UNIT 6: ELECTROCHEMISTRY

REDOX REACTIONS

• Redox is short for ‘oxidation and reduction’• Oxidation refers to substances that combine with

oxygen• Iron rusting, campfire burning

• Reduction refers to converting metal ore to pure metal• Oxidation-reduction reactions are reactions that

exchange electrons (one loses and one gains them)

DEMO

• Copper wire in silver nitrate• Cu(s) + 2AgNO3(aq) Cu(NO3)2 (aq) + 2 Ag(s)

Ionic Equation• Cu(s) + 2Ag+

(aq) + 2NO3-(aq) → Cu2+

(aq) + 2NO3-(aq) + 2Ag(s)

Overall Net Ionic Equation• Cu(s) + 2Ag+

(aq) → Cu2+(aq) + 2Ag(s)

What changed?• Cu(s) → Cu2+

(aq) + 2e- - Copper lost electrons and was OXIDIZED

• Ag+(aq) + e- → Ag(s)- Silver gained electron and was

REDUCED

• Oxidation• the loss of electrons• Reduction• the gaining of electrons• One can’t happen without the other

LEO GER

LEOLEO

Loss of Electrons

is Oxidationthe Lion Says GER

GERGain of

Electronsis Reduction

OXIDATION NUMBERS

• Electrons are transferred from one substance to another

• We use oxidation numbers aka electronegativity to track this

Electronegativity• The ability of a bonded atom to attract shared

electrons towards itself

Oxidation Numbers• A positive or negative number assigned to an atom

in a molecule or ion that reflects a partial gain or loss of electrons

NOTES

1. The oxidation number of a pure element (by itself, and not an ion) is zeroElement Oxidation

Number

Na 0

H2 0

O2 0

P4  0

IonicCompound Ions Charge Oxidation

Number

NaClNa+ +1 +1

Cl- -1 -1

Mg3N2

Mg+2 +2 +2

N-3 -3 -3

2. Oxidation number of anion is equal to its charge

• 3. oxidation number is hydrogen is always +1

Compound element OxidationNumber

HClH +1

Cl -1

H2SH +1

S -2

4. OXIDATION NUMBER IS OXYGEN IS ALMOST ALWAYS -2

• The exceptions:• peroxides, such as hydrogen peroxide. In peroxides oxygen has an

oxidation number of -1.• when oxygen is combined with fluorine it's oxidation number is +2

Compound element OxidationNumber

MgOmagnesium oxide

Mg +2

O -2

Na2Osodium oxide

Na +1

O -2

Na2O2

sodium peroxideNa +1

O -1

• 5. sum of oxidation numbers in a compound is zero

Compound

elementOxidatio

nNumber

Numberatoms

Total

Mg3N2

Mg +2 3 +6

N -3 2 -6

    SUM 0

Mn2O7

Mn +7 2 +14

O -2 7 -14

    SUM 0

Cl2O3

Cl +3 2 +6

O -2 3 -6

    SUM 0

• 6. oxidation number of polyatomic ions are equal to it’s charge

Compound elementOxidationNumber

Numberatoms Total

NO3-

N +5 1 +5

O -2 3 -6

    SUM -1

Cr2O72-

Cr +6 2 +12

O -2 7 -14

    SUM -2

SO42-

S +6 1 +6

O -2 4 -8

    SUM -2

• It is important to note that oxidation number always refers to each individual atom in the compound, not to the total for that element.• For example in H2O - the total positive "charge"

for both hydrogen atoms will be +2 (which balances with the -2 from oxygen), but EACH HYDROGEN has an oxidation number of +1.

PRACTICE PROBLEMS

• 6.1.2- practice

• Being able to determine oxidation numbers for elements in a compound is critical before you move on to the next step