8Acids, Bases and Salts

📚
CONCEPTS USED
▢ Main Concepts:
  1. Arrhenius concept → aqueous-solution based
  2. Bronsted-Lowry concept → proton transfer; aqueous / non-aqueous
  3. Lewis concept → electron-pair transfer; solvent may / may not be involved
▢ Ammonia Terms:
❖ Liquor Ammonia: aqueous solution of ammonia / \(NH_4OH\) → Arrhenius base
❖ Liquid Ammonia: \(NH_3\) liquefied under pressure → amphiprotic; coolant
❖ Autoprotolysis: \(NH_3 + NH_3 \rightleftharpoons NH_2^- + NH_4^+\)
▢ Solvent Behavior:
❖ Levelling Solvent: water
❖ Differentiating Solvent: acetic acid
❖ Levelling Effect: all strong acids appear equally strong in water due to high dielectric constant + strong proton-accepting tendency of water
📚
ARRHENIUS CONCEPT
▢ Acid: substance ionising in water to give \(H^+\) / \(H_3O^+\)
▢ Base: substance ionising in water to give \(OH^-\)
▢ Alkali: water-soluble base
▢ Neutralisation:
❖ Definition: acid + base → salt + water
❖ Nature: exothermic
❖ Heat of Neutralisation: strong acid + strong base → \(\Delta H \approx -57.35\,kJ\,mol^{-1}\) or \(-13.7\,kcal\)
❖ Examples:
  • •
    \(HCl + NaOH \rightarrow NaCl + H_2O\)
  • •
    \(\frac{1}{2}H_2SO_4 + NaOH \rightarrow \frac{1}{2}Na_2SO_4 + H_2O\)
  • •
    weak acid / weak base involved → \(\Delta H < 57.35\,kJ\); e.g. \(CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O\)
▢ Notes:
  • •
    every H-containing compound not acid; e.g. \(CH_4\) not acid
  • •
    every OH-containing compound not base; e.g. alcohols not bases
  • •
    cannot explain gas-phase / non-aqueous acid-base reactions; e.g. \(NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)\)
▢ Limitations:
  • •
    restricted to \(H^+\) / \(OH^-\) in aqueous medium
  • •
    cannot explain reactions like \(CaO + CO_2 \rightarrow CaCO_3\)
  • •
    cannot explain amphoteric ions: \(HPO_4^{2-}, H_2PO_4^-, HS^-, HCO_3^-\)
  • •
    cannot explain acidic / basic nature of many salts
  • •
    cannot explain basic metallic oxides lacking \(OH^-\); e.g. \(Fe_2O_3, CaO\)
  • •
    cannot explain acid-base behavior in non-aqueous solvents such as liquid \(NH_3\), liquid \(SO_2\)
▢ Special Salt Examples:
❖ Acidic Salts:
  • •
    blue vitriol: \(CuSO_4.5H_2O\)
  • •
    white vitriol: \(ZnSO_4.7H_2O\)
  • •
    green vitriol: \(FeSO_4.7H_2O\)
  • •
    Epsom salt: \(MgSO_4.7H_2O\)
❖ Basic Salts:
  • •
    soda ash: \(Na_2CO_3\)
  • •
    pearl ash: \(K_2CO_3\)
📚
BASICITY OF ACID AND ACIDITY OF BASE
▢ Basicity: no. of replaceable / ionisable H atoms in an acid
▢ Acidity: no. of replaceable \(OH^-\) groups in a base
▢ Lewis Structure Rule: in oxyacids of P and S, H attached to O is ionisable; H attached directly to central atom is non-ionisable
📖
Basicity of acids
📝
Monobasic
  • •
    \(HNO_3\), phenol \(C_6H_5OH\)
  • •
    oxyacids of Cl: \(HClO, HClO_2, HClO_3, HClO_4\)
  • •
    acetic acid, glycolic acid, lactic acid
