CHEM1001 Worksheet 12: Introduction to Electrochemistry

Model 1: Redox Reactions Reduction occurs when an atom/ion gains electrons, oxidation occurs when an atom/ion loses electrons. When these two phenomena happen in a single reaction, it is called a redox reaction. Electrons which are lost by the species undergoing reduction are gained by the species undergoing oxidation: electrons are transferred. An example is shown below:

2Cu(s) + O!(g) ⇌ 2CuO(s) (1)

Critical thinking questions 1. What are the oxidation numbers of the Cu atoms in Cu and CuO?

2. What are the oxidation numbers of the O in O2 and CuO? 3. What do the change in oxidation numbers tell you about whether electrons are being lost or gained? 4. Which is the chemical species that is being oxidised and which is being reduced? 5. What do the answers above tell you about the relationship between the change in oxidation number and whether a

species has undergone reduction or oxidation? 6. How many electrons are being transferred per atom and where are they coming from and going to? 7. What do you think the terms oxidising agent and reducing agent mean? Which chemical species is which? 8. Assign oxidation numbers to all the atoms in the following reactions, and determine if they are redox reactions or

not. If so, identify the atoms which are oxidised and which are reduced, the oxidising agent and the reducing agent, and the number of electrons that are transferred between them.

(a) FeO(s) + CO(g) ⇌ Fe(s) + CO!(!)

(b) FeCO!(s) ⇌ FeO(s) + CO!(g)

(c)  2H!(aq) +Mg(s) ⇌ Mg!!(aq) + H!(g)

(d) H!(aq) + OH!(aq) ⇌ H!O(l)

The reduction and oxidation parts of a redox reaction can be written separately. For reaction (1), the half reactions are:

Cu ⇌ Cu!! +  2e! (2) O! +  4e! ⇌ 2O!! (3)

9. Show how adding these two reactions gives reaction (1) 10. Instead of the half reaction (2), use Li ⇌ Li! +  1e!. Add this to half reaction (3) and show the full redox

reaction. Hint: you will need to balance the electrons. 11. What are the two half reactions for Q8(c)? 12. Add the following pairs of half reactions to create the full redox reaction.

(a) Au!!(aq) +  3e! ⇌ Au(s) and 2Cl!(aq) ⇌ Cl!(g) +  2e!

(b) Ag!(aq) +  1e! ⇌ Ag(s) and La(s) ⇌ La!!(aq) +  3e!

13. Split the following redox reactions into their half reactions. Use the oxidation numbers to guide you.

(a) Sn!! +  2Fe!! ⇌ Sn!! +  2Fe!!

(b) Pb + 2HSO!! +  PbO! +  2H! ⇌ 2PbSO! + 4H!O