Electrolytic Cell and Electroplating

Chapter 19 Page 776-781

Chem 12

• An electrolytic cell is a device in which an external source of electrons (electrical circuit) is used to make a non-spontaneous redox reaction take place.

• Molten (liquid) state of a pure substance is the medium which the cell is made. Ex: molten NaCl

• Oxidation at anode

2Cl- Cl2 + 2e-

• Reduction at cathode

Na+ + 1e- Na

NOTE:The cathode is still under going reduction, but now

is negative.The anode is still under going oxidation, but now is

positive.

REASON:• The electrodes are connected to a power source

that pump e- from negative to positive.• The electrode that is connected to the power

source is the cathode.• Since electrons are being forced into the

cathode it becomes negative.

• An example of an electrolytic cell is when molten NaCl is electrolyzed to form liquid sodium and chlorine gas. The sodium ions migrate toward the cathode, where they are reduced to sodium metal. Similarly, chloride ions migrate to the anode and are oxided to form chlorine gas.

• E0net = E0

ox + E0red

= (-1.36 V) + (-2.71 V)

= -4.07 V• The net cell voltage of -4.07• Because it is a non-spontaneous reaction the

E0net is always less than zero.

• In the electrolytic cells using molten ionic compounds, the negative ions will be oxidized.

• The two electrodes used in an electrolytic cell do not take part in any reaction. They are non-reactive. Ex: carbon and platinum.

Electroplating

• Electroplating is a procedure that uses electrolysis to apply a thin layer of a metal over the surface of another metal.

• Electrolysis is the process of making a non-spontaneous chemical redox reaction occur by passing electricity through a substance. (electrolytic cell)

• In electroplating, the anode is made up of the metal you want to coat the surface of another metal with.

• There is also a salt solution present of the anode metal.

• While electrolysis is taking place, the anode metal is oxidized and goes into solution as positive ions.

• These positive ions are then reduced on the surface of the cathode (the metal you wish to coat).

Examples of electroplating

• Coating jewlery with thin layer of expensive metal.

• Coating chromium over steel to make rust resistant.

SAMPLE PROBLEM 1A spoon is to be plated

with silver, Ag.1. Identify the anode

and cathode.2. Write an equation for

the reaction taking place at the anode and at the cathode and indicate whether it is oxidation or reduction.

3. What electrolyte is used?

ANSWER

1. Cathode = spoon (metal object to be coated), Anode = silver electrode

2. As electrolysis takes place, the silver anode is oxidized,

Ag(s) → Ag+(aq) + 1e-• The Ag+ (aq) ions in solution travel to the spoon

cathode and are reduced to form neutral Ag(s) on the surface of the spoon (cathode):

Ag+ (aq) + 1e- → Ag(s)

3. Electrolyte solution is made of AgNO3

SAMPLE PROBLEM 2

A knife to be used as an eating utensil is to be plated with gold, Au.

a) Sketch and label a diagram of the apparatus used to electroplate the knife with gold.

b) Identify the anode and cathode.c) Write an equation for the reaction taking place

at the anode and at the cathode and indicate whether it is oxidation or reduction.

d) What electrolyte is used?