  • •
    hydrohalic acids: \(HCl, HBr, HI\)
  • •
    hypophosphorous acid \(H_3PO_2\)
  • •
    prussic acid \(HCN\)
📝
Dibasic
  • •
    \(H_2SO_4, H_2SO_3, H_2S\)
  • •
    phosphorous acid \(H_3PO_3\)
  • •
    succinic acid, tartaric acid, oxalic acid
📝
Tribasic
  • •
    phosphoric acid \(H_3PO_4\)
  • •
    citric acid
📝
Tetrabasic
  • •
    hypophosphoric acid \(H_4P_2O_6\)
  • •
    pyrophosphoric acid \(H_4P_2O_7\)
📝
General Rule
  • •
    monobasic acid → 1 salt
  • •
    dibasic acid → 2 salts
  • •
    tribasic acid → 3 salts
📝
Important Notes
  • •
    sodium tartrate used in Fehling’s solution; sodium citrate used in Benedict’s solution
  • •
    although \(H_4P_2O_6\) and \(H_4P_2O_7\) are tetrabasic, they form only 2 salts
  • •
    \(H_4P_2O_7 + NaOH \rightarrow NaH_3P_2O_7 + H_2O\) → acid salt
  • •
    \(H_4P_2O_7 + 2NaOH \rightarrow Na_2H_2P_2O_7 + 2H_2O\) → normal salt
  • •
    HF behaves as dibasic due to intermolecular H-bonding; in water exists as dimer \(H_2F_2\)
  • •
    with alkali: \(H_2F_2 + KOH \rightarrow KHF_2 + H_2O\) (Fremy’s salt); further → \(KF\)
  • •
    \(Na_2HPO_4\) is an acid salt used in test of \(Mg^{2+}\)
📖
Acidity of bases
📝
Monoacidic
alkali metal hydroxides, \(NH_4OH\)
📝
Diacidic
alkaline earth metal hydroxides, \(Fe(OH)_2, Zn(OH)_2, Sn(OH)_2, Pb(OH)_2, Cu(OH)_2\)
📝
Triacidic
\(Fe(OH)_3, Cr(OH)_3, Al(OH)_3\), group IIIA hydroxides
📝
Tetraacidic
\(Sn(OH)_4, Pb(OH)_4\)
📚
BRONSTED-LOWRY CONCEPT
▢ Acid: proton donor
▢ Base: proton acceptor
▢ Illustration: \(H_2O + NH_3 \rightleftharpoons OH^- + NH_4^+\); \(H_2O\) = acid, \(NH_3\) = base
▢ Strength: acid strength ∝ tendency to lose proton; base strength ∝ tendency to accept proton
▢ Conjugate Pair: \(HCl + H_2O \rightarrow H_3O^+ + Cl^-\); \(H_3O^+\) = conjugate acid of \(H_2O\), \(Cl^-\) = conjugate base of \(HCl\)
▢ Proton State: proton always solvated; free \(H^+\) absent
▢ Non-Bronsted Acids: dry \(HCl\), dry \(HNO_3\) are not Bronsted acids in absence of solvent
📖
Types of solvents
📝
Protogenic
proton donor; e.g. \(HCl\), \(H_2O\), glacial acetic acid
📝
Protophilic
proton acceptor; e.g. \(H_2O\), alcohol, liquid ammonia
📝
Amphiprotic
can donate or accept proton; e.g. \(H_2O\), liquid ammonia, alcohol, \(CH_3COOH\), \(HNO_3\), \(H_2SO_4\)
📝
Aprotic
can neither donate nor accept proton; e.g. \(CS_2\), \(CCl_4\), benzene
📖
Conjugate acid-base rules
  • •
    strong acid → weak conjugate base
  • •
    strong base → weak conjugate acid
  • •
    \(HClO_4\) strongest acid among common acids → \(ClO_4^-\) weakest base
  • •
    \(CH_4\) weakest acid → \(CH_3^-\) strongest base
  • •
    \(H_2\) weak acid → \(H^-\) strong base
📖
Conjugate acid-base pairs

Table 1: Common conjugate acid-base pairs

Acid
Conjugate base
\(HClO_4\)
\(ClO_4^-\)
\(H_2SO_4\)
\(HSO_4^-\)
\(HCl\)
\(Cl^-\)
\(HNO_3\)
\(NO_3^-\)
\(H_3O^+\)
