Question - 11 : - (a) Though alkaline potassium permanganate and acidic potassiumpermanganate both are used as oxidants, yet in the manufacture of benzoic acidfrom toluene, we use alcoholic potassium permanganate as an oxidant, why ?Write a balanced redox equation for the reaction.

(b) When concentrated sulphuric acid is added to an inorganicmixture containing chloride, we get a colourless pungent smelling gas HC1.However, if mixture contains bromide, then red vapours of bromine are evolved.Why ?

Answer - 11 : -

In the manufacture ofbenzoic acid from toluene, neutral medium is preferred over acidic or alkalinemedium due to following reasons.

· In the neutralmedium, neither acid or base is to be added externally. This will definitelyreduce the cost of the manufacturing process.

· Alcohol if used assolvent will help in the formation of a homogeneous mixture between toluene(non-polar) and KMnO₄(ionic). Actually alcohol has non-polaralkyl group as well as polar OH group.

(b)
A chloride such as NaClreacts with concentrated sulphuric acid to evolve hydrogen chloride gas uponheating. It has a pungent smell.

Hydrogen bromide is also expected to be formed in the same way when abromide is heated with concentrated sulphuric acid. However, the acid being astrong oxidising agent will oxidise HBr to bromine (Br₂) which getsevolved as red vapours.

Please note HC1 (g) isquite stable and is not oxidised to chlorine with concentrated sulphuric acid.

Question - 12 : -
Identify the substance oxidised, reduced, oxidising agent and reducingagent for each of the following reactions :
1.   2AgBr(s) + C₆H₆O₂(aq)→2Ag(s) + 2HBr(aq) + C₆H₄O₂(aq)
2.   HCHO(l) + 2[Ag(NH₃)₂]⁺(aq)+ 3OH^–(aq) → 2Ag(s) + HCOO^– (aq) + 4NH₃(aq)+ 2H₂O(l)
3.   HCHO(l) + 2Cu²⁺(aq)+ 5OH^–(aq)→ Cu₂(O(s) + HCOO-(aq) + 3H₂O(l)
4.   N₂H₄(l)+ 2H₂O₂(l) →N₂(g) + 4H₂O(l)
5.   Pb(s) + PbO₂(s)+ 2H₂SO₄(aq)→ 2PbSO₄(s) + 2H₂O(l)

Answer - 12 : -

AgBr : reduced ; acts as oxidising agent
C₆H₆O₂ : oxidised ; acts asreducing agent.

Question - 13 : - Consider the reactions :
2S₂O^2-₃(aq) + I₂(s) → S₄O^2-₆(aq)+ 2I^–(aq)
S₂O²(aq) + 2Br₂(l) + 5H₂O (l)→ 2SO^2-₄(aq) + 4Br-(ag) + 10H +(aq)
Why does the same reductant the osulphate react differently with iodine andbromine ?

Answer - 13 : - I₂ oxidises thiosulphate ion to tetrathionate ion i.e.,from O.S. of + 2 for S (in S₂O^2-₃ ) toS.O. of + 5/2 for S (in S₄O^2-₄ ion).
Br₂ oxidises thiosulphate ion to sulphate ion i.e., fromO.S. of + 2 for S (in S₂O^2-₃) to O.S. of + 6for S
(in SO^2-₄ ion)
This is because Br₂ is a stronger

Question - 14 : - Justify giving reactions that among halogens, fluorine is the bestoxidant and among hydrohalic compounds, hydroiodic acid is the best reductant.

Answer - 14 : - The halogens (X₂) have strong electron accepting tendency.They are therefore, powerful oxidising agents. The relative order of oxidisingpowers of halogens is :

Fluorine is the strongest oxidising agent (oxidant). It can be justifiedby the fact that it can liberate the other halogens from their respectivecompounds. For example,
2KC1 + F_{2 →} 2KF + Cl₂2KBr + F₂ → 2KF + Br₂2KI + F₂ → 2KF + I₂Among halogen acids (HX), the HI is the strongest reducing agent orreductant because its bond dissociation enthalpy is the minimum (299 kJ mol^-1)

(kJ mol^-1)
The iodination of methane for examples, is of reversible nature because HIformed in the reaction being a very strong reducing agent converts iodomethaneback to methane

However, thehalogenation with other halogen acids is not of reversible nature. Thisjustifies that HI is the strongest reducing agent in the halogen acids.

