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Unit 7: Redox &

Electrochemistry

What information does the oxidation number give you?

• Electrical production (batteries, fuel cells)

Why electrochemistry?REDOX reactions are important in …

• Purifying metals (e.g. Al, Na, Li)

• Producing gases (e.g. Cl2, O2, H2)

• Electroplating metals

• Protecting metals from corrosion• Balancing complex chemical equations• Sensors and machines (e.g. pH meter)

C3H8O + CrO3 + H2SO4 Cr2(SO4)3 + C3H6O + H2O

REDOX stands for REDuction/OXidation

Oxidation is often thought of as a combination of a substance with oxygen (rusting, burning)

Oxidation refers to a loss of e-

Reduction refers to a gain of e-

What is Redox?

Loss Electrons = Oxidation

Gain Electrons = Reduction

Remember: LEO the lions

says GERRRRRR!

Reactions

What is happening to the Fe atom?

Fe is going from 0 to +3 oxidation #

It must be losing electrons

Loss of Electrons = Oxidation

In conclusion, the iron atom is being oxidized

Reactions

What is happening to the Sulfur atom?

S is going from 0 to -2 oxidation #

It must be gaining electrons

Gain of Electrons = Reduction

In conclusion, the sulfur atom is being reduced

Oxidation Numbers- the charge an atom would have if the electrons belonged to the more EN atom

- there are a few rules to help us out

1. FREE ELEMENTSALWAYS HAVE AN OXIDATION NUMBER OF ZERO

Na Na0

H2 H20

F2 F20

Oxidation Numbers- the charge an atom would have if the electrons belonged to the more EN atom

4. HYDROGEN

H is ALWAYS +1 (for us) Disclaimer – there are compounds where H has a -1 oxidation number, but we don’t deal with them at this level of chemistry.

Oxidation Numbers- the charge an atom would have if the electrons belonged to the more EN atom

5. OXYGEN

O is ALWAYS -2 (for us)

Oxidation Numbers- the charge an atom would have if the electrons belonged to the more EN atom

2. SIMPLE IONSOXIDATION NUMBER = THE CHARGE AN ION WOULD MAKE

Sodium ion Na+1

Calcium ion Ca+2

Sulfur ion

N-3Nitrogen ion

S-2

Oxidation Numbers- the charge an atom would have if the electrons belonged to the more EN atom

3. COMPOUNDSTHE SUM OF ALL OXIDATION NUMBERS WILL EQUAL ZERO

NaCl Na+1Cl-1

AsI5 As+5I5

-1

Cu(NO3)2

H2+1Cr2

+6 O7

-2H2Cr2O7

Cu+2(N+5O3-2)2

Oxidation Numbers- the charge an atom would have if the electrons belonged to the more EN atom

5. POLYATOMIC IONSTHE SUM OF OXIDATION NUMBERS WILL EQUAL THE CHARGE OF THAT ION

(SO4)-2 (S+6O4-2)-2

(NO3)- (N+5O3-2)-

(Cr2O4)-2 (Cr2+3O4)-2

Oxidation NumbersDo the five problems on your notes sheet

a. Cr2O3

b. H2Cr2O7

c. AsCl5

d. KCl

e. Mg(OH)2

What things are conserved during a chemical reaction?

What is reduced/oxidized?Identify in the following reactions what is

oxidized and what is reduced

2K + Cl2 2KCl

K0 – goes from 0 to +1, it is oxidized

Cl0 – goes from 0 to -1, it is reduced

PracticeIdentify in the following reactions what is

oxidized and what is reduced

2NaCl + 3SO3 Cl2 + SO2 + Na2S2O7

Cl-1 – goes from -1 to 0, it is oxidized

S+6 – goes from +6 to +4, it is reduced

PracticeIdentify in the following reactions what is

oxidized and what is reduced

Zn + Pb+2(aq) Zn+2

(aq) + Pb

Zn0 – goes from 0 to +2, it is oxidized

Pb+2 – goes from +2 to 0, it is reduced

a. C + H2SO4 CO2 + SO2 + H2O

b. HNO3 + HI NO + I2 + H2O

c. KMnO4 + HCl MnCl2 + Cl2 + H2O +

KCl

d. Sb + HNO3 Sb2O5 + NO + H2O

e. HCl + MnO2 MnCl2 + H2O + Cl2

a. C + H2SO4 CO2 + SO2 + H2O

b. HNO3 + HI NO + I2 + H2O

c. KMnO4 + HCl MnCl2 + Cl2 + H2O + KCl

d. Sb + HNO3 Sb2O5 + NO + H2O

e. HCl + MnO2 MnCl2 + H2O + Cl2

1. C + 2Cl2 CCl4

Ox –

Red –

2. H2 + Cl2 2HCl

Ox –

Red –

3. 2P + 3Cl2 2PCl3

Ox –

Red –

4. C + H2O CO + H2

Ox –

Red –

5. Fe + 3Cl2 2FeCl2

Ox –

Red –

6. 2Al + 3Br2 2AlBr3

Ox –

Red –

7. Pb + 2HCl PbCl2 + H2

Ox –

Red –

8. SiO2 + 2C Si + 2CO

Ox –

Red –

9. CO2 + 2Mg 2MgO + C

Ox –

Red –

10. H2SO4 + Zn ZnSO4 + H2

Ox –

Red -

Identify what atom is oxidized and what atom is

reduced:

