Question

Answer the following questions:
1) Write down the chemistry involved in lead storage cell during charging and working.
2) What is cathodic protection? Write down the chemistry involved in rusting of iron .

Answer 1

1)During charging

Negative plate reaction: PbSO4(s) + H+(aq) + 2e– → Pb(s) + HSO4–(aq)

Positive plate reaction: PbSO4(s) + 2H2O(l) → PbO2(s) + HSO4–(aq) + 3H+(aq) + 2e–

Combining these two reactions, the overall reaction is the reverse of the discharge reaction:

2PbSO4(s) + 2H2O(l) → Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4–(aq)

During charging (working)

Negative plate reaction: Pb(s) + HSO4–(aq) → PbSO4(s) + H+(aq) + 2e–

Positive plate reaction: PbO2(s) + HSO4–(aq) + 3H+(aq) + 2e– → PbSO4(s) + 2H2O(l)

Combining these two reactions, one can determine the overall reaction:

Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4–(aq) → 2PbSO4(s) + 2H2O(l)


2) Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell.

Rusting

When a droplet of water containing a little dissolved oxygen falls on an steel pipe, the solid iron or Fe(s) under the droplet oxidizes:

Fe(s) → Fe2+(aq) + 2e-

The electrons are quickly consumed by hydrogen ions from water (H2O) and dissolved oxygen or O2(aq) at the edge of the droplet to produce water:

4e- + 4H+(aq) + O2(aq) → 2H2O(l)

More acidic water increases corrosion. If the pH is very low the hydrogen ions will consume the electrons anyway, making hydrogen gas instead of water:

2H+(aq) + 2e- → H2(g)


Hydrogen ions are being consumed by the process. As the iron corrodes, the pH in the droplet rises. Hydroxide ions (OH-) appear in water as the hydrogen ion concentration falls. They react with the iron(II) ions to produce insoluble iron(II) hydroxides or green rust:

Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)

The iron(II) ions also react with hydrogen ions and oxygen to produce iron(III) ions:

4Fe2+(aq) + 4H+(aq) + O2(aq) → 4Fe3+(aq) + 2H2O(l)

The iron(III) ions react with hydroxide ions to produce hydrated iron(III) oxides (also known as iron(III) hydroxides):

Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

The loose porous rust or Fe(OH)3 can slowly transform into a crystallized form written as Fe2O3.H2O the familiar red-brown stuff that is called "rust"