\(H_2O\)
\(HSO_4^-\)
\(SO_4^{2-}\)
\(H_3PO_4\)
\(H_2PO_4^-\)
\(CH_3COOH\)
\(CH_3COO^-\)
\(H_2CO_3\)
\(HCO_3^-\)
\(H_2S\)
\(HS^-\)
\(NH_4^+\)
\(NH_3\)
\(HCN\)
\(CN^-\)
\(C_6H_5OH\)
\(C_6H_5O^-\)
\(H_2O\)
\(OH^-\)
\(C_2H_5OH\)
\(C_2H_5O^-\)
\(NH_3\)
\(NH_2^-\)
\(CH_4\)
\(CH_3^-\)
acid strength ↑ upward; base strength ↑ downward
📖
Limitations
  • •
    cannot explain acid-base reactions in non-protonic solvents where no proton transfer occurs
  • •
    examples: liquid \(SO_2\), liquid \(BF_3\), \(AlCl_3\), \(POCl_3\)
📚
LEWIS CONCEPT
▢ Acid: electron-pair acceptor
▢ Base: electron-pair donor
▢ Bonding: donation + acceptance of lone pair → co-ordinate bond
▢ Lewis Acids: typically electrophilic / oxidising
▢ Lewis Bases: typically nucleophilic / reducing
📖
Lewis acids
📝
Incomplete Octet
  • •
    molecules / atoms with incomplete octet; e.g. \(BF_3, BCl_3, AlCl_3\)
  • •
    example adduct: \(BF_3 + :NH_3 \rightarrow F_3B \leftarrow NH_3\)
📝
Simple Cations
  • •
    \(Cu^{2+} + 4NH_3 \rightarrow [Cu(NH_3)_4]^{2+}\)
  • •
    acid strength of cation ↑ with charge density (charge ↑, size ↓)
📝
Expandable Valence Shell
  • •
    \(SiF_4, SnCl_2, SnCl_4\)
  • •
    \(SnCl_2 + 2Cl^- \rightarrow [SnCl_4]^{2-}\)
  • •
    \(SiF_4 + 2F^- \rightarrow [SiF_6]^{2-}\)
📝
Multiple Bond Molecules
  • •
    molecules with multiple bond between atoms of dissimilar electronegativity; e.g. \(CO_2, SO_2, SO_3\)
  • •
    under attack by Lewis base, \(\pi\)-electron pair shifts towards more electronegative atom
📝
Non-Metal Oxides
  • •
    acidic in nature; anhydride of corresponding oxyacid
  • •
    \(CO_2\) = Lewis acid (anhydride of \(H_2CO_3\))
  • •
    CO = neutral oxide
📖
Lewis bases
  • •
    anions: \(OH^-\), \(CN^-\), etc.
  • •
    molecules with lone pairs: \(NH_3\), oxides, ethers, alcohols
  • •
    ligands in coordination compounds; e.g. CO in \(Ni(CO)_4\)
📖
Limitations
  • •
    too general; includes many coordination reactions
  • •
    even zero oxidation state metals in complexes with \(\pi\)-acceptor ligands (CO, cyclopentadienyl, olefins) behave as Lewis acids
  • •
    does not satisfactorily explain relative strengths of acids and bases
📚
ACID AND BASE STRENGTH
📖
Hydracids / oxyacids trends
📝
Same Period Hydrides
  • •
    acidic strength: \(CH_4 < NH_3 < H_2O < HF\)
  • •
    conjugate base stability: \(CH_3^- < NH_2^- < OH^- < F^-\)
📝
Group V Hydrides
basic strength decreases: \(NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3\)
📝
Group VI Hydrides
acidic strength increases: \(H_2O < H_2S < H_2Se < H_2Te\)
📝
Same Element, Higher Oxidation State
  • •
    \(HClO < HClO_2 < HClO_3 < HClO_4\)
  • •
    \(H_2SO_3 < H_2SO_4\)
  • •
    \(HNO_2 < HNO_3\)
📝
Phosphorus Oxyacid Exception
\(H_3PO_2 > H_3PO_3 > H_3PO_4\)
📝
Same Oxidation State, Different Central Atom
  • •
    \(HClO_4 > HBrO_4 > HIO_4\)
  • •