Fe + 2HCl FeCl2 + H2

HALF REACTIONS

Cu + AgNO3 Cu(NO3)2 + Ag

Reduction:

Write both half reactions for the following reaction:

Oxidation:

Ag+ Ag

Cu Cu+2 + 2e-

Cu Cu+2

Ag+ + 1e- Ag

Cu - 2e- Cu+2

HALF REACTIONS

HNO3 + I2 HIO3 + NO2

Reduction:

Write both half reactions for the following reaction:

Oxidation:

N+5 N+4

I20 2I+5 + 10e-

I20 I+5

N+5 + 1e- N+4

I20 - 10e- 2I+5

Sn + AgNO3 Sn(NO3)2 + Ag

Reduction:

Write both half reactions for the following reaction:

Oxidation:

Ag+1 Ag0

Sn0 Sn+2 + 2e-

Sn0 Sn+2

Ag+1 + 1e- Ag0

Sn0 - 2e- Sn+2

Half Reactions

Redox Lab

Al + CuCl2

Mass

GFM

Moles

Cu + AlCl32 3 3 2

If you were to react Cu and Nickel(II) Chloride what would the products be? How much metal could you make if you

started with 2.00g of Cu?

Oxidizing and Reducing Agents

Oxidizing Agent

Reducing Agent

- causes the oxidation of another atom

- oxidation number decreases

- causes the reduction of another atom

- oxidation number increases

Ca + Cl2 CaCl2What is the O.A.? What is the R.A.?

Cl

Ca

- it is actually the atom that is REDUCED

- it is actually the atom that is OXIDIZED

PRACTICE

4HCl + MnO2 MnCl2 + 2H2O + Cl2

Oxidizing Agent:

Reducing Agent:

Mn+4

Cl-

In the equation below, identify what is oxidized what is reduced. Also identify the oxidizing and reducing agent.

Reduced:

Oxidized: Cl-

Mn+4

Deals with chemical reactions that either produce electricity or need electricity to occur!

There are 2 types of ELECTROCHEMICAL CELLS.

Some things that are the same for both types of cells:

1. The RED CAT GETS FAT!

2. Electrons always flow from the anode to the cathode!

Anorexic Ox

Electrochemical Reactions

Cu

Cu electrode

Zn

Zn electrodeLose e- (Table J)

Oxidized

Anode (-)

Gains e- (Table J)

Reduced

Cathode(+)

2e-

2e-

2e- 2e- 2e- 2e- 2e- 2e- 2e-

2e-

2e-

Zn+2

Zn+2 Cu+2

Cu+2

ZnSO4 CuSO4

Will this go on forever?

Half Reactions

Cu

Cu electrode

Zn

Zn electrodeLose e- (Table J)

Oxidized

Anode (-)

Gains e- (Table J)

Reduced

Cathode(+)

Zn+2

ZnSO4 CuSO4

Will this go on forever?

Half Reactions

ZnSO4 CuSO4

Cu

Cu electrode

Zn

Zn electrode

Lose e- (Table J)

Oxidized

Anode (-)

Gains e- (Table J)

Reduced

Cathode(+)

Na+Cl- Na+

Na+

Na+

Cl-

Cl-

Cl- Na+

Cl-

Na+

Cl-

2e-

2e-

Zn+2

Zn+2

2e- 2e- 2e-

2e-

Cu+2

Cu+2

2Cl-

2Na+

Electrochemistry

Half Reactions

In the reaction below, identify what is the oxidizing agent and

the reducing agent.

Ca + H2O CaO + H2

ZnSO4 CuSO4

Cu

Cu electrode

Zn

Zn electrode

Lose e- (Table J)

Oxidized

Anode (-)

Gains e- (Table J)

Reduced

Cathode(+)

Na+Cl- Na+

Na+

Na+

Cl-

Cl-

Cl- Na+

Cl-

Na+

Cl-

2e-

2e-

Zn+2

Zn+2

2e- 2e- 2e-

2e-

Cu+2

Cu+2

2Cl-

2Na+

Electrochemistry

Summary:

1. Voltaic Cells – are spontaneous reactions

2. Electrons travel through the wire from more reactive metal to the less reactive metal (Table J)

3. Salt Bridge – permits the flow of ions

4. Red Cat gets fat!

Electrochemistry

These are NOT spontaneous reactions – they are forced by the addition of electricity!

