Predict whether the following reactions would occur spontaneously in aqueous sol
ID: 1028240 • Letter: P
Question
Predict whether the following reactions would occur spontaneously in aqueous solution at 25°C. Assume that the initial concentrations of dissolved species are all 1.0 M. (a) Ca(s) + Cd2+(aq) Ca2+(aq) + Cd(s) Reaction is spontaneous. Reaction is not spontaneous. E° (b) 2 Br (aq) + Sn2+(aq) Br2(l) + Sn(s) Reaction is spontaneous. Reaction is not spontaneous. E° (c) 2 Ag(s) + Ni2+(aq) 2 Ag+(aq) + Ni(s) Reaction is spontaneous. Reaction is not spontaneous. E° (d) Cu+(aq) + Fe3+(aq) Cu2+(aq) + Fe2+(aq) Reaction is spontaneous. Reaction is not spontaneous. E°
Explanation / Answer
a) Write down the half cell reactions as below.
Ca2+ (aq) + 2 e- --------> Ca (s); E0 = -2.87 V …..(1)
Cd2+ (aq) + 2 e- --------> Cd (s); E0 = -0.403 V …..(2)
The given reaction is obtained by reversing equation (1) (this reverses the sign of E0) and adding to equation (2); therefore,
E0cell = E0(2) + [-E0(1)] = (-0.403 V) + [-(-2.87 V)] = 2.467 V
G0 = -n*F*E0cell = - (2 mole)*(96485 J/V.mol)*(2.467 V)*(1 kJ/1000 J) = -476.057 kJ
Since G0 < 0, hence, the said reaction is spontaneous at 25°C (ans).
b) Write down the half cell reactions as below.
Br2 (l) + 2 e- ---------> 2 Br- (aq); E0 = +1.066 V ……(1)
Sn2+ (aq) + 2 e- ---------> Sn (s); E0 = -0.14 V ……(2)
E0cell = E0(2) + [-E0(1)] = (-0.14 V) + (-1.066 V) = -1.206 V
We define G0 = -n*F*E0cell where n = 2moles is the number of moles of electrons involved in the redox reaction. Since E0cell < 0, hence, G0 > 0 and the given reaction is non-spontaneous at 25°C (ans).
c) Write down the half cell reactions as below.
Ag+ (aq) + e- -------> Ag (s); E0 = +0.7994 V …..(1)
Ni2+ (aq) + 2 e- --------> Ni (s); E0 = -0.25 V ……(2)
E0cell = E0(2) + [-E0(1)] = (-0.25 V) + (-0.7994 V) = -0.10494 V.
We define G0 = -n*F*E0cell where n = 2moles is the number of moles of electrons involved in the redox reaction. Since E0cell < 0, hence, G0 > 0 and the given reaction is non-spontaneous at 25°C (ans).
d) Write down the half cell reactions as below.
Cu2+ (aq) + e- --------> Cu+ (aq); E0 = +0.153 V ……(1)
Fe3+ (aq) + e- --------> Fe2+ (aq); E0 = +0.771 V …….(2)
E0cell = E0(2) + [-E0(1)] = (+0.771 V) + (-0.153 V) = +0.618 V
We define G0 = -n*F*E0cell where n = 1 mole is the number of moles of electrons involved in the redox reaction. Since E0cell > 0, hence, G0 < 0 and the given reaction is spontaneous at 25°C (ans).