Electrolysis Revision

Electrolysis  - process by which ionic substances are broken down into simpler substances when an electric current is passed through them.

• Ionic substances contain charged particles - ions.
• Ions must be free to move - normally when the substance is dissolved in water or when melted.
• Requires a liquid to conduct the electricity - electrolyte.
• Contain free ions
• Always needs to be a flow of electrons.

For example, if electricty is passed through molten lead bromide, the lead bromide is broken down to form lead and bromine.

• Positively charged ions move to the negative electrode during electrolysis - they receive electrons and are reduced.
• Negatively charged ions move to the positive electrode - they lose electrons and are oxidised.
• The substance broken down is the electrolyte.
• A current is passed through the electrodes; the negative terminal is attached to one which becomes the negative electrode; cathode.  The positive terminal is attached to the other, this becomes a positive electrode; anode.

Electrolysis is used to electroplate objects. This is useful for coating a cheaper metal with a more expensive one.

• The negative electrode should be the object that is to be electroplated.
• The positive electrode should be the metal that you want to coat the object with(pure metal).
• The electrolyte should be a solution of the coating metal, such as a metal nitrate or sulfate.

The object to be plated, such as a metal spoon, is connected to the negative terminal of the power supply - a piece of silver is connected to the positive terminal. The electrolyte is silver nitrate solution.

Extraction

Aluminium is the most abundant metal on Earth - expensive because of the large amounts of electricity used up in the extraction process.

Aluminium ore - bauxite.  This is purified to yield a white powder - aluminium oxide.

First, the aluminium oxide must be melted so that the electrolysis can pass through it.  It has a very high melting point so would be very expensive.  Instead, it is dissolved in molten cryolite - aluminium compound - with a lower melting point. This reduces energy costs involved in extracting aluminium.

• The electrodes are made of carbon(graphite), a good conductor of electricity.
• Alumnium forms at the negative electrode adn oxygen foms at the positive electrode.
• The oxygen then reacts with the carbon in the electrode to produce carbon dioxide.
• This means that the positive electrodes gradually burn away and so have to be replaced frequently.  So, this adds to the cost of the process.

Some metals have to be extracted if they are more reactive than carbon by electrolysis of molten compounds.

Purification

Electricity is passed through solutions containing copper compounds such as copper sulfate. The anode - positive electrode - is impure copper.  Pure copper forms on the cathode - negative electrode.

What is a half equation?

It shows you what happens at one of the electrodes during electrolysis.

A half-equation is balanced by adding, or taking away, a number of electrons equal to the total number of charges on the ions in the equation.

For example:

• Anode: 2Cl^– – 2e^– → Cl2 (oxidation)
• Cathode: 2H⁺ + 2e^– → H2 (reduction).
• Cathode: Al³⁺ + 3e^- → Al
• Anode: 2O^2- → O₂ + 4e^-

Brine is a solution of sodium chloride and water. The electrolysis of brine is a large-scale process used to manufacture chlorine from salt - two other useful chemicals are obtained during the process; sodium hydroxide and hydrogen.

Uses of the chemical

Chlorine

• disinfectant and purifier
• manufacture of hydrochloric acid
• making plastics

Sodium Hydroxide

• processing food products
• removing pollutants from water
• manufacture of paper

Hydrogen

• manufacture of hydrochloric acid

During Electrolysis:

• Chlorine gas forms at the anode (positive electrode).
• Hydrogen gas forms at the cathode (negative electrode).
• A solution of sodium hydroxide forms.

At the cathode, two hydrogen ions accept two electrons to become one hydrogen molecule.

At the anode, two chloride ions lose their electrons and become one chlorine molecule.  The sodium ions stay in solution because they're more reactive than hydrogen. Hydroxide ions from water are left behind.

Positively charged ions gain electrons - reduction; the ions have been reduced.  Metal ions and hydrogen ions are positive; whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the reactivity series.

• Metal will be produced if it less reactive than hydrogen
• Hydrogen will be produced if the metal is more reactive than hydrogen

For example, the electrolysis of copper chloride solution produces copper at the negative electrode, but the electrolysis of sodium chloride solution produces hydrogen.