Everything Science Grade 12

4.4 CHAPTER 4. ELECTROCHEMICAL REACTIONS

Example 5: Calculating the emf ofa cell

QUESTION

Calculate the cell potential of the electrochemical cell in which the following

reaction takes place, and represent the cell using standard notation.

Mg(s) + 2H+(aq)→ Mg2+(aq) + H 2 (g)

SOLUTION

Step 1 : Write equations for thetwo half reactions involved

Mg2++ 2e−� Mg (E◦V =− 2. 37 )

2H++ 2e−� H 2 (E◦V = 0. 00 )

Step 2 : Determine which reaction takes place at the cathode and which

is the anode reaction

From the overall equation, it is clear that magnesium is oxidised

and hydrogen ions arereduced in this reaction. Magnesium is

therefore the anode reaction and hydrogen is the cathode reac-

tion.

Step 3 : Represent the cell using standard notation

Mg|Mg2+||H+|H 2

Step 4 : Calculate the cell potential

E◦(cell)= E◦(cathode) - E◦(anode)

= 0. 00 − (− 2 .37)

= +2. 37 V

Predicting whether a reaction will take place spontaneously

Look at the following example to help you tounderstand how to predict whether a

reaction will take placespontaneously or not. In the reaction,

Tip

You will see that the half
reactions are written as
they appear in the table
of standard electrode po-
tentials. It may be useful
to highlight the reacting
substance in each half
reaction. In this case, the
reactants are Pb2+and
Br−ions.

Pb2+(aq) + 2Br−(aq)→ Br 2 (l) + Pb(s)

the two half reactions are as follows:

Pb2++ 2e−� Pb (− 0. 13 V)

Br 2 + 2e−� 2Br−(+1.06 V)