4<NHX3<HX2O<HF\ce{CH4}<\ce{NH3}<\ce{H2O}<\ce{HF}.
  • Not Lewis for water's amphoteric nature. It is Brønsted: water gives HX+\ce{H+} to NHX3\ce{NH3} and takes it from HX2S\ce{H2S}. HCOX3X−\ce{HCO3-} is amphiprotic too.
Lewis acids and bases
1× asked, last 2021
BFX3+NHX3→FX3B−NHX3\ce{BF3 + NH3 -> F3B-NH3}
Acid accepts an electron pair (electron-deficient: BFX3\ce{BF3}, AlClX3\ce{AlCl3}, cations); base donates a lone pair.
FFFBHHHNlone pair donatedacid: acceptsbase: donates
BFX3\ce{BF3} has an empty orbital; NHX3\ce{NH3} gives its lone pair: FX3B←NHX3\ce{F3B<-NH3}
  • Lewis acids: electron-deficient molecules (BFX3\ce{BF3}, AlClX3\ce{AlCl3}), cations (HX+\ce{H+}, FeX3+\ce{Fe^{3+}}), octet-expanders (SiFX4\ce{SiF4}). Lewis bases: NHX3\ce{NH3}, HX2O\ce{H2O}, OHX−\ce{OH-}, CNX−\ce{CN-}.
Ionic product of water
3× asked, last 2025
Kw=[HX+][OHX−]=1.0×10−14 at 298 K,pH+pOH=pKa+pKb=pKwK_w = [\ce{H+}][\ce{OH-}] = 1.0\times10^{-14}\ \text{at 298 K},\qquad \text{pH}+\text{pOH} = \text{p}K_a+\text{p}K_b = \text{p}K_w
KwK_w depends only on TT. Neutral: pH = pKwK_w/2, 7 only at 298 K. KaKb=KwK_aK_b=K_w only for an acid and its own conjugate base.
tt (°C)pH of pure water255010076.13still neutral: [HX+]=[OHX−][\ce{H+}]=[\ce{OH-}]
Ionisation of water is endothermic, so KwK_w rises with TT and neutral pH falls (standard KwK_w data)
  • Not 'pH of water rises on heating'. Ionisation of water is endothermic, so KwK_w rises with TT and the pH of pure water falls below 7. It is still neutral, since [HX+]=[OHX−][\ce{H+}]=[\ce{OH-}].

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17 more concepts in 5 topics

pH of strong acids and bases; Weak acids and bases; Hydrolysis of salts; Buffers, indicators and titrations; and Solubility product. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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pH of strong acids and bases

  • pH scale and dilution7×
  • Mixing strong acids and bases5×

Weak acids and bases

  • Ostwald's dilution law2×
  • pH of a weak acid or base7×
  • Polyprotic acids3×
  • Common ion effect on ionisation1×

Hydrolysis of salts

  • Salt of weak acid + strong base2×
  • Salt of strong acid + weak base1×
  • Salt of weak acid + weak base3×

Buffers, indicators and titrations

  • Henderson–Hasselbalch equation9×
  • Buffer by partial neutralisation9×
  • Buffer action and capacity2×
  • Acid–base indicators4×
  • Titration curves and choice of indicator5×

Solubility product

  • Solubility product and molar solubility17×
  • Common ion effect on solubility6×
  • Precipitation and selective precipitation9×

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