Redox ReactionsRedox Reactions

Redox ReactionsRedox Reactions Redox is the abbreviated way to write Redox is the abbreviated way to write reduction-reduction-

oxidationoxidation reaction. reaction. Redox reactions are a very Redox reactions are a very broadbroad type of reaction type of reaction

and many of the types we have already discussed and many of the types we have already discussed can also be classified as a redox reaction.can also be classified as a redox reaction.

In a redox reaction, In a redox reaction, reductionreduction and and oxidationoxidation must must occur together.occur together.

But what is reduction and oxidation?But what is reduction and oxidation? ReductionReduction – chemical change in which electrons – chemical change in which electrons

are are gainedgained.. OxidationOxidation – a chemical change in electrons are – a chemical change in electrons are

removedremoved..

So, we need to Review So, we need to Review Oxidation NumbersOxidation Numbers

What is the oxidation number?What is the oxidation number? Book definition – the number of Book definition – the number of electronselectrons

that must be that must be addedadded to or to or removedremoved from from an atom in a combined state to convert an atom in a combined state to convert the atom into the elemental form.the atom into the elemental form.

In other words, the number of electrons In other words, the number of electrons gainedgained or or lostlost when a chemical bond is when a chemical bond is made.made.

Where can we find possible oxidation Where can we find possible oxidation numbers? numbers?

The Periodic TableThe Periodic Table

Assigning Oxidation Numbers – Assigning Oxidation Numbers – The steps from page 606The steps from page 606

1.1. Identify the Identify the chemical formulachemical formula

If no formula is provided, write the If no formula is provided, write the formula of the formula or ion from the formula of the formula or ion from the name.name.

2.2. Assign known Assign known oxidation numbersoxidation numbers

- The oxidation number of an atom of any - The oxidation number of an atom of any freefree element in atomic or molecular form element in atomic or molecular form is is zerozero..

Examples: Examples: Cu, HCu, H22, Ag, Na, S, Ag, Na, S88

- The oxidation number of - The oxidation number of monatomicmonatomic ion is ion is equal to the equal to the chargecharge of the ion of the ion

Assigning Oxidation Numbers – Assigning Oxidation Numbers – ContinuedContinued

- - FluorineFluorine always has an oxidation always has an oxidation number of number of -1-1 since it is the since it is the mostmost electronegative element.electronegative element.

- In a compound: - In a compound:

Group 1 atoms =Group 1 atoms =

Group 2 atoms =Group 2 atoms =

Aluminum =Aluminum =

- Hydrogen = - Hydrogen = +1+1, unless it is with a , unless it is with a metalmetal, then it is , then it is -1-1

+1

+2+3

- Oxygen is usually Oxygen is usually -2-2, exceptions:, exceptions:- If combined with If combined with FluorineFluorine, Oxygen’s , Oxygen’s

oxidation number becomes oxidation number becomes +2+2

- In In peroxidesperoxides, H, H22OO22, Oxygen’s oxidation , Oxygen’s oxidation number becomes number becomes -1-1

- When comparing When comparing nonmetalsnonmetals, the more , the more electronegativeelectronegative atom (page 194) gets the atom (page 194) gets the negative charge based on the column it negative charge based on the column it is located in on the periodic table. is located in on the periodic table.

- Group 17 =Group 17 = , Group 16 = , Group 16 = , Group , Group 15 = __15 = __

Assigning Oxidation Numbers – Assigning Oxidation Numbers – ContinuedContinued

-1 -2 -3

Calculate any remaining Calculate any remaining numbersnumbers, and , and verify results.verify results.– Use the total oxidation number of each Use the total oxidation number of each

element’s atoms = element’s atoms = oxidationoxidation numbernumber times times subscriptsubscript

– The The sumsum of the oxidation numbers for of the oxidation numbers for all the atoms in a all the atoms in a moleculemolecule is is zerozero..

– The The sumsum of the oxidation numbers for of the oxidation numbers for all the atoms in a all the atoms in a polyatomicpolyatomic ionion is is equal to the equal to the chargecharge on that ion. on that ion.

Assigning Oxidation Numbers – Assigning Oxidation Numbers – ContinuedContinued

ExamplesExamples

NHNH44++ PbSOPbSO44 Ca(OH)Ca(OH)22

AlAl HH22 FeFe22(CO(CO33))33

HH22OO KClOKClO33 HH22POPO44-1-1

PbPb+2+2 BFBF33 SS22OO77-2-2

-3 +4 = +1

-3 +1

+2 +6 -8= 0

+2 +6 -2

+2 -2 = 0

+1 -2

0 0

+2+3 -3 = 0

+3 -1

+12 -14 = -2

+6 -2

+1 +5 -6 = 0

+1 +5 -2

+2 -2 = 0

+1 -2

+6 +12 -18 = 0

+3 +4 -2

+2 +5 -8 = -1

+1 +5 -2

Now that we reviewed oxidation Now that we reviewed oxidation numbers, what do we do with numbers, what do we do with

them?them? Remember, in a redox reaction, Remember, in a redox reaction,

something has to be oxidized (something has to be oxidized (losesloses electrons, oxidation number electrons, oxidation number increasesincreases) ) and something else has to be reduced and something else has to be reduced ((gainsgains electrons, oxidation number electrons, oxidation number decreasesdecreases).).

If we assign oxidation numbers to all If we assign oxidation numbers to all atoms in a chemical equation, we can atoms in a chemical equation, we can determine whether it is a redox reaction.determine whether it is a redox reaction.

Are the following redox Are the following redox reactions?reactions?

2 H2 H33OO++ + Zn + Zn H H22 + 2H + 2H22O + ZnO + Zn+2+2

CC33HH88 + 5O + 5O2 2 3CO 3CO22 + 4H + 4H22OO

2KI + Pb(NO2KI + Pb(NO33))22 PbI PbI22 + 2KNO + 2KNO33

2K + Cl2K + Cl22 2KCl 2KCl

Some other termsSome other terms OxidizingOxidizing agent – the agent – the substancesubstance that that

gainsgains electrons in an redox reaction electrons in an redox reaction and is and is reducedreduced..

ReducingReducing agent - the substance that agent - the substance that losesloses electrons in an redox reaction electrons in an redox reaction and is and is oxidizedoxidized..

Notice that the Notice that the oxidationoxidation agent agent oxidizesoxidizes another substance and becomes another substance and becomes reducesreduces in the process and visa versa in the process and visa versa

Also note that these refer to the Also note that these refer to the wholewhole compound or ion.compound or ion.

Let’s look at the following Let’s look at the following redox reactionredox reaction

First, write the oxidation number for each First, write the oxidation number for each atom:atom:

8 H8 H33OO++ + MnO + MnO44-- + 5Fe + 5Fe+2+2 Mn Mn+2+2 + 5Fe + 5Fe+3+3 + 12 H + 12 H22OO

What was reduced?What was reduced?

What was oxidized?What was oxidized?

What is the oxidizing agent?What is the oxidizing agent?

What is the reducing agent?What is the reducing agent?

Mn

FeMnO4

-

Fe+2

HomeworkHomework

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