Free-energy change, G , is related to cell potential, E , by the equation G = n

Question

Free-energy change, G, is related to cell potential, E, by the equation

G=nFE

where n is the number of moles of electrons transferred and F=96,500C/(mol e) is the Faraday constant. When E is measured in volts, G must be in joules since 1 J=1 CV.

Part A

Calculate the standard free-energy change at 25 C for the following reaction:

Mg(s)+Fe2+(aq)Mg2+(aq)+Fe(s)

Express your answer to three significant figures and include the appropriate units.

3.71×105

Since E is positive and G is negative, this reaction is spontaneous under standard conditions.

Part B

Calculate the standard cell potential at 25 C for the reaction

X(s)+2Y+(aq)X2+(aq)+2Y(s)

where H = -787 kJ and S = -241 J/K .

Express your answer to three significant figures and include the appropriate units.

Part A was correct, but I'm miscalulating part B.

3.71×105

Explanation / Answer

T = 273 + 25 = 298 K

H = -787 kJ

S = -241 J/K . = - 241 x 10^-3 kJ /K

Go   = H - T S

            = -787 - 298 x (- 241 x 10^-3 )

            = -715.2 kJ /mol

Go   = - n F Eo

here = n = 2 electrons used up

-715.2 = - 2 x 96485 x 10^-3 x Eo

Eo = 3.71 V

(here 10^-3 is used for kJ conversion)

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