Chapter 20

Oxidation – ReductionReactions

What are they?

A family of reactions that are concerned with the transfer of electrons between species Redox reactions are a matched set -- you don't have an oxidation reaction without a reduction reaction happening at the same time

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2Fe2O3 + 3C 4Fe + 3CO2

Oxidation refers to the complete or partial loss of electrons or the gain of oxygen

Carbon is oxidizedReduction refers to the complete or partial gain of electrons or the loss of oxygen

Fe2O3 is reduced

Oxidation NumbersDef - A positive or negative number assigned to an atom to indicate its degree of oxidation or reductionGeneral rule – a bonded atom’s oxidation number is the charge that it would have if the electrons in the bond were assigned to the atom of the more electronegative element

Water – H2O (H = +1) (O = -2)

Some rules to followFree elements (non combined) are assigned an oxidation state of zero (ie. Al, Na, Fe, H2, O2, N2)

The oxidation state for any simple one-atom ion is equal to its charge (ie. Na+ = +1)Group 1A metals in compounds are always +1 (H, Li, Na, K)Group 2A metals in compounds are always +2 (Be, Mg, Ca)

Oxygen in compounds is assigned an oxidation state of -2, exception is peroxide where it is -1 (eg. H2O2)

Hydrogen is +1 unless it is in a metal hydride such as NaH; then it is -1The sum of the oxidation states of all atoms in a species must be equal to the charge on the species

eg. HClO4 = 0 [+1(H)+7(Cl)-2*4=-8(O)]

Determine the oxidation number of chlorine in each of the following substances

KClO3, Cl2, Ca(ClO4)2, Cl2O

KClO3 (Cl = +5)

Cl2 (Cl = 0)

Ca(ClO4)2 (Cl =+7)

Cl2O (Cl = +1)

Assign the following oxidation #’sS2O3, Na2O2, P2O5, NO3

-

S = +3, O = -2Na = +1, O = -1P = +5, O = -2N = +5, O = -2

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Examples - assigning oxidation numbers

Assign oxidation states to all elements:

H2 SO3 SO42-

K+ NH3 MnO4-

Cr2O72- CH3OH PO4

3-

ClO3- HSO3

- Cu

H2 = 0

SO3 = S +6, O -2

SO4-2 = S +6, O -2

K+ = +1

NH3 = N -3, H +1

MnO4- = Mn +7 , O -2

Cr2O7-2 = Cr +6 , O -2

CH3OH = C -2 , O -2, H +1

PO43- = P +5, O -2

ClO3- = Cl +5, O -2

HSO3- = H +1, S +4 , O -2

Cu = 0

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an increase in oxidation number of an atom signifies oxidation

a decrease in oxidation number of an atom signifies reduction

+2 to +4

0 to -1

Changes in oxidation number

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SnCl2 + PbCl4 SnCl4 + PbCl2

CuS + H+ + NO3- Cu+2 + S + NO + H2O

-1+2 -1+4 -1+4 -1+2

+2 = Oxidation

-2 = Reduction

-2+2 +1 -2+5 +2 0 -2+2 -2+1

-3 = Reduction

+2 = Oxidation

Oxidizing and Reducing Agents

Now the confusing part…CuO + H2 Cu + H2O

Cu goes from +2 to 0Cu is reduced, therefore the compound it comes from, CuO, is called an oxidizing agent because it causes some other substance to be oxidized

H goes from 0 to +1H is oxidized, therefore H2 is called a reducing agent because it causes some other substance to be reduced.

Identifying Agents in an Equation

CuO + H2 Cu + H2O

Reduction: CuO is the oxidizing agent

Oxidation: H2 is the reducing agent

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In each reaction, look for changes in oxidation state.

If changes occur, identify the substance being reduced, and the substance being oxidized.

Identify the oxidizing agent and the reducing agent.

