LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- define electrolysis and identify cell components
- predict products of molten and aqueous electrolysis
- write ionic half-equations
- explain electroplating and copper purification
- evaluate hydrogen–oxygen fuel cells
THE BIG IDEA
Explain how electricity decomposes ionic substances and how electrochemical cells transfer energy.
Electrolysis uses electrical energy to force a non-spontaneous chemical change. Ions move through the electrolyte while electrons move through the external circuit.
Reduction occurs at the cathode and oxidation occurs at the anode.
SECTION 01
Electrolysis and charge transfer
Electrolysis is the decomposition of an ionic compound, molten or aqueous, by an electric current. The electrolyte contains mobile ions.
The cathode is negative and attracts cations; the anode is positive and attracts anions. At electrodes, ions gain or lose electrons and become neutral products.
KEY IDEAS
- Cathode: reduction by electron gain.
- Anode: oxidation by electron loss.
- Electrons move in wires; ions move through the electrolyte.
SECTION 02
Molten and aqueous products
A molten binary compound contains only its ions, so the metal forms at the cathode and the non-metal at the anode.
An aqueous electrolyte also contains H⁺ and OH⁻ from water. At the cathode, hydrogen forms instead of a metal more reactive than hydrogen. At the anode, concentrated halide ions form halogens; otherwise oxygen commonly forms.
Molten lead(II) bromide gives grey lead at the cathode and brown bromine vapour at the anode. Concentrated sodium chloride produces hydrogen, chlorine and sodium hydroxide solution. Dilute sulfuric acid produces twice the volume of hydrogen as oxygen.
SECTION 03
Copper electrolysis and electroplating
With inert electrodes in aqueous copper(II) sulfate, copper forms at the cathode and oxygen at the anode; the blue solution becomes paler as Cu²⁺ is removed.
With copper electrodes, copper dissolves at the anode and is deposited at the cathode, keeping Cu²⁺ concentration nearly constant. This principle purifies copper.
For electroplating, the object is the cathode, the coating metal is the anode and the electrolyte contains ions of the coating metal.
KEY IDEAS
- Electroplating improves appearance and corrosion resistance.
- Clean the object so the metal coating adheres evenly.
SECTION 04
Hydrogen–oxygen fuel cells
A hydrogen–oxygen fuel cell converts chemical energy directly to electrical energy and produces water as its only chemical product.
Advantages for vehicles include no carbon dioxide at the point of use and quick refuelling. Disadvantages include hydrogen storage, limited infrastructure, energy needed to manufacture hydrogen and possible fossil-fuel emissions during production.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Ions carry charge in electrolytes and electrons in wires.
- Reduction occurs at the cathode; oxidation at the anode.
- Aqueous products depend on competing ions.
- Electroplating places the object at the cathode.
- Fuel-cell impacts depend on hydrogen production and storage.