Occur within one container, not two separate cells!

These reactions are used to plate metals, purify metals and separate compounds.

Electrochemistry

Sn

Fe

Electrochemistry

CathodeBecomes negative

Picks up + ions from solution

Gets plated with the metal ion from the

solution

Anode

Becomes positive

Loses positive ion (Sn+2) to solution

During a laboratory activity, a student reacted a piece of zinc

with 0.1M HCl(aq). Based on Reference Table J, identify one

metal that does not react spontaneously with HCl(aq).

K

K electrode

Fe

Fe electrode

NaBr

K+1Fe+2

Write out the oxidation and reduction half reactions for the voltaic cell below. (Do not need drawing in notes)

Electrochemical Cell Differences

spontaneous

non-spontaneous

Anode - negative Anode - positive

Electrochemistry

Needs two containers

Needs one container

Packet Review16.

K

K electrode

Fe

Fe electrode

NaBr

K+1Fe+2

Packet Review16.

Ag

Ag electrode

Al

Al electrode

Na2SO4

Al+3 Ag+1

Cu

Zn

Anode Cathode

Electrons ALWAYS flow from Anode to Cathode

Becomes negative

Picks up + ions from solution

Cu+2

Gets plated with the metal ion from the

solution

Becomes positive

Loses positive ion (Cu+2) to solution

Electrochemistry

Electrochemistry

And review packet

Cu

Zn

Anode Cathode

Electrons ALWAYS flow from Anode to Cathode

Becomes negative

Picks up + ions from solution

Cu+2

Gets plated with the metal ion from the

solution

Becomes positive

Loses positive ion (Cu+2) to solution

Electrochemistry

Electrochemistry

Balancing Net Ionic Equations

Done on the board

Electrochemistry

Electrolysis Simulation

Ionic equation balancing

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Balancing equations using oxidation numbers

C3H8O + CrO3 + H2SO4 Cr2(SO4)3 + C3H6O + H2O

BALANCING REACTIONS- conservation of mass and charge

- we must make sure that the e- that one atom loses must equal the e- that another atom gains

HNO3 + I2 HIO3 + NO2 + H2O

Try to balance this one:

BALANCING REACTIONSHNO3 + I2 HIO3 + NO2 + H2O

1. Assign ox #’s, write ½ reactions and cross out spectators

N+5 + 1e- N+4

I20 - 5e- I+5

2. Balance each ½ reaction with respect to atoms and then e-

N+5 + 1e- N+4

I20 - 10e- 2I+5

10 )(

3. Distribute to all parts of the ½ reaction

1 )(

10N+5 + 10e- 10N+4

I20 - 10e- 2I+5

BALANCING REACTIONS3. Carry everything down and cross out e-

10N+5 + 10e- 10N+4

I20 - 10e- 2I+5

10N+5 + I20 10N+4 + 2I+5

4. Put coefficients back into equation and balance what is left.

10HNO3 + 1I2 10HIO3 + 2NO2 + H2O4

BALANCING REACTIONSSb + HNO3 Sb2O5 + NO + H2O

1. Assign ox #’s, write ½ reactions and cross out spectators

Sb0 Sb2+5 + 5e-

N+5 + 3e- N+2

2. Balance each ½ reaction with respect to atoms and then e-

3 )(

3. Distribute to all parts of the ½ reaction

10 )(

6Sb0 3Sb2+5 + 30e-

10N+5 + 30e- 10N+2

2Sb0 Sb2+5 + 10e-

N+5 + 3e- N+2

Most missed Part 2 Questions1. You have a voltaic cell with copper and aluminum as the electrodes. As the cell operates, the mass of the Al electrode decreases. Explain, in terms of particles, why this decrease in mass occurs.

2. Explain, in terms of electrical energy, how the operation of a voltaic cell differs from the operation of an electrolytic cell used in the Hall process. Include both the voltaic cell and the electrolytic cell in your answer.

3. Explain, in terms of ions, why molten cryolite conducts electricity.

[Cryolite = Na3AlF6]

6Sb + 10HNO3 3Sb2O5 + 10NO + H2O

6Sb0 3Sb2+5 + 30e-

10N+5 + 30e- 10N+2

BALANCING REACTIONS3. Carry everything down and cross out e-

6Sb0 +10N+5 3Sb2+5 + 10N+2

4. Put coefficients back into equation and balance what is left.

5

Balance the following

S + HNO3 SO2 + NO + H2O

The Statue of Liberty is made of an iron framework covered by copper metal. Over time, a thin green layer(patina) forms on the outside. Where the iron came into contact with the copper a

reaction occurred where the iron was oxidized. Why did this happen? Use

your Reference Tables.