Redox Reactions and Electrochemistry
- If the molar conductivity (_m) of a 0.050 mol L^-1 solution of a monobasic weak acid is 90 S cm^2 mol^-1, its extent (degree) of dissociation will be[Assume ^°_+ = 349.6 S cm^2 mol^-1 and ^°_- = 50.4 S cm^2 mol^-1.] Answer: 3. 0.225 Key idea: α=Lambda_m/Lambda^°_m for a weak electrolyte. Lambda^°_m=349.6+50.4=400 S cm² mol⁻¹. α=90/400=0.225 . Option (3). 1
- Consider the following compounds : KO_2, H_2O_2 and H_2SO_4.The oxidation states of the underlined elements in them are, respectively, Answer: 1. +1, -1, and +6 Key idea: assign oxidation states from known charges. KO₂ is potassium superoxide, K^+ and O₂^- : K = +1. H₂O₂ is... 1
- Mass in grams of copper deposited by passing 9.6487 A current through a voltmeter containing copper sulphate solution for 100 seconds is:(Given : Molar mass of Cu : 63 g mol^-1, 1F = 96487 C) Answer: 3. 0.315 g Key idea: Q=It , then use Faraday's law for mathrmCu²⁺+2e^-→ Cu . Q=9.6487×100=964.87 C, so moles of e^- = 964.87/96487 =... 1
- Match List I with List II.List I (Conversion): A. 1 mol of H_2O to O_2 B. 1 mol of MnO_4^- to Mn^2+ C. 1.5 mol of Ca from molten CaCl_2 D. 1 mol of FeO to Fe_2O_3List II (Number of Faraday required): I. 3F II. 2F III. 1F IV. 5FChoose the correct answ… Answer: 2. A-II, B-IV, C-I, D-III Key idea: Faradays = moles of electrons transferred. A: 2H₂O→ O₂+4H^++4e^- , so 1 mol H₂O needs 2 F (II).... 1
- Which reaction is NOT a redox reaction? Answer: 1. BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl Key idea: a redox reaction has a change in oxidation numbers. BaCl₂+Na₂SO₄→ BaSO₄+2NaCl is a doubl... 1
- On balancing the given redox reaction,a Cr_2O_7^2- + b SO_3^2-(aq) + c H^+(aq) → 2a Cr^3+(aq) + b SO_4^2-(aq) + c2H_2O()the coefficients a, b and c are found to be, respectively - Answer: 2. 1, 3, 8 Key idea: Balance by the ion-electron (or oxidation-number) method. Cr goes from +6 to +3: gain of 3 e ^- per Cr, 6 e ^-... 1
- Given below are two statements : one is labelled as Assertion A and the other is labelled as Reason R :Assertion A : In equation Δ_r G = -nFE_cell, value of Δ_r G depends on n.Reasons R : E_cell is an intensive property and Δ_r G is an extensive prop… Answer: 2. Both A and R are true and R is the correct explanation of A. Key idea: Δ_rG = -nFE_cell links an extensive quantity to an intensi... 1
- The conductivity of centimolar solution of KCl at 25°C is 0.0210 ohm^-1 cm^-1 and the resistance of the cell containing the solution at 25°C is 60 ohm. The value of cell constant is - Answer: 4. 1.26 cm ⁻¹ Key idea: Cell constant = kappa × R . G^* = 0.0210\ Ω⁻¹mathrmcm⁻¹ × 60\ Ω = 1.26\ mathrmcm⁻¹ . Option 4. 1
- Find the emf of the cell in which the following reaction takes place at 298 KNi(s) + 2Ag^+ (0.001\ M) → Ni^2+ (0.001\ M) + 2Ag(s)(Given that E^°_cell = 10.5 V, 2.303 RT/F = 0.059 at 298 K) Answer: Dropped in the official answer key: full marks to every candidate Key idea: use the Nernst equation E=E^°-0.059/nlog Q . Reaction: m... 1
- Given below are half cell reactions :MnO_4^- + 8H^+ + 5e^- → Mn^2+ + 4H_2O,E^°_Mn^2+/MnO_4^- = -1.510 V1/2O_2 + 2H^+ + 2e^- → H_2O,E^°_O_2/H_2O = +1.223 VWill the permanganate ion, MnO_4^- liberate O_2 from water in the presence of an acid ? Answer: 2. Yes, because E^°_cell = +0.287 V Key idea: a reaction is spontaneous if E^°_cell>0 , using the reduction potentials of the two ha... 1
- At 298 K, the standard electrode potentials of Cu^2+/Cu, Zn^2+/Zn, Fe^2+/Fe and Ag^+/Ag are 0.34 V, -0.76 V, -0.44 V and 0.80 V, respectively.On the basis of standard electrode potential, predict which of the following reaction can not occur ? Answer: 1. 2CuSO₄(aq) + 2Ag(s) → 2Cu(s) + Ag₂SO₄(aq) Key idea: a metal can displace another metal from its salt only if its standard reducti... 1
