Question

(3) (4 pts) Nitrous acid will not disproportionate spontaneously to the gases nitrogen dioxide and nitric oxide. Show this by writing the balanced chemical reaction and evaluating the standard emf (in V) for the reaction.

Answer 1

HNO_{​​​​2} ------------> NO + NO_{​​​​​2}

Oxidation state of N in HNO_{​​​​2} is 1(+1) +X + 2 (-2) = 0

X = +3

Oxidation state of NO is X+1(-2) = 0

X = +2

Oxidation state of NO_​​​2 is

x +2(-2) = 0

x = +4

Oxidation state N decreases from +3 to +2. It's reduction.

Oxidation state N increases from +3 to +4. It's oxidation.

Hence it is a disproportionation reaction.

Balanced equation is

2 HNO₂ ------------> NO + NO_{​​​​​2} +H_{​​​​​​2}​​​O

Half reactions are

NO_{​​​​​2}+ 2 H⁺ + 2 e ------->2 HNO₂ ; E^{​​​​​​0} =  1.065 V

HNO​​​_{​​​​​2} + + e ----------> NO + OH^{​​​​​-} E^{​​​​​​0} = –0.46 V

E^{​​​​​​}_{​​​​​cell} = E⁰_{​​​​Cathode} - E⁰_Anode

E_{​​​​​​Cell} = -0.46 V - 1.065 = -1.525 V

Since E_{​​​​​​cell} is negative, the reaction does not occur spontaneously.

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