    \(HClO > HBrO > HIO\)
  • •
    \(H_2SO_3 > H_2SeO_3\)
📖
Factors increasing acidity
  • •
    resonance stabilisation of conjugate base; phenol > alcohol
  • •
    electron-withdrawing group (−I effect) ↑ acidity
  • •
    greater % s-character ↑ acidity: \(sp > sp^2 > sp^3\)
  • •
    for carboxylic acids, +I effect ↓ acidity
📖
Illustrative orders of acid strength
  • •
    \(HClO_4 > HClO_3 > HClO_2 > HClO\)
  • •
    \(HI > HBr > HCl > HF\)
  • •
    \(HClO_3 > HBrO_3 > HIO_3\)
  • •
    \(CCl_3COOH > CHCl_2COOH > CH_2ClCOOH > CH_3COOH\)
  • •
    \(HCOOH > CH_3COOH > C_2H_5COOH\)
  • •
    \(HC\equiv CH > CH_2=CH_2 > CH_3-CH_3\)
  • •
    \(CH_4 < H_2S < HI\)
  • •
    formic acid > benzoic acid > acetic acid > phenol > \(H_2O\) > alcohol > ethyne > ethene > ethane
📖
Illustrative orders of base strength
  • •
    \(KOH > NaOH > Ca(OH)_2 > NH_4OH\)
  • •
    \((CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3\)
  • •
    \((C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3\)
  • •
    \(NaOH > NH_3 > H_2O\)
  • •
    \(NH_3 > NH_2NH_2 > NH_2OH\)
  • •
    \(NH_3 > C_5H_5N > C_6H_5NH_2\)
  • •
    \(NH_3 > H_2O > HF\)
  • •
    \(F^- > Cl^- > Br^- > I^-\)
  • •
    \(O^{2-} > S^{2-}\)
  • •
    \(CH_3NH_2 > NH_3 > NF_3\)
📖
Organic substituent effects
📝
+I Effect
decreases acidity; \(HCOOH > CH_3COOH > C_2H_5COOH\)
📝
−I Effect
increases acidity; \(ICH_2COOH < BrCH_2COOH < ClCH_2COOH < FCH_2COOH\)
📝
Polyhalogen Effect
\(Cl_3CCOOH > Cl_2CHCOOH > ClCH_2COOH > CH_3COOH\)
📝
Phenols
  • •
    \(p\)-nitrophenol > \(o\)-nitrophenol > \(m\)-nitrophenol > \(H_2O\) > alcohols
  • •
    phenol > \(m\)-methyl phenol > \(p\)-methyl phenol > \(o\)-methyl phenol
  • •
    \(m\)-methoxy phenol > phenol > \(o\)-methoxy phenol > \(p\)-methoxy phenol
  • •
    \(o\)-fluorophenol > \(p\)-fluorophenol
📝
Benzoic Acids
  • •
    \(o\)-nitrobenzoic acid > \(p\)-nitrobenzoic acid > \(m\)-nitrobenzoic acid > benzoic acid
  • •
    \(o\)-methoxy benzoic acid > \(m\)-methoxy benzoic acid > benzoic acid > \(p\)-methoxy benzoic acid
📝
Anilines (Basic Order)
  • •
    aniline > \(m\)-nitroaniline > \(p\)-nitroaniline > \(o\)-nitroaniline
  • •
    \(p\)-methyl aniline > \(m\)-methyl aniline > aniline > \(o\)-methyl aniline
  • •
    ortho effect: all \(o\)-substituents ↑ acidity of benzoic acids and ↓ basicity of anilines
📖
Levelling and differentiating
📝
Water as Levelling Solvent
  • •
    \(HClO_4, HNO_3, HCl, HBr\) in water all form \(H_3O^+\)
  • •
    thus these acids appear equally strong in water
📝
Acetic Acid as Differentiating Solvent
acid strengths become distinguishable; order in text: \(HNO_3 < HCl < H_2SO_4 < HBr < HClO_4\)
📚
SOME IMPORTANT POINTS
▢ Amphiprotic Species:
  • •
    \(HCO_3^-\) acts as acid and base
  • •
    \(HSO_4^-\) acts as acid and base
  • •
    water acts as acid and base
▢ Examples:
  • •
