redox electrochemistry. spontaneous redox reactions can transfer energy –electrons (electricity)...

REDOX

electrochemistry

• Spontaneous redox reactions can transfer energy– Electrons (electricity)– Heat

• Non-spontaneous redox reactions can be made to happen with electricity.

Redox reactions involve the transfer of the electron.

Trends in Oxidation and Reduction

Active metals: Lose electrons easily Are easily oxidized Are strong reducing

agents

Active nonmetals: Gain electrons easily Are easily reduced Are strong oxidizing agents

Oxidation

- is defined as the loss of an electron

• (LEO - loss of electron; oxidation)

- when oxidation happens, charge becomes more positive

• Fe → Fe2+ + 2e-

4 Fe (s) + 3 O2 (aq) → 2 Fe2O3 (s)

Reduction

• is defined as the gain of an electron• (GER- gain of electrons – reduction• - when reduction happens, charge becomes

more negative• Fe2+ + 2e- → Fe• - term is derived from the observation that metal

oxides lose mass when refined to produce the pure metal:

• 2 Fe2 O3 (s) → 4 Fe (s) + 3 O2 (aq)

• N2O → NO• Has the nitrogen gained or lost

electrons? • N2O → NO +1 +2Nitrogen has lost one electron and is

therefore being oxidized.

11

2

00

22

ClNaClNa

Each sodium atom loses one electron:

Each chlorine atom gains one electron:

eNaNa10

10 CleCl

Lose Electrons = Oxidation

Gain Electrons = Reduction

• These pairs of reactions are called half-reactions. For example, in the reaction

• Cu2+ (aq) + Fe (s) → Cu (s) + Fe2+ (aq)• two half-reactions occur; iron is oxidized

while the copper (II) ion is reduced. Seen another way, the copper (II) ion causes the oxidation of the iron; it is the oxidizing agent. Likewise, the iron is the reducing agent for the copper (II) ion.

There are five basic rules for the determination of oxidation number:

• Rule 1: The oxidation number for any atom in its elementary state is 0.

• Rule 2: The oxidation number for any simple ion is the change on the ion.a. The oxidation number of alkali metals in compounds is 1+

(Li1+ , Na1+ , K1+ , Rb1+ , Cs1+ , Fr1+ ).b. The oxidation number of alkaline-earth metals in compounds is 2+

(Mg2+ , Ca2+ , and Ba2+ ).

Find the oxidation number for magnesium and chlorine

12

2

00

ClMgClMg

22 ClMgClMg

• Rule 3: The oxidation number for oxygen usually is 2-. In peroxides, it is 1-.

2

2

1

OH 2

1

2

1

OHwater peroxide

• Rule 4: The oxidation number for hydrogen is 1+ in all its compounds except in metallic hydrides like NaH or BaH2 , where it is 1-.

• Rule 5: All other oxidation numbers are assigned so that the sum of oxidation numbers equals the net charge on the molecule or polyatomic ion.

Find the oxidation numbers

• H2S Ca(OH)2

2

2

1

SH2(+1) + (-2) = 0 H O

2

122

)(

HOCa(+2) + 2(-2) + 2(+1) = 0 Ca O H

3

2?

ONX + 3(-2) = -1N O

24

2?

OS

X = +5 X = +6

X + 4(-2) = -2S O

Find the oxidation numbers

Find the oxidation number of Cr:

• Ex: Cr2O72-

2X + 7(-2) = -2Cr O

X = +6

Write the oxidation numbers for each atom:

• S8

• SO2

• S2O32-

• SO42-

• MgSO4

• H2SO4

0

S = +4 and O = -2

S = +2 and O = -2

S = +6 and O = -2

Mg = +2, S = +6 and O = -2

H = +1, S = +6 and O = -2

Find the oxidation numbers

• P4

• P2O5

• PCl5• H3PO4

• PO43-

• Na3PO4

0

P = +5 and O = -2

P = +5 and Cl = -1

H = +1, P = +5 and O = -2

P = +5 and O = -2

Na = +1, P = +5 and O = -2

A summary of terminology for oxidation-reduction (redox) reactions

X Y

e-

transfer or shift of electrons

X loses electron(s) Y gains electron(s)

X is oxidized Y is reduced

X is the reducing agent Y is the oxidizing agent

X increases its oxidation number

Y decreases its oxidation number

Not all reactions are redox

)()()()( 3

2511111

3

251

aqONNasClAgaqClNaaqONAg

)()()()(22

2

1

4

26

2

1

4

26

2

1121

lOHaqOSNaaqOSHaqHONa

Reactions in which there has been no change in oxidation number are not redox reactions.

• Just like the Bronsted-Lowry theory of acids, oxidation and reduction also happens in pairs; a species cannot donate an electron unless another species gains the electron. Since oxidation and reduction always happen together, these are most often called redox reactions.

Reducing and oxidizing agents

The substance reduced is the oxidizing agent The substance oxidized is the reducing agent

eNaNa10

10 CleCl

Sodium is oxidized – it is the reducing agent

Chlorine is reduced – it is the oxidizing agent

• 2H2(g) + O2(g) → 2H2O(g)

• 2H2(g) + O2(g) → 2H2O(g)

0 0 +1 -2

The O.N. of H increases; it is oxidized; it is the reducing agent.

The O.N. of O decreases; it is reduced; it is the oxidizing agent.

FIND THE OXIDATION NUMBERS:

Find the oxidation numbers:

2Al(s)+ 3H2SO4(aq) Al2(SO4)3(aq) + 3H2(g)

2Al(s) + 3H2SO4(aq) Al2(SO4)3(aq) +3H2(g)

0 0+6+1 -2 +3 +6 -2

The O.N. of Al increases; it is oxidized; it is the reducing agent.

The O.N. of H decreases; it is reduced; it is the oxidizing agent.

Find the oxidation numbers

• PbO(s) + CO(g) → Pb(s) + CO2(g)

• PbO(s) + CO(g) → Pb(s) + CO2(g)

+2 -2 +2 -2-2 0 +4

The O.N. of C increases; it is oxidized; it is the reducing agent.

The O.N. of Pb decreases; it is reduced; it is the oxidizing agent.