Question

A voltaic cell is constructed from a standard Cd? Cd half cell (Ered = -0.403V) and a standard Ag Ag half cell (Ered = 0.799V The anode reaction is: The cathode reaction is: The spontaneous cell reaction is: The cell voltage is

Answer 1

1) Since, E° value of Cd is more negative than Ag. Hence, Cd will act as reducing agent and it will reduce Ag. Ag will act as oxidizing agent and it will oxidize Cd.

At cathode (Reduction reaction);

Ag+ + e- ----> Ag

At Anode (oxidation reaction);

Cd ----> Cd2+ + 2e-

Cell reaction;

2Ag+ + Cd ----> 2Ag + Cd2+

E°(cell) = E(cathode) - E(anode)

E°(cell) = 0.799 - (-0.403) = 1.202 Volts ...Answer

2) E° for Al is more negative than E° for Zn. Thus, Al is reducing agent and will reduce Zn. Zn is oxidizing agent and will oxidize Al.

At Cathode (Reduction);

Zn2+ + 2e- -----> Zn

At Anode (Oxidation)

Al -----> Al3+ + 3e-

Cell reaction;

3Zn2+ + 2Al -----> 3Zn + 2Al3+

E°(cell) = E°(cathode) - E°(Anode)

E°(cell) = - 0.763 - (-1.660) = 0.897 volts ....Answer

3) N2 + 3H2 ----> 2NH3

A) This reaction is exothermic reaction. Thus, Delta H(rxn) is negative i.e. less than zero.

B) Number of moles of NH3 (2) is less than number of moles of reactants (3+1 = 4). Thus, Delta S(rxn) is negative i.e. less than zero.

C) We know that;

Delta G = Delta H - T * Delta S

Delta H and Delta S both are negative.

Delta G = (-ve) - T * (-ve)

If temperature is high, then Delta G is positive i.e. Delta G > 0

If temperature is low, then Delta G is negative i.e. Delta G < 0

D) Delta S(universe) = Delta S(system) + Delta S(surroundings)

Since, reaction system and surroundings are same temperature. Thus Delta S(universe) = 0

Let me know if any doubts.