How to balance simple redox reactions.

OXIDATION STATE RULES

Oxidation numbers/states are used to balance redox reactions. Balancing more complicated redox-reactions can be quite challanging. First you must master the rules.

What is the oxidation number of an element uncombined with other elements? (For instance Cl₂) ?

Zero, zipp, nada.

What is the sum of all the oxidation numbers of all the atoms in a molecule/a spieces?

It is equal to the total charge. So for instance, MgO is an uncharged molecule. Since O almost always have the oxidation number (-2) when combined with other spieces (see below) Mg must have the oxidation number (+2) in this case.

What is the oxidation number of H when combined with other spieces?

(+1). Consider the H₂O molecule – O has the oxidation number (-2), hence the two hydrogens must contribute with one (+1) charge each!

What is the oxidation number of elements in group 1 and 2?

It is equal to their group number!!!

What is the oxidation number of the halogens?

(-1) – unless combined with O or another halogen. F is (-1) in all compounds.

What is the oxidation number of oxygen?

It is (-2) in most of its compounds. However, in combination with F which always has the oxidation number (-1). Other exceptions : peroxides (O₂^2-), superoxides and ozonides. These molecules are so uncommon that I doubt you'll have to remember it for now.

What does an increase in the oxidation number/state of a spieces indicate?

Oxidation! (loss of electrons = increased oxidation state)

What does a decrease in the oxidation number/state of a spieces indicate?

Reduction! (gain of electrons)

TEST YOUR SELF :

What is the oxidation number of S in H₂S?

(-2)

What is the oxidation number of P in P₄O₆?

(+3) – because the sum of the oxidation numbers of O₆ is 6 ^x(-2) is -12 and the molecule is uncharged the P₄-part of the molecule must equal 12. 12 divided over 4 molecules is of course +3.

What is the oxidation number of Cl in ClO^-?

(+1)

What is an oxidation agent? What happens to it during a redox-reaction?

An oxidation agent (or oxidant) causes oxidation!!! It makes the other spieces in the reaction go through oxidation because the oxidation agent stels electrons. Hence the oxidation agent (or oxidant) gets reduced in the process!!!

What is a reducing agent (or reductant)? What happens to it during a redox-reaction?

It causes reduction, by giving away its electrons. Hence it gets oxidized in the process.

How can you identify the oxidant and reductant in a redox reaction?

You consider the rules listed above. You then compare the oxidation numbers of the spieces before and after the reaction (on each side of the →). I an element has NOT undergone a change in oxidation number it has not been reduced nor oxidized.

Test your self : consider the following reaction, called the claus process :

2H₂S_(g) + SO_2(g) → 3S_{( s)} + 2H₂O Identify the oxidant and reductant!

S in H₂S_(g) goes from (-2) to (0), so it is oxidized and thus the reducing agent.

S in SO_2(g) goes from (+4) to (0), so it is reduced and thus the oxidation agent.

What is the most important rule to consider when balancing redox reactions?

Electrons cannot be lost or created – so the number of electrons that the reducing agent loses must add up to the number of electrons gained by the oxidant.

Balance the reaction : Cu_(s) + Ag⁺→ Cu²⁺ + Ag_(s)

This reaction appears to be balanced because the number of atoms is the same on both sides of the arrow. However the number of electrons is not the same on both sides!!!

Thus we must multiply the spieces with insufficient number of electrons with the number 2 (in this case) to make it all add up. The balanced reaction is thus :

Cu_(s) + 2Ag⁺→ Cu²⁺ + 2Ag_(s)

Write a balanced redox-reaction from the following skeleton reactions:

NO₂ + O₃ → N₂O₅ + O₂?

2NO₂ + O₃ → N₂O₅ + O₂

S₈ + Na → Na₂S?

S₈ + 16Na → 8Na₂S

Cr²⁺ + Sn⁴⁺ → Cr³⁺ + Sn²⁺ ?

2Cr²⁺ + Sn⁴⁺ → 2Cr³⁺ + Sn²⁺

Ok, so tomorrow I will move on to more complicated reactions, taking place in acidic /basic solutions. Then I'll cover galvanic cells and its relation to the equilibrium constant. Ciao Manhattan for now!