Question

Which of the following redox reactions do you expect to occur spontaneously in the reverse direction? (Hint: The reactions are occurring under standard conditions (1 M for the aqueous ions).)

Check all that apply.

	2Ag+(aq)+Ni(s) ? 2Ag(s)+Ni2+(aq)
	Ca2+(aq)+Zn(s) ? Ca(s)+Zn2+(aq)
	2Al(s)+3Pb2+(aq) ? 2Al3+(aq)+3Pb(s)
	Fe(s)+Cr3+(aq) ? Fe3+(aq)+Cr(s)

Answer 1

Occuring spontaneously in the reverse direction means that
it's nonspontaneous in the forward reaction

Ecell = Ecathode-Eanode

Spontaneous when Ecell negative, when cathode potential is greater than anode's

nonspontaneous when anode potential is greater than cathode's.

1. Ag is getting reduced and Ni is getting oxidized, so, in the table,
Ni (anode) should be above Ag(cathode) in order to be nonspontaneous.

The reaction is spontaneous. Does not occur spontaneously in the reverse direction.

2. Zn oxidized, Ca reduced.

Zn should be above Ca, yes.

Occurs spontaneously in the reverse direction.

* tip: usually, to see if it's spontaneous in a forward reaction,

I skim the equation to see which one is going through reduction/oxidation.

then, I expect to see reduction half reaction to be located above the oxidation reaction.

Ecathode - Eanode = Ecell

+ (positive) - (less positive) = will always be positive!

3. Al oxidized, Pb reduced.

Al should be above Pb, no.

Does not occur spontaneously in the reverse direction.

4. Fe is oxidized and Mn is being reduced.

Fe (anode) should be higher than Mn(cathode) = yes

Occurs spontaneously in the reverse direction.