lecture 21- electrochemical cells

BELLWORK

Balance the following redox reaction using the “half-reaction” method

HNO2 + I- NO + I2

2e- + 2H+ + 2HNO2 2NO + 2H2O

2I- I2 + 2e-

2H+ + 2HNO2 + 2I- 2NO + 2H2O + I2

Electrochemical cells produce an electric current from redox

reactions

CAN ALSO BE CALLED A

Voltaic cell Galvanic cell Battery

A simple battery separates the reduction reaction from the

oxidation reaction so that the electrons must travel through a wire

Oxidation e- e- e- e- e- e- e- Reduction

Lose e- e- e- e- e- e- e- e- e- e- e- e- Gain e-

Redox reactionZn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

Separate into 2 half-reactions and balance

Zn(s) Zn2+ + 2e- oxidation

2e- + Cu2+(aq) Cu(s) reduction

How to make a battery or an electrochemical cell

e- e- e- e- e- e- e- e- e- e- e- e-

e- e- e- e- e- e- e- e- e-

Create each half reaction in a separate chamber called a half cell and connect them by a wire

Oxidation Reduction

half cell half cell

To make the Zinc half cell Add a strip of Zinc metal to a

solution containing Zn2+

1M ZnSO4

Zn(s)

Zinc metal

To make the copper half cell Add a strip of copper metal to

a solution containing Cu2+

1M CuSO4

Cu(s)

Copper metal

The metal strips are electrodesThe solutions are electrolytes

oxidation occurs at the anode

Zn(s) Zn2+ + 2e- metal is lost

reduction occurs at the cathode

2e- + Cu2+(aq) Cu(s) metal is plated

An Ox Red Cat

When a wire connects the half cells an electrical current is created.

This can’t occur for long because positive charge will build up at the anode and negative charge will build up at the cathode

A salt bridge is used to keep the two cell compartments neutral

The salt bridge contains a salt (positive and negative ions) that flows into each beaker as

needed to keep the charge neutral

The salt bridge completes the circuit and the current will continue to flow

SALT BRIDGE

Gotta have it!!!

How do you determine the voltage of a battery?

Use the table of reduction potentials.• All of the half-reactions are for reduction• Eº is the electrical potential of the ½ reaction in Volts• If you need the oxidation potential, flip the equation and change

the sign of Eº. All reduction potentials are reversible.• The top of the table = elements that are easily reduced, and not

easily oxidized. They take electrons easily.• The bottom of the table= elements that are easily oxidized and

not easily reduced. They give electrons easily.

To determine the voltage of a battery you add the voltages of the two half-reactions

Flip the reaction for the oxidized element and change the sign of its voltage.

Ex. What is the voltage of the Zn-Cu battery?Cu2+ + 2e- Cu(s) 0.34V Zn2+ + 2e- Zn(s) -0.76V oxidized because it is the

lowest on the table of reduction potentials

Flip itZn(s) Zn2+ + 2e- 0.76V

The element at the top of the table will be reduced. The lower one will be oxidized.

ADD THESE FOR VOLTAGE

Describe the voltaic cell represented as:

Al(s) | Al2(SO4)3(aq) || NiSO4(aq) | Ni(s)