C3.4 - Electrolysis

Electrolysis is a process in which an electric current is passed through a compound, causing a chemical change.

• An electrolyte. • Two electrodes. • An electrical supply such as a power pack or battery.

An electrolyte is a compound in its liquid state or in solution which contains mobile ions and conducts electricity.

• Electrical conductors used in electrolysis.There are two electrodes used in electrolysis: • The negative electrode - The cathode • The positive electrode - The anode.

• Positive ions (cations) gain electrons at the cathode and become atoms - They become discharged. • Negative ions (anions) lose electrons at the anode and become atoms - They become discharged. • If the atoms formed are non-metal atoms, covalent bonds may form between them, making molecules.

A binary ionic compound contains just two elements.

• Ions move to an oppositely charged electrode during electrolysis. • The concentration of ions close to each electrode goes down as ions gain or lose electrons to become atoms. • Other ions in the electrolyte can move to replace them by diffusion and convection. • Cannot happen which ionic compounds in the solid state and therefore cannot conduct electricity.

• Inert electrodes are not changed during electrolysis. • Usually made from unreactive metals such as copper or platinum, or from graphite. • Inert electrodes can be used for the electrolysis of many ionic compounds in their liquid state or in aqueous solution.

Water is naturally partially ionised - It contains small concentrations of hydrogen ions and hydroxide ions. At the cathode hydrogen ions are discharged - Reduced as they gain electrons - Producing hydrogen in the gas state. At the anode hydroxide ions are discharged - Oxidised as they lose electrons - Forms water and oxygen.

• 4H+ (aq) + 4e- -> 2H2 (g) - 4OH- (aq) -> 2H2O (l) + O2 (g) + 4e-

• Hydrogen is produced at the cathode, but if ions from a less reactive metal than hydrogen are present, they are discharged more easily and therefore that metal is produced instead. • Oxygen is produced at the anode, but if ions from an element in Group 7 are present at a high enough concentration, the Group 7 element is produced instead.

• Coating an object with a metal by electrolysis. - Type of electrolysis that uses non-inert electrodes.

Electrodes that are changed during electrolysis.

• The cathode is the object you want to coat. • The anode is a piece of metal you want to coat the object with. • The electrolyte is a solution containing ions of the coating metal.

• Metal ions from the electrolyte are discharged on the surface of the object (the cathode). • These ions are replaced by metal ions leaving the surface of the anode. • Overall, metal leaves the anode and is deposited on the object. • The process continues until the anode is used up.

• Cathode of pure copper. • Anode of impure copper. • Electrolyte of copper (II) sulphate solution. • Power supply connected to the electrodes.

• Cathode gains copper atoms and increases in mass. • Copper is deposited on the cathode. • Anode loses copper atoms and decreases in mass. • Impurities from the anode fall off and collect underneath it.