9.1.10 Electrolytic Cells

• An ionic compound conducts electricity when it is molten or in solution
• The current causes the ionic compound to split up and form new substances.
• This process is called electrolysis, a word which comes from Greek and means “splitting by electricity”
• Electrolysis has many uses, including:
  • purifying copper
  • plating metals with silver and gold
  • extracting reactive metals, such as aluminium
  • making chlorine, hydrogen and sodium hydroxide

Electrolysis cells

• Electrolysis cells can be constructed using a beaker or crucible as the cell depending whether the ionic compound is in solution or molten
• For Standard Level Chemistry we only need to look at the electrolysis of molten ionic compounds

Electrolysis of a molten ionic compound

• In electrolysis, the substance that the current passes through and splits up is called the electrolyte
• The electrolyte contains positive and negative ions
• What happens to these ions during electrolysis?
  • Negative ions move to the anode and lose electrons - this is oxidation
  • Positive ions move to the cathode and gain electrons - this is reduction
  • Electrically neutral atoms or molecules are released

Electrolysis of molten lead bromide

• The reactions which take place at the  electrodes can be shown by half equations
• When the positive lead ions move to the cathode, they gain electrons in a reduction reaction:

Pb²⁺(l) + 2e^-  ⇌ Pb (l)

• Similarly, when the negative bromide ions move to the anode they lose electrons in an oxidation reaction:

 2Br^-(l) - 2e^-  ⇌ Br₂ (l) 

• Sometimes oxidation reactions are written with '+2e^-' on the right of the arrow instead of '-2e' on the left
• In this case, the alternative half equation is:

 2Br^-(l) ⇌ Br₂ (l) + 2e^- 

• Since metals are always cations and non-metal anions, it is easy to predict the products of electrolysis of molten salts:
  • Metals will always be formed at the cathode and non-metals at the anode

Comparing Voltaic & Electrolytic Cells Summary Table