    \(NH_2^- + H^+ \rightarrow NH_3\) → conjugate acid of \(NH_2^-\) is \(NH_3\)
  • •
    conjugate acid of \(HPO_4^{2-}\) is \(H_2PO_4^-\)
  • •
    strongest base among \(ClO^-, ClO_2^-, ClO_3^-, ClO_4^-\) is \(ClO^-\)
  • •
    tribasic acid \(H_3PO_4\) furnishes \(H_2PO_4^-\), \(HPO_4^{2-}\), \(PO_4^{3-}\)
▢ Hydrolysis Clues:
  • •
    \(CH_3COONa\) = salt of weak acid + strong base → alkaline solution
  • •
    cation of weak base hydrolyses acidic: \(NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+\)
  • •
    anion of weak acid hydrolyses basic: \(CN^- + H_2O \rightleftharpoons HCN + OH^-\)
▢ Auto-Protolysis:
  • •
    \(NH_3 + NH_3 \rightleftharpoons NH_4^+ + NH_2^-\)
  • •
    \(H_2SO_4 + H_2SO_4 \rightleftharpoons H_3SO_4^+ + HSO_4^-\)
▢ Applicability:
  • •
    Arrhenius → aqueous only
  • •
    Bronsted-Lowry + Lewis → aqueous and non-aqueous
  • •
    all Arrhenius acids are Bronsted acids; not all Arrhenius bases are Bronsted bases
  • •
    all Bronsted bases are Lewis bases; not all Bronsted acids are Lewis acids
▢ Miscellaneous:
  • •
    EDTA behaves as Arrhenius acid, Bronsted base, Lewis base
  • •
    solution with pH = 0 → strongly acidic
  • •
    stomach acidic, small intestine basic → aspirin mostly unionised in stomach, ionised in intestine
  • •
    no acid stronger than \(H_3O^+\) can exist in water; no base stronger than \(OH^-\) can exist in water
  • •
    \(CCl_4\) and \(CH_4\) are neither Lewis acids nor Lewis bases
  • •
    \(NH_3\) is Lewis base; \(AlCl_3\), \(BF_3\) are Lewis acids
  • •
    aqueous \(FeCl_3\) is acidic
  • •
    aqueous acetic acid contains \(CH_3COO^-\), \(H_3O^+\) and \(CH_3COOH\)
  • •
    \(HClO_4\) strongest acid known; \(CsOH\) strongest base known in text
📚
READ & DIGEST
▢ Lewis Acid Strength:
  • •
    group IIIA trihalides: \(BF_3 < BCl_3 < BBr_3 < BI_3\)
  • •
    for analogous halides across group: \(BX_3 > AlX_3 > GaX_3 > InX_3 > TlX_3\)
▢ Basicity of Hydroxides:
  • •
    subgroup A: along period → basicity decreases; down group → basicity increases
  • •
    subgroup B: along period → basicity increases; down group → basicity decreases
▢ Metals with Dilute Nitric Acid: Mn and Mg are the only metals mentioned giving \(H_2\) with dilute \(HNO_3\)
📚
SALT
▢ Definition: product of acid-base neutralisation
▢ Types:
  • •
    Normal salt
  • •
    Acid salt
  • •
    Base salt
  • •
    Mixed salt
  • •
    Double salt
  • •
    Complex salt
📖
Normal salt
📝
General
formed by complete reaction between acid and base; may be neutral, acidic or basic
📝
Neutral Normal Salt
📄
Formation
strong acid + strong base
📄
Behavior
no hydrolysis; only hydrated ions in water
📄
Examples
\(NaCl, KCl, NaNO_3\)
📝
Acidic Normal Salt
📄
Formation
strong acid + weak base
📄
Hydrolysis
cationic hydrolysis; solution acidic; blue litmus → red
📄
Examples
\(CuSO_4, FeCl_3, NH_4Cl, LiNO_3, BeCl_2, ZnCl_2, ZnSO_4, HgCl_2\)
