Electrolysis Examples

• The copper ions (Cu²⁺) will go into the electrolyte solution of copper sulfate.
• The Cu²⁺ ions will be attracted to the cathode and pure copper will form.
  • Cu²⁺_(aq) + 2e^- → Cu_(s)

• The impurities of the anode will fall to the bottom of the beaker.
• When the mass of the cathode is constant, the purification process is complete.

To measure the mass of copper formed on the cathode over time:

• Record the initial mass of the electrodes. The cathode must be cleaned for the copper to stick to it
• Use a variable resistor because the current may vary during the process. It should be kept constant.

• You should wash electrodes to remove any copper sulfate. You should then dry them.
• You should also wash the electrodes with a more volatile solvent to make them dry faster. You could use propanone, for instance.

• Measure the final mass of the electrodes to work out the change in mass: change in mass = final mass - initial mass.

• When we use graphite electrodes to apply electricity across molten lead bromide, we form lead (metal) at the cathode and bromine (non-metal) at the anode.
  • Lead bromide → lead + bromine
• We know lead is present if a shiny metal forms around the cathode.
• Bromine is observed as a brown gas because of the high temperatures reached in the process.

• Applying electricity across hydrochloric acid with graphite electrodes produces hydrogen gas at the cathode and chlorine gas (non-metal) at the anode.
• We can detect hydrogen at the cathode using a lit splint. A squeaky pop is heard.
• We can detect chlorine gas by its smell. It also bleaches litmus paper.

• The electrolysis of aqueous solutions is a little more complex, as you have to consider the water.
• In the case of aqueous sodium chloride, hydrogen is formed instead of sodium.
• Applying electricity across aqueous sodium chloride produces hydrogen gas at the cathode and chlorine gas (non-metal) at the anode.
• The electrodes are inert graphite or platinum.
• The formation of hydrogen at the cathode can be detected using a lit split. A squeaky pop is heard.
• The chlorine gas can be detected by its smell. It also bleaches litmus paper.

• When dilute sulfuric acid is electrolysed, hydrogen is produced from the acid and oxygen from the water.
• Applying electricity to sulfuric acid with graphite electrodes produces hydrogen gas at the cathode and oxygen at the anode.
• The formation of hydrogen at the cathode can be detected using a lit split. A squeaky pop is heard.
• The oxygen gas produced at the anode can also be detected using a glowing splint. The splint will relight in oxygen gas.

• Oxidation reactions happen i.e. negatively charged ions lose electrons.
  • E.g. 2Br^- → Br₂ + 2e^-

• Reduction reactions happen i.e. positively charged ions gain electrons.
  • E.g. Pb²⁺ + 2e^- → Pb