Question

Determine the EMF of this equation 2Fe + 6HCl ----> 2FeCl3 + 3H2

and what is that Reduction and Oxidation.

Answer 1

In an oxidation process there is always an increase in oxidation state. Reduction is a decrease in oxidation state. To determine the elements oxidized and reduced, look at the oxidation states.

2Fe + 6HCl ----> 2FeCl₃ + 3H₂

Oxidation sate of Fe in FeCl3 = +3

Oxidation sate of H in HCl = +1

Oxidation sate of solid Fe = 0

Oxidation sate of H in H2 = 0

In the above reaction oxidation sate of Fe has increased from 0 to +3, i.e., Fe in oxidized. Oxidation state of H is decreased from +1 to 0 ,i.e., HCl is reduced to H2.

So the half reactions are:

Anode reaction(Oxidation) : 2Fe = 2Fe³⁺ + 6e

Cathode reaction(Reduction) : 6H⁺ + 6e = 3H2

You can set up a cell now: Fe│FeCl₃ (a₁) ││ HCl (a₂) │ H₂ │ Pt

You need to know the concentration of FeCl₃ (a₁) and HCl (a₂) to determine the formal EMF.

You have not given the concentration values, so you can calculate only the standard EMF value( when a₁ = a₂ = 1 and H₂ pressure is 1 atm in the cell) for the cell. For that you need E⁰_Fe3+/Fe and E⁰_H+/H2 .

Standard EMF of the cell

= E⁰_cathode - E⁰_anode

= E⁰_H+/H2 - E⁰_Fe3+/Fe