- The molar conductivity of 0.007 M acetic acid is 20 S cm^2 mol^-1. What is the dissociation constant of acetic acid ? Choose the correct option.^°_H^+ = 350 S cm^2 mol^-1, ^°_CH_3COO^- = 50 S cm^2 mol^-1 Answer: 3. 1.75×10⁻⁵ mol L ⁻¹ Key idea: α=Lambda_m/Lambda^°_m , and K_a=Cα²/1-α≈ Cα² . Lambda^°=350+50=400 ; α=20/400=0.05 . K_a≈0.007×(0.05... 1
- The molar conductance of NaCl, HCl and CH_3COONa at infinite dilution are 126.45, 426.16 and 91.0 S cm^2 mol^-1 respectively. The molar conductance of CH_3COOH at infinite dilution is. Choose the right option for your answer. Answer: 2. 390.71 S cm ² mol ⁻¹ Key idea: Kohlrausch's law of independent migration of ions. Lambda^°(CH₃COOH)=Lambda^°(CH₃COONa)+Lambda^°(H... 1
- Which of the following reactions is the metal displacement reaction ? Choose the right option. Answer: 2. Cr₂O₃ + 2Al xrightarrowΔ Al₂O₃ + 2Cr Key idea: a metal displacement reaction is one where a more reactive metal displaces a less... 1
- The number of Faradays(F) required to produce 20 g of calcium from molten CaCl_2 (Atomic mass of Ca = 40 g mol^-1) is : Answer: 1. 1 Electrode reaction: mathrmCa²⁺+2e^-→ Ca . One mole of Ca needs 2 F. Moles of Ca =20/40=0.5 mol. Charge =0.5×2=1 F. Answer: opti... 1
- What is the change in oxidation number of carbon in the following reaction ?CH_4(g) + 4Cl_2(g) → CCl_4(l) + 4HCl(g) Answer: 3. - 4 to +4 Find carbon's oxidation number on each side. In CH₄ : x+4(+1)=0 , so x=-4 . In CCl₄ : x+4(-1)=0 , so x=+4 . The change... 1
- On electrolysis of dil.sulphuric acid using Platinum (Pt) electrode, the product obtained at anode will be : Answer: 2. Oxygen gas In dilute H₂SO₄ with inert Pt electrodes, water is oxidised at the anode more easily than the sulphate ion. Anode: 2H₂... 1
- For a cell involving one electron E^_cell = 0.59 V at 298 K, the equilibrium constant for the cell reaction is :[Given that 2.303 RT/F = 0.059 V at T = 298 K] Answer: 3. 1.0×10¹⁰ At 298 K, E^(ominus)_cell = 0.059/nlog K . log K = dfracnE^(ominus)0.059 = 1×0.59/0.059 = 10 , so K = 1.0×10¹⁰ . Option... 1
- For the cell reaction2Fe^3+(aq) + 2I^-(aq) → 2Fe^2+(aq) + I_2(aq)E^_cell = 0.24 V at 298 K. The standard Gibbs energy (Δ_rG^) of the cell reaction is :[Given that Faraday constant F = 96500 C mol^-1] Answer: 1. -46.32 kJ mol ⁻¹ Formula: Δ_rG^(ominus) = -nFE^(ominus)_cell . In the reaction 2mathrmFe³⁺ + 2I^- → 2mathrmFe²⁺ + I₂ , 2 electron... 1
- Which of the following reactions are disproportionation reaction ?(a) 2Cu^+ → Cu^2+ + Cu^0(b) 3MnO_4^2- + 4H^+ → 2MnO_4^- + MnO_2 + 2H_2O(c) 2KMnO_4 Δ K_2MnO_4 + MnO_2 + O_2(d) 2MnO_4^- + 3Mn^2+ + 2H_2O → 5MnO_2 + 4H^Select the correct option from th… Answer: 1. (a) and (b) only A disproportionation reaction is one in which the same species is both oxidised and reduced. (a) mathrmCu^+ → Cu... 1
- For the redox reactionMnO_4^- + C_2O_4^2- + H^+ Mn^2+ + CO_2 + H_2Othe correct coefficients of the reactants for the balanced equation are(columns: MnO_4^-, C_2O_4^2-, H^+) Answer: 3. 2, 5, 16 Balance by the ion-electron method. Reduction: mathrmMnO₄^-+8H^++5e^-→ Mn²⁺+4H₂O . Oxidation: mathrmC₂O₄²⁻→2CO₂+2e^- . M... 1
- Consider the change in oxidation state of Bromine corresponding to different emf values as shown in the diagram below :BrO_4^- 1.82\ V BrO_3^- 1.5\ V HBrO 1.595\ V Br_2 1.0652\ V Br^-Then the species undergoing disproportionation is Answer: 4. HBrO A species disproportionates if the emf for its reduction (to the species on one side) is higher than the emf for its oxidati... 1
- The correct order of N-compounds in its decreasing order of oxidation states is Answer: 1. HNO ₃ , NO, N ₂ , NH ₄ Cl Find the oxidation state of N in each compound. HNO₃ : +5 NO: +2 N₂ : 0 NH₄Cl : -3 The decreasing order... 1
- Ionic mobility of which of the following alkali metal ions is lowest when aqueous solution of their salts are put under an electric field ? Answer: 3. Li Key idea: in aqueous solution ionic mobility falls with greater hydration, and smaller ions hydrate more. Hydrated size order... 1