Question - 15 : - Why does the following reaction occur ?
XeO^4-₆ (aq) + 2F^–(aq) + 6H⁺(aq)→XeO₃(g) + 3H₂O(l)
What conclusion about the compound Na₄XeO₆ (of whichXeO^4-₆ is a part can be drawn from the reaction ?

Answer - 15 : -

In the reaction, O.N.of Xe decreases from + 8 to + 6. Therefore, XeO₆ isreduced.O.N. of F increases from – 1 to 0. Therefore, F^– isoxidised. The redox reaction occurs because Na₂XeO₆ (ofwhich XeOg“ is a part) is a stronger oxidising agent than F₂.

Question - 16 : - Consider the reactions :
(a) H₃PO₂ (aq)+ 4AgNO₃ (aq) + 2H₂O (l)→H₃PO₄ (aq)+ 4Ag (s) + 4HNO₃ (aq)
(b) H₃PO₂ (aq) + 2CuSQ₄ (aq) + 2H₂O(l)→ H₃PO₄ (aq) + 2Cu (s) + 2H₂SO₄ (aq)
(c) C₆H₅CHO (I) + 2[Ag(NH₃)₂⁺ (aq)+ 3OH^– (aq)→ C₆H₅COO (aq) + 2Ag (s) + 4NH₃ (aq)+ 2H₂O (l)
(d) C₆H₅CHO (l) + 2Cu²⁺(aq) + SOH^–(aq)→Nochange observed
What inference do you draw about the behaviour of Ag⁺ and Cu²⁺ ionsfrom these reactions ?

Answer - 16 : -

(a) Hypophosphorusacid (H₃PO₂) reduces Ag⁺ ions to Ag whichgets precipitated
(b) Hypophosphorus acid (H₃PO₂)reduces Cu²⁺ ions to Cu which gets precipitated
(C) Benzaldehyde (C₆H₅CHO)reduces Ag+ions to Ag which gets precipitated
(d) Benzaldehyde (C₆H₅CHO)is not in a position to reduce Cu²⁺ ions to Cu.
This shows that Ag⁺ ions are stronger oxidising agent than Cu²⁺ ionsin all the reactions. This is quite evident from their relative positions inthe reactivity series.

Question - 17 : - Balance the following equations byion-electron method

Answer - 17 : -

Question - 18 : - Balance the following equations in basic medium by ion-electron methodand oxidation number method and identify the oxidising agent and the reducingagent.
(a) p₄(s) + OH⁺(aq) —> PH₃(g) + H₂PO^–₂ (aq)
(b) N₂H₄(7) + CIO^–₃ (aq) —>NOfe) + C l^–(g)
(c) C1₂O₇(g) + H₂O₂(aq) —> CIO^–₂(aq)+ O₂(g) + H^–

Answer - 18 : -

Question - 19 : - Write four informations about the reaction :
(CN)₂ (g) + 2OH^– (aq)—> CN^– (aq)+ CNO (aq) + H₂O (l).

Answer - 19 : -

1. The reaction involvesthe decomposition of cyanogen (CN)₂ in the alkaline medium.

2. Both (CN)₂ i.e.cyanogen and CN^– i.e. cyanide ions are pseudo halogens innature i.e. they behave like halogens in characteristics.

3. Cyanogen undergoesdisproportionation in the reaction. It undergoes simultaneous increase anddecrease in O.N.of the species involved.

4. It is an example ofredox reaction.

Question - 20 : - The Mn³⁺ ion is unstable in solution and under goesdisproportionation to give Mn²⁺, MnO₂ and H⁺ ion.Write balanced ionic equation for the reaction.

Answer - 20 : -

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