U4 S2 L2 E cell

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U4 S2 L2

Standard Reduction Potentials

Its all about ~~ SHE!

Textbook Readings MHR page 761: Introducing Cell Potentials page 768: Standard Cell Potentials pages 769-774: Calculating Standard Cell Potentials

Textbook Practice Items MHR page 773: items 5, 6, 7 and 8 page 774: Thought Lab page 775: items 1-4, and 6

Upon completion of this lesson, you should be able to:

• define half-cell voltage, standard half-cell, cell voltage, E°

• use a standard reduction potential table to predict cell voltage and to predict if the redox reaction is spontaneous (occurs as it is written)

• develop a table of redox half-reactions from experimental results

• Electrons spontaneously flow from a position of higher potential energy

(anode) to a position of lower potential energy (cathode).

• The moving electrons can do work; light bulbs, motors….

• Electric potential of a cell:

E p 761

– The difference between the potential energy of the anode and the cathode

.

• Measured in volts (V)

• Commonly called – cell voltage or cell potential

• A cell potential of 0 V (zero) means the cell has no electric potential.

– If the electrodes are the same, then there is no difference in there potential and will not generate any electric potential (same height)

– Electrodes must be different.

Standard cell potentials

p 768

The standard half cell table of reduction potentials was created by comparing all half cell reactions with the standard hydrogen electrode (SHE)

The SHE has H

1.0 mol/l HCl

2(g)

(aq). bubbled over a Pt electrode in

This half cell has been given a E

0

0.00 V.

=

Standard reduction potentials of half cells

Table 19.1 p 768 or Table E.14 p 848

These are standard values which means:

1. all elements in the table are in their standard states at

25 o

C and 101.3 kPa

.

2. And all ions have a standard molar concentration of

1.0 mol/L

.

Keep in mind that all half cells are written as reduction.

– For oxidation half reactions we need to reverse the equation and change the sign!

• Strongest reducing agent is lowest on the right on the table.

• Strongest oxidizing agent is highest on the left

• Calculate the E

o

for the following cell:

Zn

(s)

| Zn

2+

(1.0mol/L)

|| Cu

2+

(1.0mol/L)

| Cu

(s)

Formula method:

o

E cell

o

E cathode

o

E anode

Half cell method:

• Calculate the E

0 for this reaction:

Which half cell will be the anode?

Zn | Zn

2+

(1 mol/L)

|| H

+

(1 mol/L)

|Pt,H

2

Spontaneity

– will electrons flow!

p 770

• If a cell is to produce an electric current the

E

0 cell must be positive .

That is, the cathode must have a higher reduction potential than the anode

.

Ie: On the table, the cathode must be higher on the table than the anode

.

Calculate the cell potential for each: Indicate if a current can be produced.

Cd | Cd

2+

|| Cu

2+

| Cu

I

2

| 2 I

-

|| 2 Cl

-

| Cl

2

• a)

Reduction table trends:

Which will react spontaneously with Br

2 but not with I

2

?

Ie: On the table, the cathode must be higher on the table than the anode

.

Cr

2+ b) F

c) Fe

2+ d) Mn

2+

• P 775 #2c

• P 775 #6

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