Electrolytic Cells (DP IB Chemistry)

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

Electrolytic cells

• Electrolytic cells can be constructed using a beaker or crucible as the cell depending on whether the ionic compound is in solution or molten

Lead bromide electrolytic cell

Electrolysis of a binary ionic compound, when molten, produces the component elements

• 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 the 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⁺(aq) + 2e^-  ⇌ Pb(s)

• 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