OXIDATION-REDUCTION REACTIONS

Settle in, this is going to take a while…

What is redox?

Reaction where there is a transfer of electrons between reactants

Oxidation involves the loss of electrons (OIL) Oxidation number/state of the element increases Oxidized element is the reducing agent

Reduction involves the gain of electrons (RIG) Oxidation number/state of the element decreases Reduced element is the oxidizing agent

Example

Complete Reaction:Mg + Zn(NO3)2 Mg(NO3)2 + Zn

Net-ionic Reaction:Mg + Zn2+ Mg2+ + Zn

The magnesium metal was oxidized by the zinc and the zinc was reduced by the magnesium.

Do what?!?!

The oxidation state of the magnesium changed from 0 to +2 Oxidation state increased = oxidation Because magnesium gave its electrons

away, it is the reducing agent The oxidation state of zinc changed from

+2 to 0 Oxidation state decreased = reduction Because zinc took the electrons, it is the

oxidizing agent

How do you know oxidation states? The oxidation number for any pure element

is zero. Group 1 metals form +1 ions, group 2

metals form +2 ions, group 13 metals form +3 ions.

Transition metals can be all kinds of oxidation numbers (ranging from +1 to +7)

Transition metal oxidation states can be determined based on the nonmetal(s) it’s bonded to…

Nonmetal oxidation states

Fluoride is ALWAYS -1, the other halides are usually -1.

Oxide is usually -2, except when it’s in the peroxide ion (-1) or bonded to fluorine (+2)

Hydrogen is +1, unless it is the hydride ion (-1)

Putting it all together

The total charge on a compound is zero, so all oxidation numbers must cancel out.

The total charge of elements in a polyatomic ion must add to the charge on the ion

Practice

What is the oxidation number of each element in the following compounds?

1. Zn(NO3)2

2. H2SO4

3. KMnO4

4. N2O4

5. PCl3

What’s the point?

When an element gains electrons, another element must accept those electrons (Newton’s 3rd law).

If you separate the reaction into half-reactions, you can exploit this electron transfer to generate electricity.

The study of this is electrochemistry, but more on that later…

Half-Reactions?

You can separate a redox reaction into the reduction reaction and the oxidation reaction.

First you have to identify which element is oxidized and which is reduced.

So let’s practice identification first:

Practice

Determine the oxidation states of all elements in the following reactions and then identify which element is oxidized and which is reduced.

N2 + 3H2 2NH3

2MnO2 + Zn + 2H2O 2MnO(OH) + Zn(OH)2

AgNO3 + Cu Cu(NO3)2 + Ag

N2 + 3H2 2NH3

2MnO2 + Zn + 2H2O 2MnO(OH) + Zn(OH)2

AgNO3 + Cu Cu(NO3)2 + Ag

Separating reactions

Once the oxidized and reduced elements have been identified, separate the reactions.

Use net ionic reactions instead of complete reactions

2AgNO3 + Cu Cu(NO3)2 + 2Ag

2Ag+1 + Cu Cu2+ + 2Ag

2Ag+1 + Cu Cu2+ + 2Ag

The silver is reduced, so that is the reduction reaction:

2Ag+1 2Ag The masses are balanced, but the

charges are not, so add the electrons being transferred:

2Ag+1 + 2e- 2Ag Notice that the reduction half reaction

has electrons as reactants

2Ag+1 + Cu Cu2+ + 2Ag

The copper is oxidized, so that is the oxidation reaction:

Cu Cu2+

The masses are balanced, but the charges are not, so add the electrons being transferred:

Cu Cu2+ + 2e-

Notice that the oxidation half reaction has electrons as products

2Ag+1 + Cu Cu2+ + 2Ag

When you put the 2 half-reactions together they equal the complete reaction:

2Ag+1 + 2e- 2Ag Cu Cu2+ + 2e-

the electrons cancel out to make the net ionic reaction

More about balancing

Sometimes you need more than electrons to balance a half reaction.

Oxygens and hydrogens can be balanced by the addition of H+, OH-, and H2O depending on if the solution is acidic or basic.

Balancing Acidic Redox

Balance all elements that are not oxygen or hydrogen

Balance O by adding H2O where needed Balance H by adding H+ where needed Balance charges by adding e- to the

more positive side

Balancing Basic Redox

Follow the steps for acidic balancing first, then…

Count the H+ used to balance the reaction and add the same number of OH- to both sides.

Combine OH- and H+ to make water. Combine/cancel water molecules as

needed.

A LOT of practice

Balance the following equation in both an acidic and a basic solution:

HOCl + NO(g) Cl2(g) + NO3-

More practice

Balance this reaction in an alkaline environment:

MnO2(s) + Zn(s) + H2O 2MnO(OH)(s) + Zn(OH)2(s)

Homework Problems

Pick a few homework problems to work through together