📄
Example Reaction
\(FeCl_3 + 3H_2O \rightarrow Fe(OH)_3 + 3HCl\)
📝
Basic Normal Salt
📄
Formation
strong base + weak acid
📄
Hydrolysis
anionic hydrolysis; solution basic; red litmus → blue
📄
Examples
\(Na_2CO_3, K_2CO_3, CH_3COONa\), borax \(Na_2B_4O_7.10H_2O\)
📄
Example Reaction
\(CH_3COONa + H_2O \rightleftharpoons CH_3COOH + NaOH\)
📝
Weak Acid + Weak Base Salts
📄
Nature
may be acidic / basic / neutral depending on relative strengths
📄
Examples
  • •
    \(CH_3COONH_4\) → neutral (pH ≈ 7)
  • •
    \(HCOONH_4\) → acidic
  • •
    \((NH_4)_2CO_3\) → basic
📖
Buffer systems
📝
Buffer
  • •
    weak acid + salt of its strong base
  • •
    weak base + salt of its strong acid
📝
Simple Buffer Examples
  • •
    \(CH_3COOH + CH_3COONH_4\)
  • •
    \(NH_4OH + CH_3COONH_4\)
📖
Acid salt
📝
Definition
formed by incomplete replacement of H of di / tri / tetra basic acid
📝
Other Name
bi-salt
📝
Nature
may be acidic, basic or neutral
📝
Examples
📄
Neutral Acid Salt
\(NaHSO_4\)
📄
Basic Acid Salts
\(NaHCO_3, NH_4HCO_3, Na_2HPO_4, NaH_2PO_4\)
📄
Acidic Acid Salt
\(NH_4HSO_4\)
📖
Base salt
📝
Definition
formed by incomplete replacement of \(OH^-\) from polyacidic base
📝
Nature
always basic
📝
Possible Groups
may contain \(OH\) group or oxygen group
📝
Examples
  • •
    \(Sn(OH)Cl, Pb(OH)Cl\)
  • •
    malachite: \(CuCO_3.Cu(OH)_2\) → basic copper carbonate
  • •
    azurite: \(2CuCO_3.Cu(OH)_2\) → basic copper carbonate
  • •
    basic zinc carbonate: \(ZnCO_3.Zn(OH)_2\)
  • •
    white lead: \(2PbCO_3.Pb(OH)_2\)
  • •
    \(BiOCl\) (pearl white)
📝
Illustration
  • •
    \(Sn(OH)_2 + HCl \rightarrow Sn(OH)Cl + H_2O\) → base salt
  • •
    \(Sn(OH)_2 + 2HCl \rightarrow SnCl_2 + 2H_2O\) → normal salt
📖
Mixed salt
📝
Definition
contains >1 cation and/or >1 anion; on dissolution gives simple ions
📝
Examples
  • •
    bleaching powder: \(CaOCl_2\) or \(Ca(OCl)Cl\) → \(Ca^{2+}, OCl^-, Cl^-\)
  • •
    Rochelle’s salt: sodium potassium tartrate
  • •
    microcosmic salt: \(NaNH_4HPO_4.4H_2O\)
📝
Notes
  • •
    sodium gives golden yellow flame test
  • •
    all double salts are mixed salts, but all mixed salts are not double salts
📖
Double salt
📝
Definition
formed by crystallisation / mixing of two simple salts in equimolar solution; not by chemical reaction
📝
Property
dissolves in water to give simple ions
📝
Examples
  • •
    Mohr’s salt: \(FeSO_4.(NH_4)_2SO_4.6H_2O\)
  • •
    all alums
  • •
    all pseudoalums
  • •
    carnallite: \(KCl.MgCl_2.6H_2O\)
  • •
    fusion mixture: \(Na_2CO_3.K_2CO_3\)
  • •
    dolomite: \(CaCO_3.MgCO_3\)
📝
Mohr Salt Note
\(FeSO_4 + (NH_4)_2SO_4 \rightarrow FeSO_4.(NH_4)_2SO_4.6H_2O\)
📖
Alums and pseudoalums
📝
Alum Formula
\(M_2SO_4.M'{}_2(SO_4)_3.24H_2O\)
📝
Where
📄
M
monovalent alkali metal / \(NH_4^+\) (Li excluded in text)
📄
M'
trivalent metal; e.g. \(Fe^{3+}, Cr^{3+}, Al^{3+}, Co^{3+}\)
📝
