Zumdahl*s Chapter 17

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GALVANIC CELL

SAHAR ADHAM

LECTUERE OF PHYSICAL CHEMISTRY

Comparison of Electrochemical Cells

galvanic electrolytic produces electrical current anode (-) cathode (+) two electrodes conductive medium salt bridge vessel

E ° cell

> 0.

need power source anode (+) cathode (-)

E ° cell

< 0.

ELECTRON TRANSFER REACTIONS

Electron transfer reactions are oxidation-reduction or redox reactions.

Results in the generation of an electric current (electricity) or be caused by imposing an electric current.

Therefore, this field of chemistry is often called

ELECTROCHEMISTRY.

YOU CAN’T HAVE ONE… WITHOUT THE OTHER!

 oxidation: loss of electrons

 reduction: gain of electrons

LEO the lion says GER !

GER!

WHY STUDY ELECTROCHEMISTRY?

Batteries

Corrosion

Industrial production of chemicals such as Cl

2 and Al

, NaOH, F

2

Biological redox reactions

The heme group

GALVANIC CELLS

One ½ cell rxn. occurs in each compartment.

Zn

Zn 2+ the anode.

+ 2e – in

Cu 2+ + 2e – cathode.

Cu in

But not without a connection.

Zn

SO

4

2

Zn

2+

SO

4

2

Cu

2+

Cu

Zn + Cu

2+

Zn

2+

+ Cu

ION (“SALT”) BRIDGE

But even with a connection of the electrodes, no current flows.

We need to allow neutrality in the solutions with a salt bridge to shift counterions.

Zn

2e

SO

4

2

Zn

2+

SO

4

2

Cu

2+

2e

Cu

Zn + Cu

2+

Zn

2+

+ Cu

Cell Potential

•Cell Potential

or

Electromotive

Force

(emf): The “pull” or driving force on the electrons.

STANDARD REDUCTION POTENTIALS,

E

°

The voltage generated by the Zn/Cu galvanic cell is

+1.1V under

standard conditions

.

Standard conditions are:

T = 25°C and P = 1 bar for gases.

Solids and liquids are pure.

Solutions are 1 M in all species.

E

° cell is sum of ½ cell

E

° values.

CELL POTENTIALS AND REDUCTION POTENTIALS

E

°cell

E

°cell

= E

°reduced - E°oxidized

= E

°cathode - E°anode

½ CELL REDUCTION POTENTIALS

All ½ cells are catalogued as reduction reactions & assigned reduction potentials,

E

°.

The lower reduction potential ½ rxn is

reversed

become the oxidation.

E

° oxidation

= –

E

° reduction

That makes spontaneous

E

° cell

> 0.

But

E

° red can’t be found w/o

E

° ox

!

to

ORIGIN FOR REDUCTION POTENTIALS

We need a standard electrode to make measurements against!

The Standard Hydrogen Electrode (SHE)

2H

+

(

aq

) + 2e

H

2

(1 bar)

E

°

0 V

1 bar H

2

E

° cell

E

°

SHE flows over a Pt electrode, and the full is

assigned

to the

other

electrode.

= 0 V.

E.g., standard

calomel

electrode:

Hg

2

Cl

2

(

s

) + 2e

2 Hg( l

) + Cl

– E

°

SCE

= +0.27V

a more physically convenient reference.

GALVANIC LINE NOTATION

Shorthand

the form: for a complete redox cell is of

Anode | anodic soln. || cathodic soln. | Cathode

So making a cell of Cu corrosion,

Cu | Cu

2+ || NO

3

– , NO(

g

), H + |Pt where all ions should be suffixed (

aq

) and both metals should have (

s

).

TYPES OF GALVANIC CELLS

Primary Battery

: can not be recharged e.g.

Mercury Battery

Secondary Battery : rechargeable (storage batteries) e.g.

Ni-Cad Battery

Fuel Cell : reactants supplied from an external source e.g.

H2/O2 fuel cells.

MERCURY BATTERY

Anode:

Zn is reducing agent under basic conditions

Cathode

:

HgO + H

2

O + 2e- ---> Hg + 2 OH

can not be recharged

NI-CAD BATTERY

Anode (-)

Cd + 2 OH

-

Cathode (+)

---> Cd(OH)

2

+ 2e-

NiO(OH) + H

2

O + e- ---> Ni(OH)

2 rechargeable

+ OH

-

WHY RECHARGEABLE?.

