IB Chemistry Power Points

Topic 09

Voltaic Cells SL

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ELECTROCHEMISTRY

Deals with chemical changes produced by an electric

current and with the production of electricity by chemical

reactions

All electrochemical reactions involve transfer of electrons

and are redox reactions

Electrochemical reactions take place in electrochemical

cell (an apparatus that allows a reaction to occur through

an external conductor)

ELECTROCHEMICAL CELLSTwo types:

Voltaic cells: produce electrical energy

Also called galvanic cells. In these cells spontaneous chemical reactions generate electrical energy and supply it to an external circuit.

Electrolytic cells: require electrical energy

These are cells in which an external electrical source forces a nonspontaneous reaction to occur.

Electrochemical Terms – all cells

Half-cell: a metal electrode in contact with a solution of its own ions

Electrode: A conductor used to establish contact with a nonmetallic part of a circuit, such as an electrolyte

Anode: The electrode where oxidation takes place

Cathode: The electrode where reduction takes place

VOLTAIC CELLS• Cells in which spontaneous reactions produceselectrical energy

• The two half-cells are separated so that electron transfer occurs through an external circuit

• Each half-cell contains the oxidized and reducedforms of a species in contact with each other

• Half-cells are linked by a piece of wire and a salt bridge

A TYPICAL VOLTAIC CELL

The Zinc-Copper cell

Composed of two half-cells:

1. A strip of copper immersed in 1 M CuSO4

2. A strip of zinc immersed in 1 M ZnSO4

Experimentally we see:

Initial voltage is 1.10 volts

The mass of the zinc electrode decreases

The mass of the copper electrode increases

[Zn2+] increases and [Cu2+] decreases

Zinc – Copper Cell - notation

Zn Zn2+(1.0 M) Cu 2+ (1.0 M) Cu

Electrode

Salt bridge

Species (withconcentrations) in contact with electrodes

A salt bridge (or porous partition) has three

functions:

1. The salt bridge allows for the flow of ions and

therefore electrical contact between the two

half-cells

2. As a result of electrical contact, the salt

bridge maintains the electrical neutrality in

each half-cell as ions flow into and out of the

salt bridge

3. The salt bridge prevents mixing of the

electrode solutions

Current flows spontaneously from negative to the positive electrode (oxidation electrode to reduction electrode)

In voltaic cells, voltage drops as the reaction proceeds. When voltage = 0, the reaction is at equilibrium

Electrical neutrality in each half cell is important!

anions flow into the oxidation half-cell to counter the build-upof positive charge and vice versa

If this did not happen, current would stop flowing

Voltaic Cells - Summary

Voltaic cells: Electrochemical cells in which a spontaneous redox reaction can be harnessed to produce an electric current.

Anode: oxidationnegative

Cathode: reductionpositive

Example: The Zinc – Copper Cell