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Oxidation Numbers &Oxidation-Reduction

Mr. Shields Regents Chemistry U14 L01

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Oxidation NumbersOxidation NumbersWe’ve talked about oxidation nos. before. What is the Definition of an atoms oxidation number?

The number of electrons an atom tends to give up or gain in aChemical reaction

Where can we find an elements oxidation number?

What are the oxidation nos. for O and S in the compound SO2 ?

Looking at the reference table what is oxygen’s oxidation no.If Oxygen is -2 Sulfur must be +4 (Why?)

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Oxidation NumbersOxidation Numbers

But what if we had looked up the oxidation no. of Sulfur first?

The table say’s sulfurs most common oxidation no. is -2 soOxygen would be +1 (why).

But that can’t be. There is no +1 for oxygen.

So how do we figure this out?

There are a few RULES we need to learn when assigning Oxidation Numbers to atoms in compounds.

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Ox. Number RulesOx. Number RulesRule 1: Uncombined atoms are always 0Rule 2: Group 1 atoms are always +1Rule 3: Group 2 atoms are always +2Rule 4: Oxygen is always - 2

exception: Peroxides like H2O2, Na2O2 oxygen is -1 In OF2 oxygen is (know these) +2

Rule 5: Fluorine is always - 1Rule 6: The other Halogens are “usually” - 1Rule 7: Hydrogen combined with non-metals +1

example: HBrRule 8: Hydrogen combined with metals (hydrides) - 1

example: CaH2

Rule 9: Total of the charges for all atoms in a compound is zero (0)Rule 10: Total of the charges for all atoms in a polyatomic is equal

to the charge of the polyatomic ion

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Oxidation NumbersOxidation Numbers

Example: What is the oxidation number of each element in the following compound.

SrCl2

Sr is group 2 so it always has an oxidation no. of +2

The sum of all the charges of a compound is zero sothe 2 Cl must be -2.

Therefore each of the Cl has an Ox. Number of -1

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Oxidation NumbersOxidation Numbers

Example: What is the oxidation number of each element in the following compound.

PbCrO4

First you need to recognize that CrO4 is a polyatomic with a charge of -2

Since Oxygen is always -2, Cr must be +6 (why?)

Since CrO4 has a charge of -2, Pb must be +2

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Oxidation NumbersOxidation Numbers

Problem: What is the oxidation number of each element in NH4NO3

In this compound there are 2 polyatomics (NH4+, NO3

-)

NH4+:

H +1 (Always)N - 3 why?

NO3-:

O - 2 (Always)N +5 why?

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Find the oxidation #’s for each element in the compound H2S2O7

H is always +1 when bonded to a non-metal

O is always -2 (except in H2O2 and OF2)

To find Sulfur, the sum of charges must equal 0

2 (+1) + 2X + 7 (-2) = 0 2 + 2X + -14 = 0

2X - 12 = 02X = 12 X = 6

Sulfur = +6

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RedoxRedox

We’re now going to discuss a topic that involves something calledOxidation & Reduction

- These reactions are commonly known as REDOX REACTIONS

SO … what is oxidation and what is reduction?

Originally oxidation referred to any reaction in which Oxygen was one of the reactants.

Ex: 4Fe + 3O2 2Fe2O3

2Cu + O2 2CuOCH4 + 2O2 CO2 + 2H2O

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RedoxRedoxOpposite of Oxidation is Reduction . Originally, this referredto any reaction in which Oxygen was removed from a reactant.

Ex: 2Fe2O3 + 3C 4Fe + 3CO2

CuO + H2 Cu + H2O

Today, oxidation & Reduction still refer to these type of reactions but both have taken on a much broader meaning

Recall that Oxygen is a very electronegative element- EN = 3.5 on a scale of 0 - 4- This means it very strongly draws electrons to itself (how many electrons does it want ?)

Right… 2e- : O + 2e- O-2

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RedoxRedoxSince Oxygen wants electrons, substances that react with oxygenHave to lose electrons

- Cu: + O Cu+2O-2

From this comes the broader definition of Oxidation/Reduction

Oxidation is the process by which substances lose 1 or more e-

Reduction is the process by which substances gain 1 or more e-

HOW CAN WE REMEMBER THIS?

- Remember the phrase OIL RIG

Oxidation Is Lose of electrons / Reduction Is Gain of electrons

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RedoxRedoxNOTE: Oxidation and reduction ALWAYS occur together

Electrons can’t just be lost they need to be accepted by some Other substance.

Metals are typically oxidized (lose electrons)

Na Na+ +1e-

Non-metals are typically reduced (gain electrons)

Cl + 1e- Cl-

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Half-cell reactionsHalf-cell reactions

The Equation that show how an element either gains or losesElectrons is called a HALF CELL reaction

For example what is oxidized and what is reduced in thefollowing equation and what are the 2 half cell reactions?

Fe + S FeS

sulfur is reduced (why?)S0 + 2e- S-2 This is known as the REDUCTION half cell

iron is oxidized (why?)Fe0 Fe+2 + 2e- This is known as the OXIDATION half cell

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Half cell rxnsHalf cell rxns

Oxidation half cell Rxns lose electrons (they shows up on the product side)

2 F- F2 (g) + 2 e - Mn+2 (s) Mn+7 + 5 e –

Reduction half cell Rxns gain electrons (they show up on the reactant side)

F2(g) + 2 e - 2 F- Mn+7 + 5 e - Mn+2 (s)

Oxidation Oxidation

Reduction Reduction

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Reducing AgentReducing Agent

When something is Oxidized:

- It’s called the Reducing Agent

When something is oxidized itLoses electrons (“OIL”)

Something that accepts these Electrons is reduced (“RIG”)

Therefore what is oxidized Facilitates the reduction ofSomething else

metals

Non-metals

So … It’s the Reducing AGENT!

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Reducing agentReducing agent

Let’s look at an example of a reducing agent at work

Fe + O2 Fe2O3

What is the Reducing Agent? And what is Reduced?

The first question to ask is which is “WHAT’S OXIDIZED?”

Iron is losing electrons: Fe0 Fe+3 so Fe is oxidized.

But Fe has to give it’s electrons to something else. What is it?

Oxygen. So O2 is reduced. Therefore, Fe is the Reducing Agent

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Oxidizing AgentOxidizing AgentWe said that when something is Oxidized it’s the Reducing Agent.

So… When something is Reduced It’s called the Oxidizing Agent.

We know that when something GAINS electrons it’s reducedThose electrons must come from another Atom or Ion.The atom or ion that lost those electrons is therefore oxidized.

For example: 2H2 + O2 H20 (what is the Oxidizing agent?)

Oxidation half cell: H0 H+1 + 1e- Reduction half cell: O0 + 2e- O-2

Since Oxygen is reduced; it’s the “agent” that oxidizes Hydrogen, so it’s the Oxidizing agent.

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Problem: for the following Reaction

C(s) + Cl2(g) CCl4 (l)

A) What is reduced? B) What gets oxidized? C) What is the reducing agent? D) What is the oxidizing agent?

Problem: for the following Reaction

2 MgO 2 Mg + O2

A) What is reduced? B) What gets oxidized? C) What is the reducing agent? D) What is the oxidizing agent?E) Write the Oxidation and Reduction Half cell reactions.

Cl0

C0

C0

Cl0

Mg+2

O-2

O-2

Mg+2