Examples
  • •
    potash alum: \(K_2SO_4.Al_2(SO_4)_3.24H_2O\)
  • •
    chrome alum: \(K_2SO_4.Cr_2(SO_4)_3.24H_2O\)
  • •
    soda alum: \(Na_2SO_4.Al_2(SO_4)_3.24H_2O\)
  • •
    ammonium alum: \((NH_4)_2SO_4.Al_2(SO_4)_3.24H_2O\)
📝
Properties
  • •
    isomorphous: same crystalline structure and same no. of atoms
  • •
    undergo cationic hydrolysis due to acidic salt character
  • •
    potash alum used as water purifier and antiseptic
📝
Pseudoalums
  • •
    ferrous alum: \(FeSO_4.Al_2(SO_4)_3.24H_2O\)
  • •
    ferric alum: \(FeSO_4.Fe_2(SO_4)_3.24H_2O\)
  • •
    magnesium alum: \(MgSO_4.Al_2(SO_4)_3.24H_2O\)
  • •
    also isomorphous
📖
Complex salt
📝
Definition
formed by Lewis acid-base reaction; central atom = Lewis acid, ligand = Lewis base
📝
Nature
may be anionic, cationic or neutral
📝
Examples
  • •
    \(K_4[Fe(CN)_6]\), \(K_3[Fe(CN)_6]\)
  • •
    alkaline \(K_2[HgI_4]\) (Nessler’s reagent)
  • •
    \(Ni(CO)_4\)
  • •
    \([Fe(H_2O)_5NO]SO_4\) (brown ring complex)
  • •
    \(Na_2[Fe(CN)_5NO]\) (sodium nitroprusside)
📝
Complex Types
  • •
    \([Fe(CN)_6]^{4-}\) → anionic complex
  • •
    \([Cu(NH_3)_4]^{2+}\) → cationic complex
  • •
    \(Ni(CO)_4\) → neutral complex
📝
Coordination
bond between central atom and ligand is co-ordinate bond; no. of such bonds = coordination number

Table 1: Complex salts: coordination number and no. of ions

Compound
Coordination no.
No. of ions
\(K_4[Fe(CN)_6]\)
6
5
\(Ni(CO)_4\)
4
0
\([Cu(NH_3)_4]SO_4\)
4
2
\([Fe(H_2O)_5NO]SO_4\)
5 + 1 = 6
2
\(Na_2[Fe(CN)_5NO]\)
5 + 1 = 6
3
greater the no. of ions produced → greater the molar conductivity
📚
ACIDS, BASES AND SALTS
▢ Acid-Base Concepts:

Table 1: Main Concepts

Concept
Acid
Base
Arrhenius
Gives H+ / H3O+ in water
Gives OH− in water
Bronsted-Lowry
Proton donor
Proton acceptor
Lewis
Electron-pair acceptor
Electron-pair donor
▢ Conjugate Acid-Base Pair:
❖ Definition: Two species differing by one proton
❖ Examples:
  • •
    NH3 / NH4+
  • •
    H2O / OH−
  • •
    HCl / Cl−
  • •
    HCO3− / CO3^2−
  • •
    HPO4^2− / H2PO4−
❖ Rule: Strong acid → weak conjugate base; weak acid → strong conjugate base
▢ Lewis Acid and Base:
❖ Lewis Acids:
  • •
    BF3
  • •
    AlCl3
  • •
    Ag+
  • •
    SnCl2
  • •
    Electron-deficient boron compounds
❖ Lewis Bases:
  • •
    OH−
  • •
    NH3
  • •
    H2O
  • •
    Cl−
  • •
    I−
❖ Example: I2 + I− → I3−; I− acts as Lewis base
▢ Solvent Types:

Table 1: Solvent Classification

Solvent Type
Meaning
Example
Protophilic
Proton-loving / proton acceptor
NH3
Protogenic
Proton donor
HF
Amphiprotic
Can donate and accept H+
H2O
Aprotic
Does not donate proton
Benzene
▢ Strength of Acids and Bases:
❖ Acid Strength: Higher ionization tendency → stronger acid
❖ Base Strength: Greater proton affinity / electron-pair donation → stronger base
❖ Oxyacid Rule: For oxyacids of same central atom, more oxygen atoms → stronger acid
