Title: Lesson 6 Electrolytic Cells

Learning Objectives:– Describe electrolytic cells

– Identify at which electrode oxidation and reduction takes place

– Understand how current is conducted in electrolytic cells

– Deduce the products of electrolysis of a molten salt

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A particular voltaic cell is made from magnesium and iron half-cells. The overall equation for the reaction occurring in the cell is

Mg(s) + Fe2+(aq) → Mg2+(aq) + Fe(s)

Which statement is correct when the cell produces electricity?A. Magnesium atoms lose electrons.B. The mass of the iron electrode decreases.C. Electrons flow from the iron half-cell to the magnesium half-cell.D. Negative ions flow through the salt bridge from the magnesium half-cell to the iron half-

cell.

For each incorrect statement, explain why it is wrong.

Electrolytic Cells• Opposite of voltaic cells• Voltaic – uses spontaneous redox reactions to generate electricity• Electrolytic – uses electricity to bring about a non-spontaneous redox reaction

• Electro-lysis electricity is used to bring about reactions of chemical breakdown

• The reactant in the process of electrolysis is present in the electrolyte. This can be a molten ionic compound or a solution of an ionic compound. Current is passed through the electrolyte and redox reactions occur at the electrodes. Removes charges from ions to become neutral, these ions are discharged during this process.

• Reactive metals (e.g. magnesium, sodium, aluminium etc.) are found naturally in compounds such as NaCl or Al2O3 where they exist as positive ions.

• Extraction would involve reduction… But the Eө values are so low this is a problem… There are no good reducing agents available!

Electrolysis is the only means of extracting these metals from their ores…

Components of the electrolytic cell

• Electric power source is battery or DC power source (see diagram of battery for terminal information)

• Electrodes immersed in electrolyte and connected to power supply. Made from metal or graphite. (Inert when not taking part in redox)

• Electric wires connect electrodes to power supply.

• Power source pushes electrons towards negative electrode (cathode)• Current is passed through the electrolyte, by the ions that are mobile and migrate to the electrodes.• Electrons are released at positive electrode (anode) and returned to the source.• Chemical reactions occurring at each electrode remove ions from the solution.

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An Electrolytic Cell:

M+

M+

M+

M

M

M

X-X-

X-

X X

X X

CATHODE(-)

ANODE (+)

Anions move to anode

Cations move to cathode

Layer of metal formed

Bubbles of gas formed

MOLTEN SALTor

SALT SOLUTION

e- e-

Redox reactions occur at the electrodesIn the electrolyte:• The ions in the electrolyte migrate to the electrodes by attraction of opposite charges.• Positive ions (cations) cathode (negative electrode)• Negative ions (anions) anode (positive electrode)

At the electrodes:• OXIDATION (Anode)• REDUCTION (Cathode)

In aqueous solutions:• Water can be oxidized to oxygen (anode), and reduced to hydrogen (cathode)

• Cathode (negative): M+ + e- M (Cations gain, so are reduced)• Anode (positive): A- A + e- (Anions lose, so are oxidized)

Charges on the electrode - Reminder

• Charges on the electrodes are inverted in an electrolytic cell compared with a voltaic cell• Redox reaction (not the electrical charge) defines the electrode:• OXIDATION occurs at the ANODE• REDUCTION occurs at the CATHODE

• Electrons flow from anode to cathode. THIS NEVER CHANGES!

Determining the products in electrolytic cells

The electrolysis of molten salts• When the electrolyte is a molten salt, the only ions present are those from the compound

itself.• Only one ion migrates to the electrode… More straightforward to predict the reactions that

will occur!

Making it economical• Most ionic compounds have very high melting points, so to make it more economical, a compound can be added to lower

the melting point.

• E.g. For the electrolysis of molten sodium chloride, calcium chloride can be added to the electrolyte. This brings the melting point down. (801oC to 580oC).

• But you need to check that the presence of CaCl does not interfere with the discharge of sodium at the cathode.

• We can check this by looking at the Eө values:

• From the higher Eө value, we can deduce that Na+ will be reduced in preference to Ca2+, so addition of CaCl will not interfere with Na production at the cathode.

Solutions

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Practical Demonstration

Complete the electrolysis of molten zinc chloride in the fume hood. Instructions here: http://www.nuffieldfoundation.org/practical-chemistry/electrolysis-zinc-chloride