Everything Science Grade 12

CHAPTER 4. ELECTROCHEMICAL REACTIONS 4.5

Although you don’t actually know what productis formed when

hydrogen peroxide is reduced, the most logicalproduct is OH−.

The reduction half reaction is:

H 2 O 2 → OH−

After the atoms and charges have been balanced, the final equa-

tion for the reduction half reaction is:

H 2 O 2 + 2e−→ 2OH−

Step 5 : Multiply each half reaction by a suitable number so that the

number of electrons released in the oxidationhalf reaction is made

equal to the number ofelectrons that are accepted in the reduction half

reaction.

We need to multiply the oxidation half reaction by two so that

the number of electronson both sides are balanced. This gives:

2Co(NH 3 )2+ 6 → 2Co(NH 3 )3+ 6 + 2e−

Step 6 : Combine the two halfreactions to get a finalequation for the

overall reaction.

2Co(NH 3 )2+ 6 + H 2 O 2 → 2Co(NH 3 )3+ 6 + 2OH−

Step 7 : Do a final check to make sure that the equation is balanced

Exercise 4 - 7

1. Balance the following equations.

(a) HNO 3 + PbS→ PbSO 4 + NO + H 2 O

(b) NaI + Fe 2 (SO 4 ) 3 → I 2 + FeSO 4 + Na 2 SO 4

2. Manganate(VII) ions(MnO− 4 ) oxidise hydrogen peroxide (H 2 O 2 ) to oxygen gas.
   The reaction is done in an acid medium. Duringthe reaction, the manganate(VII)
   ions are reduced to manganese(II) ions (Mn2+). Write a balanced equation for
   the reaction.


3. Chlorine gas is prepared in the laboratory byadding concentrated hydrochloric
   acid to manganese dioxide powder. The mixture is carefully heated.

(a) Write down a balanced equation for the reaction which takes place.

(b) Using standard electrode potentials, show by calculations why this mixture

needs to be heated.

(c) Besides chlorine gaswhich is formed during the reaction, hydrogen chloride

gas is given off when the concentrated hydrochloric acid is heated. Explain

why the hydrogen chloride gas is removed fromthe gas mixture when the

gas is bubbled through water.

(IEB Paper 2, 2004)