# U4 S2 L2 E cell U4 S2 L2

Standard Reduction Potentials

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:

E o cell

E o cathode

 o

E anode

– Half cell method:

• Calculate the E 0 for this reaction:

Which half cell will be the anode?

2+

(1 mol/L)

+

(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

## 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 .

a) Cr 2+ b) F c) Fe 2+ d) Mn 2+

• P 775 #2c