Everything Science Grade 12

CHAPTER 4. ELECTROCHEMICAL REACTIONS 4.4

Look at the electrode potential for the first half reaction. The negative value shows
that lead loses electronseasily, in other words itis easily oxidised. The reaction would
normally proceed fromright to left (i.e. the equilibrium lies to the left), but in the
original equation, the opposite is happening. It isthe Pb2+ions that are being reduced
to lead. This part of thereaction is therefore notspontaneous. The positive electrode
potential value for the bromine half-reaction shows that bromine is more easily reduced,
in other words the equilibrium lies to the right. The spontaneous reaction proceeds
from left to right. This is not what is happeningin the original equationand therefore
this is also not spontaneous. Overall it is clear then that the reaction will not proceed
spontaneously.

Tip

Another way of predict-

ing whether a reaction

occurs spontaneously, is

to look at the sign of the

emf value for the cell. If

the emf is positive then

the reaction is sponta-

neous. If the emf is nega-

tive, then the reaction is

not spontaneous.

Example 6: Predicting whether a reaction is spontaneous

QUESTION

Will copper react withdilute sulphuric acid (H 2 SO 4 )? You are given the fol-

lowing half reactions:

Cu2+(aq) + 2e−� Cu(s) (E^0 = +0. 34 V)

2H+(aq) + 2e−� H 2 (g) (E^0 = 0 V)

SOLUTION

Step 1 : For each reaction, lookat the electrode potentials and decide in

which direction the equilibrium lies

In the first half reaction, the positive electrodepotential means

that copper does not lose electrons easily, in other words it is

more easily reduced andthe equilibrium positionlies to the right.

Another way of saying this is that the spontaneous reaction is the

one that proceeds from left to right, when copperions are reduced

to copper metal.

In the second half reaction, the spontaneous reaction is from right

to left.

Step 2 : Compare the equilibrium positions to the original reaction

What you should noticeis that in the original reaction, the reac-

tants are copper (Cu) and sulphuric acid (2H+). During the reac-

tion, the copper is oxidised and the hydrogenions are reduced.

But from an earlier step, we know that neitherof these half reac-

tions will proceed spontaneously in the directionindicated by the

original reaction. The reaction is therefore not spontaneous.