20.2 Predicting direction of redox reactions

Question 1

It is possible to use standard electrode potentials to decide on the feasibility of a reaction.
Why

Answer 1

When you connect a pair of electrodes, the electrons will flow from the more negative species to the more positive species
So the signs of the electrodes tell us the direction of the redox reaction

Question 2

With electrodes
Zn2+(aq) + 2e- —> Zn(s) with Eº=-0.76V

Cu2+(aq) + 2e- –> Cu(s) with Eº= +0.34V

What direction would electrons move, so what way will the two half reactions go

Why does the reverse reaction not occur

Answer 2

.Electrons move from more negative to less negative
So go from zinc to copper so zinc is the better reducing agent, so will oxidise itself, whilst the copper ions are reduced.

. So the half reactions are:
Zn(s) —> Zn2+ 2e-
and Cu2+ + 2e- —> Cu(s)

So the overall reaction is
Cu2+ + Zn2+ –> Zn(s) + Cu2+

The reverse reaction wouldn’t occur because it would require electrons to flow from positive to negative

Question 3

How would a cell with only metal ions but no metal work .
How would you write it

Eg Fe2+ and Fe3+

Answer 3

Beaker containing all the relevant ions and a platinum electrode to make electrical contact is used in order to measure Eº by connecting a hydrogen electrode

. In one beaker, the H+ ions with hydrogen gas entering it
. In the other beaker, the mixture of Fe2+ and Fe3+ ions with a platinum electrode

So diagram would be
Pt/H2(g)/2H+(aq)//Fe3+(aq), Fe2+(aq)/Pt

There is a comma between ions in the same state

Question 4

Does oxidation occur at the anode or cathode

What about reduction

Answer 4

It always occurs at the anode

Remember that they are both vowels so go together

Reduction occurs at the cathode