Examples - labeling redox reactions

H2 + CuO Cu + H2O0 -2+2 0 -2+1

= +1 (H is oxidized)

= -2 (Cu is reduced)

(H2 is the reducing agent)

(oxidizing agent is CuO)

SnCl2 + PbCl4 SnCl4 + PbCl2 Sn +2 to +4 so oxidized so SnCl2 is reducing agentCl -1 to -1 no changePb +4 to +2 so reduced so PbCl4 is oxidizing agent

2Sb + 3 Cl2 2SbCl3

Sb 0 to +3 so oxidized = reducing agentCl 0 to -1 so reduced so Cl2 is oxidizing agent

CuS + H+ + NO3- Cu2+ + S + NO + H2O

Cu +2 to +2 no changeS -2 to 0 so oxidized so CuS is reducing agentH+ +1 to +1 no changeN +5 to +2 so reduced so NO3

- is oxidizing agentO -2 to -2 no change

Assign OX#’s & determine what is oxidized/reduced/OA/RA

NaI + H2SO4 H2S + I2 + Na2SO4 + H2ONa +1 to +1 no changeI -1 to 0 oxidized so NaI is reducing agentH +1 to +1 no changeS +6 to -2 so reduced so H2SO4 is oxidizing agentO -2 to -2 no change

SO3 + H2O H2SO4

S +6 to +6 no change0 -2 to -2 no changeH +1 to +1 no changeNot a redox reaction

Assign OX#’s & determine what is oxidized/reduced/OA/RA

Redox rxn’s that form ionswriting half reactions2 Mg(s) + O2(g) 2 MgO(s)

Oxidation: Mg Mg2+ + 2é

Mg is the reducing agentReduction: O2 + 2é O2-

O2 is the oxidizing agent

Writing Half-ReactionsCa0 + 2 H+1Cl-1 Ca+2Cl-1

2 + H20

Oxidation: Ca0 Ca+2 + 2e-

Reduction: 2H+1 + 2e- H20

The two electrons lost by Ca0 are gained by the two H+1 (each H+1 picks up an electron).

PRACTICE SOME!

Practice Half-ReactionsDon’t forget to determine the charge of each species first!

Mg + S MgS

Oxidation Half-Reaction: Mg0 Mg+2 + 2e-

Reduction Half-Reaction: S0 + 2e- S-2

Al + P AlP

Oxidation Half-Reaction: Al0 Al+3 + 3e-

Reduction Half-Reaction: P0 + 3e- P-3

Ni+2 + Fe Fe+2 + NiNi+2 + 2é Ni0

Fe0 Fe+2 + 2é

Ca + Se CaSeCa0 Ca+2 + 2éSe0 + 2é Se-2

Be + Se BeSeBe0 Be2+ + 2éSe0 + 2é Se-2

Write the half-reactions for the following equations

Identify these processes as either oxidation or reduction

2I- I2 + 2é

Oxidation – loss of electronsIodine charge changes from -1 to 0

Zn2+ + 2é ZnReduction – gain of electrons

Zinc charge changes from +2 to 0

Identify as either oxidation or reduction

I2 + 2é 2I-

Reduction iodine goes from 0 to -1

Cu Cu2+ + 2éOxidation copper goes from 0 to +2

O2 + 4é 2O2-

Reduction oxygen goes from 0 to -2

Al Al3+ + 3éOxidation aluminum goes from 0 to +3

Corrosionrusting – spontaneous

oxidation

Disintegration of a metal into its constituent atomsWater speeds up the processMost structural metals have reduction potentials that are less positive than O2 so in the presence of O2 they will rust

Water

Rust

Iron Dissolves-

Fe Fe+2

e-

salt speeds up process by increasing conductivity making electron transfer easier

O2 + 2H2O +4e- 4OH-

Fe2+ + O2 + 2H2O Fe2O3 + 8 H+

Fe2+

Preventing CorrosionGalvanizing – putting on a zinc coatAlloying – bonding with another metal that forms an oxide coatCoating – covering a metal with oil, paint or plastic to prevent corrosionElectroplating – putting a thin metal coat over another metal to protect it

Not all metals corrode, some form a protective coating to prevent it (Al, Cr)

Other metals (gold & platinum) resist losing their electrons; therefore, don’t corrode