Acids, Bases, Salts and Redox Questions for NDA

Theories of Acids and Bases

Three main theories define acids and bases. Arrhenius theory: acids give H+ ions in water, bases give OH- ions. Simple but limited to water solutions only. Bronsted-Lowry theory: acid is a proton (H+) donor, base is a proton acceptor. Works beyond water. A conjugate acid-base pair differs by just one proton. Lewis theory: acid is an electron pair acceptor, base is an electron pair donor. Broadest definition, covers reactions without H+ at all.

• Arrhenius: HCl gives H+, NaOH gives OH- in water
• Bronsted-Lowry: NH3 is a base (accepts H+), HCl is an acid (donates H+)
• Lewis: BF3 is an acid (accepts electron pair), NH3 is a base (donates electron pair)
• Conjugate pair: acid loses H+ to form conjugate base. Example: HCl and Cl-
• Lewis theory is the broadest — all Arrhenius acids are Bronsted-Lowry acids, and those are Lewis acids too
• Amphoteric substances act as both acid and base: water, Al2O3, ZnO

pH Scale and Strength of Acids and Bases

pH measures hydrogen ion concentration. Scale runs 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is basic. Strong acids fully dissociate in water (HCl, H2SO4, HNO3). Weak acids partially dissociate (CH3COOH, H2CO3). Similarly, NaOH and KOH are strong bases; NH4OH is a weak base. Remember: lower pH means more acidic, not just more H+ — a common confusion.

• pH = -log[H+]. If [H+] = 0.01 M, pH = 2
• pH + pOH = 14 at 25 degrees Celsius
• Strong acids: HCl, HBr, HI, H2SO4, HNO3, HClO4
• Weak acids: CH3COOH, H2CO3, H3PO4, HF
• Strong bases: NaOH, KOH, Ca(OH)2. Weak base: NH4OH
• Acidic solution: [H+] > [OH-]. Basic: [OH-] > [H+]

Salts and Their Hydrolysis

A salt is formed by neutralisation of acid and base. The nature of the salt (acidic, basic, or neutral) depends on which parent acid and base made it. Strong acid + strong base gives neutral salt (NaCl). Strong acid + weak base gives acidic salt (NH4Cl). Weak acid + strong base gives basic salt (CH3COONa). Weak acid + weak base depends on Ka and Kb values. Salt hydrolysis is the reverse of neutralisation — the salt reacts with water to regenerate the parent acid or base characteristics.

• NaCl: neutral salt, pH = 7
• NH4Cl: acidic salt, pH less than 7 (weak base NH4OH + strong acid HCl)
• CH3COONa: basic salt, pH greater than 7 (weak acid + strong base NaOH)
• Buffer solutions resist change in pH — made from weak acid + its salt, or weak base + its salt
• CH3COOH + CH3COONa = acidic buffer. NH4OH + NH4Cl = basic buffer
• Normal salt has no replaceable H or OH; acid salt still has H (NaHSO4); basic salt still has OH (Mg(OH)Cl)

Redox Reactions: Oxidation and Reduction

Redox means simultaneous oxidation and reduction. Oxidation is loss of electrons, increase in oxidation number, or gain of oxygen/loss of hydrogen. Reduction is gain of electrons, decrease in oxidation number, or loss of oxygen/gain of hydrogen. The oxidising agent causes oxidation in other substances and itself gets reduced. The reducing agent causes reduction and itself gets oxidised. Oxidation number rules help identify what is oxidised or reduced.

• OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)
• Oxidation number of free element = 0. Of monatomic ion = its charge
• Oxidation number of O = -2 (except in peroxides it is -1, in OF2 it is +2)
• Oxidation number of H = +1 (except in metal hydrides it is -1, like NaH)
• Sum of oxidation numbers in a neutral compound = 0; in a polyatomic ion = charge of ion
• Disproportionation: same element is simultaneously oxidised and reduced

Important Oxidising and Reducing Agents

NDA often asks you to identify common oxidising and reducing agents, and the products of reactions involving KMnO4 and K2Cr2O7. Knowing a few key agents by heart saves time. KMnO4 (potassium permanganate) and K2Cr2O7 (potassium dichromate) are strong oxidising agents used in titrations. Non-metals like F2, Cl2 are strong oxidising agents. Metals like Na, Zn and H2S, SO2 are good reducing agents.

• Strong oxidising agents: KMnO4, K2Cr2O7, HNO3 (conc.), H2SO4 (conc.), F2, Cl2, O3
• Strong reducing agents: H2, C, CO, SO2, H2S, Na, Zn, Fe
• KMnO4 in acidic medium: Mn goes from +7 to +2 (colourless MnSO4 formed)
• KMnO4 in neutral/basic medium: Mn goes to +4 (brown MnO2 formed)
• K2Cr2O7 in acidic medium: Cr goes from +6 to +3 (green Cr2(SO4)3 formed)
• Bleaching powder (CaOCl2) acts as oxidising agent due to active Cl

• Confusing the oxidising agent with the substance being oxidised — the oxidising agent itself GETS REDUCED. Always check what happens to the agent itself.
• Using Arrhenius theory for all acids/bases. NDA asks about Lewis acids (like BF3) which have no H at all — Arrhenius fails there.
• Forgetting that HF is a weak acid despite fluorine being the most electronegative element. Strong bond between H and F reduces dissociation.
• Mixing up salt types: NH4Cl is acidic (not basic) because NH4OH is a weak base. Students reverse this frequently under exam pressure.
• Assuming pH = -log[acid concentration] always. This only works for strong acids that fully dissociate. For weak acids you need Ka and degree of dissociation.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain — the single most important memory hook for all redox questions.
• LEO GER: Losing Electrons is Oxidation, Gaining Electrons is Reduction — alternative to OIL RIG, use whichever sticks.
• SAAN WABB for salt pH: Strong Acid + Strong base = Neutral. Weak Acid + Strong Base = Basic. Weak base + strong Acid = Acidic.
• PANIC for strong acids: Perchloric, hydrochloric (HCl), Acetic... wait, acetic is weak. Strong ones: HClO4, HNO3, H2SO4, HCl, HBr, HI — memorise these 6, everything else is weak acid.
• Lewis BASE donates electron pair — both Base and Bond start with B, a base forms a bond by donating its pair.

• NDA typically asks 1-2 direct definition questions on theories of acids and bases — know which theory covers which situation. Lewis theory covering non-aqueous and non-proton systems is a favourite trap.
• pH calculation questions are straightforward — usually one step using pH = -log[H+] or the pH + pOH = 14 relation. Practice doing these in under 30 seconds.
• Oxidation number finding is a high-frequency question type. Expect to find oxidation number of Mn in KMnO4, Cr in K2Cr2O7, S in Na2S2O3, or Cl in various compounds — these repeat in PYQs.
• Redox identification questions show a reaction and ask which substance is oxidised, which is the reducing agent, etc. Solve by tracking oxidation numbers of each element before and after reaction.
• Buffer solution concept and which combination makes a buffer appear once per few papers. Remember: weak acid + salt of weak acid with strong base = acidic buffer.