Atoms and Molecules

Question 35

Given the following standard redox potentials,

Fe³⁺/Fe²⁺ = 0.771 V
Cu²⁺/Cu⁺ = 0.15 V
2O₂ , 2H⁺/H2O = 0.816 V

which of the following statements are true?

(a) Fe³⁺ can oxidise water
(b) Fe²⁺ can reduce oxygen
(c) Cu²⁺ can reduce oxygen
(d) Cu⁺ is the strongest reducing agent
(e) the following reaction will occur spontaneously
        Fe³⁺ + Cu⁺ = Fe²⁺ + Cu²⁺
(f) Fe³⁺ is the strongest oxidising agent

Answer

Remember that reducing agents give up their electrons to oxidising agents and consequently are oxidised, and oxidising agents remove electrons from reducing agents and consequently are reduced. Electrons go from the reduced part of a redox pair of more negative (or less positive) redox potential to the oxidised part of a redox pair of more positive (or less negative) redox potential.

Thus electrons will travel

            Cu⁺ Fe³⁺        ; i.e. Cu⁺ reduces Fe³⁺ to Fe²⁺    ;  Fe³⁺ oxidises Cu⁺ to Cu²⁺
              Fe²⁺ O₂         ; i.e. Fe²⁺ reduces O₂ to H₂O       ;  O₂ oxidises Fe⁺⁺ to Fe³⁺
            Cu⁺ O₂            ; i.e. Cu⁺ reduces O₂ to H₂O         ; O₂ oxidises Cu⁺ to Cu²⁺

The stronger oxidant part of a redox pair of more positive (or less negative) redox potential pulls the electrons from the reduced part of a redox pair of more negative (or less positive) redox potential.

(a) No. Fe³⁺ is a weaker oxidising agent than O₂
(b) Yes. 4Fe²⁺ + O₂ + 4H⁺ 4Fe³⁺ + 2H₂O
(c) No. Cu²⁺ is not a reducing agent as it cannot give up any electrons.
(d) Yes.
(e) Yes. Then comes Fe²⁺ then comes H₂O (which is a very weak reducing agent)
(g) No. O₂ is the strongest oxidant then comes Fe³⁺ then comes Cu²⁺

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This page was last updated by Martin Chaplin
on 29 August, 2003