It is

because

the products of the reaction are solids that the Ni-Cd battery can be recharged

The solid hydroxides are sticky, and remain in place.

If current is applied, the reaction can be driven backwards

!

HOW TO CHARGE?

When you charge a battery, you are forcing the electrons backwards

(from the + to the -). To do this, you will need a higher voltage backwards than forwards

.

WHY NOT RECHARGEABLE ?

But in

mercury battery the ZnO is not sticky, and doesn’t remain attached to the electrode. This battery is not rechargeable

H

2

AS A FUEL

Cars can use electricity generated by H

2

/O

2 fuel cells.

H

2 carried in tanks or generated from hydrocarbons

Fuel Cells

Galvanic cells for which the reactants are continuously supplied.

anode:

2H

2

+ 4OH

4H

2

O + 4e

cathode :

4e

+ O

2

+ 2H

2

O

4OH

2H

2

(

g

) + O

2

(

g

)

2H

2

O(

l

)

17-44

Mercury batteries take advantage of the

high

density of Hg to be quite small: used in watches, hearing aids, calculators, etc.

Lithium-iodine batteries are particularly small and lightweight, but also very long-lived

Often used in pacemakers, where they can last for 10 years

THE END

V1.06

QUESTION 1

For a galvanic cell, the electrode at which reduction occurs is called the:

A: Anode B: Cathode

ANSWER

For a galvanic cell, the electrode at which reduction occurs is called the:

B: Cathode

QUESTION 2

For a galvanic cell, the electrode with negative polarity is called the:

A: Anode B: Cathode

ANSWER

For a galvanic cell, the electrode with negative polarity is called the:

A: Anode

32

QUESTION 3

Which of the following statements is

incorrect

a.

b.

In a galvanic cell, reduction occurs at the anode.

The cathode is labeled "+" in a voltaic cell.

c.

d.

Oxidation occurs at the anode in a voltaic cell.

Electrons flow from the anode to the cathode in all electrochemical cells.

ANSWER a.

In a galvanic cell, reduction occurs at the anode.

34

QUESTION 4

Consider the following notation for an electrochemical cell

Zn|Zn

2+

(1M)||Fe

3+

(1M), Fe

2+

(1M)|Pt

What is the balanced equation for the cell reaction?

a.

Zn(s) + 2Fe

3+

(aq)

→ 2Fe

2+

(aq) + Zn

2+

(aq) b.

Zn

2+

(aq) + 2Fe

2+

(aq)

→ Zn(s) + 2Fe

3+

(aq) c.

Zn(s) + 2Fe

2+

(aq)

→ 2Fe

3+

(aq) + Zn

2+

(aq) d.

Zn(s) + Fe

3+

(aq)

→ Fe

2+

(aq) + Zn

2+

(aq) e.

Zn(s) + Fe

2+

(aq)

→ Fe(s) + Zn

2+

(aq)

ANSWER

Zn(s) + 2Fe

3+

(aq

) → 2Fe

2+

(aq) + Zn

2+

(aq)

QUESTION 5

What is the oxidation state of nitrogen in

HNO

3

?

A: +3 B: +4

C: +5 D: -5

ANSWER

What is the oxidation state of nitrogen in

HNO

3

?

C: +5

38

QUESTION 6

Consider the following electrode potentials:

Mg

V

2+

2+

Cu

2+

+ 2e

+ 2e

+ e

Cu

Mg

V E

+

°

E

=

E

°

–1.18 V

°

=

–2.37 V

= 0.15 V

Which one of the reactions below will proceed spontaneously from left to right?

a. Mg

2+

+ V

 b. Mg c. V

2+

2+

+ 2Cu

+

+ 2Cu

+

 d. V + 2Cu

2+

V

2+

V

V

+2

2+

+ Mg

2Cu

2+

+ Mg

+ Cu

+ 2Cu

2+

+ e. none of these

ANSWER

 d. V + 2Cu

2+

V

2+

+ 2Cu

+

QUESTION 7

What is the oxidative state of iodine in IO

3

?

A: +7 B: +6

C: +5 D: +4

ANSWER

What is the oxidative state of iodine in IO

3

?

C: +5

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