❖ Examples:
  • •
    HClO4 > HClO3 > HClO2 > HClO
  • •
    KOH > NaOH > Mg(OH)2 > Al(OH)3
  • •
    NH3 is more basic than PH3
▢ Salts and pH:

Table 1: Salt Hydrolysis and pH

Salt Type
pH Nature
Strong acid + strong base
Neutral, pH ≈ 7
Strong acid + weak base
Acidic, pH < 7
Weak acid + strong base
Basic, pH > 7
Weak acid + weak base
Depends on Ka and Kb
▢ High-Yield Points:
  • •
    B(OH)3 is not a protonic acid; it behaves as Lewis acid
  • •
    BF3 is Lewis acid due to electron deficiency
  • •
    Water is amphiprotic
  • •
    HCO3− is amphiprotic because it can donate and accept proton
  • •
    Rain during thunderstorm has slightly lower pH due to formation of acidic oxides/acids
  • •
    For diprotic acid H2X, Ka1 > Ka2
Q1.
In the reaction NH3 + H2O → NH4+ + OH−, which of the following constitutes a conjugate acid-base pair?
📅MOE Model
Q2.
According to Bronsted-Lowry concept, an acid is said to be strong if
📅MOE 2061
Q3.
Which of the following is the strongest conjugate base?
📅MOE 2052
Q4.
Alum is
📅IOM 2001
Q5.
A salt formed from strong acid and strong base generally has pH
📅IOM 1999
Q6.
In HCl + H2O → H3O+ + Cl−, H2O is base according to
📅IOM 2008
Q7.
According to Arrhenius, acids are
Q8.
According to Lewis concept, an acid is a substance which
Q9.
BF3 molecule is
Q10.
Water is a
Q11.
Which of the following is not a protonic acid?
Q12.
According to Lewis concept, which one of the following is not a base?
Q13.
An example of Lewis acid is
Q14.
In the reaction SnCl2 + 2Cl− → [SnCl4]^2−, the Lewis acid is
Q15.
Cl− is the conjugate base of which acid?
Q16.
The conjugate acid of H2PO4− is
Q17.
The conjugate acid of HPO4^2− is
Q18.
Ammonia gas dissolves in water to give NH4OH. In this reaction, water acts as
Q19.
Which of the following species is an acid and also a conjugate base of another acid?
Q20.
Which of the following is the strongest acid?
Q21.
With reference to protonic acids, which statement is correct?
Q22.
Which of the following has highest proton affinity?
Q23.
The strongest Lewis base among the following is
Q24.
The strongest Bronsted base among the following anions is
Q25.
Boron compounds behave as Lewis acids because of their
Q26.
Ionization of HCl in water results in formation of
Q27.
Conjugate base of HCO3− is
Q28.
Which of the following is the weakest base?
Q29.
Which of the following is the strongest acid?
Q30.
Which of the following is the strongest acid?
Q31.
Which is the strongest base among the following?
Q32.
In the reaction I2 + I− → I3−, the Lewis base is
Q33.
Which of the following can act both as Bronsted acid and Bronsted base?
Q34.
BF3 is acid according to
Q35.
When rain is accompanied by thunderstorm, the collected rain water will have pH value
Q36.
Which one is not a Lewis acid?
Q37.
For every diprotic acid of the type H2X, which relation is true?