Question

Given the E⁰ values of the following two half-reactions:

Zn à Zn²⁺ + 2e^-               E⁰ = 0.763 volt

Fe à Fe²⁺ + 2e^-                E⁰ = 0.441 volt

a)Write a balanced complete oxidation-reduction reaction?

b)Explain whether the corrosion of an iron pipe (i.e., Fe Fe²⁺) in the presence of Zn/Zn²⁺ is possible or not (thermodynamically)?

c)Explain whether or not Zn will protect the corrosion of iron pipe if metallic Zn is in contact with the iron pipe?

Answer 1

The electrochemical series is built up by arranging various redox equilibria in order of their standard electrode potentials (redox potentials). The most negative E° values are placed at the top of the electrochemical series, and the most positive at the bottom.

Metals at the top of the series are good at giving away electrons. They are good reducing agents.

Metal ions at the bottom of the series are good at picking up electrons. They are good oxidising agents.

So in summary the more positive the E° value, - the less readily the metal loses electrons, and the more readily its ions pick them up again. The more positive the value, the stronger oxidising agent the metal ion is.

So from the Eo values given here Zn acts as an oxidising agent and Fe will act as reducing agent

so the balanced equation will be

Fe(s) + Zn²⁺ Fe²⁺ + Zn(s)

Zince does not corrode iron pipes it in turns protect it from corrosion so Zinc galvanised iron pipes are available.

Zince protect iron and steel pipes by the following mechanism.

• By a barrier effect, i.e. they prevent oxygen and moisture from reaching the steel surface.
• By giving cathodic protection at scratches, chips, edges etc. Zinc is a metal with a relatively low galvanic potential and a small tendency to corrode. However, the corrosion rate is low in most environments, since the surface of the coating is quickly covered with corrosion products, which subsequently protect against further corrosion.