The following diagram, called a Latimer diagram, summarizes the standard reducti

Question

The following diagram, called a Latimer diagram, summarizes the standard reduction potentials for various nitrogen-containing compounds and ions in acidic aqueous solution. It contains useful data for this HW set.

An arrow represents the “properly balanced reduction half-reaction” between the oxidized species on the left and the reduced species on the right of the arrow. The potential value over each arrow is the standard reduction potential E 0 red for that properly balanced half-reaction. For example, from this diagram, you can write NO3 – (aq) + e– + 2 H+ (aq) NO2(g) + H2O(l); E 0 red = +0.79 V

(2) (16 pts) Use the Latimer diagram above for this question.

(a) (6 pts) NO2(g) disproportionates spontaneously into nitric acid and nitrous acid in acidic media. Show this by writing the balanced chemical reaction and evaluating the standard emf for the reaction.

(b) (6 pts) Nitrous acid will not disproportionate spontaneously to nitrogen dioxide and nitric oxide. Show this by writing the balanced chemical reaction and evaluating the standard emf for the reaction.

(c) (4 pts) Given that the definition of 1 volt = 1 joule/coulomb, show that the reduction of the nitrate ion to nitrogen gas in acidic media has a standard reduction potential of +1.25 V.

Explanation / Answer

a) Write down the oxidation and reduction reactions (in acidic medium – we can only use H+ and H2O) as below.

NO2 (g) + H2O (l) ---------> NO3- (aq) + 2 H+ (aq) + e-; E01 = -0.79 V (this is the reverse process of reduction and hence, the reduction potential must be reversed to give the oxidation potential) …(1)

NO2 (g) + H+ (aq) + e- --------> HNO2 (aq); E02 = 1.12 V ……..(2)

Add equations (1) and (2) to get the balanced equation for the disproportionation as

2 NO2 (g) + H2O (l) --------> NO3- (aq) + HNO2 (aq) + H+ (aq)

E0cell = E01 + E02 = (-0.79 V) + (1.12 V) = 0.33 V (ans).

We know that

G0 = -n*F*E0cell where n = number of electrons involved in the redox process and F = 1 Faraday of electricity = 96500 Coulombs. Since E0cell is positive, G0 is negative.

The condition for spontaneity of a reaction is that G0 < 0. Consequently, the disproportionation of NO2 to NO3- and HNO2 is spontaneous (ans).

b) Write down the balanced chemical equations (in acidic medium) for the oxidation and reduction processes as below.

HNO2 (aq) ---------> NO2 (g) + H+ (aq) + e-; E01 = -1.12 V (oxidation potential is negative) …..(3)

HNO2 (aq) + H+ (aq) + e- ---------> NO (g) + H2O (l); E02 = 1.00 V ……(4)

Add equations (3) and (4) to get the balanced equation for the disproportionation as

2 HNO2 (aq) -------> NO2 (g) + NO (g) + H2O (l)

E0cell = E01 + E02 = (-1.12 V) + (1.00 V) = -0.12 V (ans).

Since E0cell is negative, G0 is positive, i.e, > 0. Hence, the disproportionation reaction is non-spontaneous (ans).

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