Question

1. Quinone (Q) is the representative species that undergoes a single two-electron reduction in an aqueous media. Simultaneous protonation generates hydroxyquinone (CH2) at pH 0. (a) Explain the principle of single two-electron reduction. (b) The reduction potential of quinone is +0.669 V at pH 0. Predict the reduction potential at pH 7 and 14. (c) The measured reduction potentials of quinone are +0.433 V and +0.11 V at pH 7 and 14, respectively. Explain the difference between the theory and experiment.

Answer 1

OH کم ay 2 H+ + 20 + OH quinone (a) Hydroxyquinone (H₂) we can simple write , Q+ 2 H+ 420 H2Q - electrode is The reduction potential of this ginen Nernest equation by o a RT E = € en H₂Q - (0) 2F aa 2 H+ where 60 → Molar E Reduction potential of the electrode standard reduction potential of the electrode gan Constant Kelvin temperature & Faraday's constant a H₂2., a Q is a H+ the activity H₂Q, Q respectively. are € HT sleeg so, quinone in acid medium takes tung two electron & reduced to proton (14+) is Hydroquinone (H₂O): This is the basic principle.

b ) wrrite equation (i) Es RT 2 F en H₂Q 2. a a a E = EL RT en a H₂Q 2F RT 2F ln Q a 2 144 - £: E a RT 2F en H₂Q ook nang - R T x 1-2 ln aut) → E = £° – RT en a H₂O H 2R RT P (11) 2 [:- lna 'H+ =P Epl] at pH=0 ; £ + 0.669 V oc putting this thro values in equation () we get, 0.6.69 (fºr the en primele ) tö) so equation (ii) & (in) can simply written an E = 0.669 RT 4 P F - E= 0.669 The value of RT F at 25°C 0.059 pH is 0.059 so, at pH = 7 E = 0. 669 -0,059 X 7 = 0.256 v at pH = 14 E = 0.669 0.059x14 =-0.157 v

E value (theoritical) € value (experimental) pH = 7 0.256 +0.433 v H -0.157 v e14 +O. || v In Both experimental value is much theoritical. This is due to the fact that greater than equation (ü) 0.66) E-RT en a H₂O - lin 2 2F actually, €° = 0.669 v ) odbrane (at 25°c) but theorith cally take the whole value of right side is equal to 0. 669 v. assumption Because theoritically we made that a H₂Q Sa . But we Cannot made practically tuno > the concentration activity equal rather we made equal. So, thon difference Occours reduction potential (E) valier. between Again banie Condition pt= =16 partial ) ionisation Ocourers of H2O H₂Q occown. H₂Q HQ + Ht so, the Concentration crell as activity of Ha is decre ares. So value increases practically.