Two half-reactions are proposed for an electrochemical process carried out at 298 K

CHEMISTRY · Multi-part · A Level · Chemistry · Electrode potentials · Electrode potentials

Problem

Two half-reactions are proposed for an electrochemical process carried out at 298 K. Half-reaction 1: P

^{2+}

+ 2 e

^{-}

\;\longrightarrow\;

P(s)

E^{\circ} = +0.32\;\text{V}

Half-reaction 2: Q

^{3+}

+ 3 e

^{-}

\;\longrightarrow\;

Q(s)

E^{\circ} = -0.15\;\text{V}

A chemist combines the reactants so that the tentative overall reaction is 3 P

^{2+}

+ 2 Q(s)

\;\longrightarrow\;

3 P(s) + 2 Q

^{3+}

Assume all species are at unit activity (1.0 M for dissolved ions and 1 bar for gases). (a) Determine the balanced overall redox reaction that would be thermodynamically favored under standard conditions, if any. Clearly identify what is oxidized and what is reduced. (b) Calculate the standard cell potential

E^{\circ}_{\text{cell}}

for the favored reaction identified in part (a). (c) Using

n=6

mol e

^{-}

and

F = 9.65\times10^{4}\,\text{C mol}^{-1}

, compute the standard free-energy change

\Delta G^{\circ}

(in kJ mol

^{-1}

) for the reaction in part (a). (d) The chemist measured the standard enthalpy change for the same reaction and obtained

\Delta H^{\circ} = +48\,\text{kJ mol}^{-1}

. Discuss how the signs of

E^{\circ}_{\text{cell}}

\Delta G^{\circ}

, and

\Delta H^{\circ}

together inform the chemist about the driving force and energetic profile of the process.

Related theory

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