Electrochemistry

This chapter asks for three uses of electrolysis and the process that uses electrolysis to treat wastewater p. 176. Superior car batteries for electric cars produce and store a lot of energy, charge rapidly, and are durable and light p. 177.

6.1 Electrolytic Cell

Electrochemistry is the study of the relationship between electrical and chemical phenomena in two types of electrochemical cells p. 178. In an electrolytic cell, electric current flows through an electrolyte to produce a chemical reaction, so electrical energy is converted to chemical energy p. 178. In a chemical cell, chemical changes produce an electric current, so chemical energy is converted to electrical energy p. 178. Electrolysis is the decomposition of a compound in the molten or aqueous state into its constituent elements when electric current flows through it p. 178.

An electrolytic cell has an electrical source such as a battery, an anode connected to the positive terminal, a cathode connected to the negative terminal, and an electrolyte containing cations and anions p. 178. Electrolysis cannot take place if there is no electric current flowing through the electrolyte p. 179. Electrolytes are substances in the molten or aqueous state which allow electric current to flow and undergo chemical changes, while non-electrolytes do not allow electric current to flow p. 179. Molten lead(II) bromide, molten sodium chloride, sodium hydroxide solution and copper(II) sulphate solution are electrolytes, while naphthalene, acetamide, glucose solution and ethanol are non-electrolytes p. 179. Sodium chloride is an ionic compound made up of sodium ions, Na⁺, and chloride ions, Cl⁻ p. 179.

During electrolysis, positively charged cations move to the cathode and negatively charged anions move to the anode p. 180. In molten lead(II) bromide, lead(II) ions, Pb²⁺, move to the cathode while bromide ions, Br⁻, move to the anode p. 180. Electrolytes in the solid state cannot conduct electricity because there are no free-moving ions p. 180. In Experiment 6.1, a light bulb shows whether solid lead(II) bromide, molten lead(II) bromide and copper(II) sulphate solution conduct electricity p. 180. The electrolysis of lead(II) bromide is a teacher’s demonstration carried out in a fume chamber p. 181.

Three factors affect the selection of ions discharged at the electrodes in aqueous solutions: the position of ions in the electrochemical series, the concentration of electrolyte and the type of electrode p. 182. When a positive ion is discharged it receives electrons and becomes neutral, and when a negative ion is discharged it donates electrons p. 182.

In the electrochemical series, the higher the position of a metal, the easier it is for the metal to donate electrons p. 183. Ions at the bottom of the electrochemical series, such as the silver ion and the hydroxide ion, have higher tendencies to be discharged p. 183. In sodium sulphate solution, hydrogen ions are discharged at the cathode to give hydrogen gas, and hydroxide ions are discharged at the anode to give oxygen gas p. 183. In copper(II) sulphate solution, copper(II) ions are discharged because they are less electropositive than hydrogen ions, so copper is deposited at the cathode and oxygen gas is produced at the anode p. 184. In Experiment 6.2, the hypothesis is that the lower the position of an ion in the electrochemical series, the easier it is for the ion to be discharged p. 184. Gases from magnesium nitrate and sodium sulphate solutions are tested with a glowing wooden splinter and a burning wooden splinter p. 185.

Negative ions which are more concentrated in an electrolyte are more likely to be discharged at the anode p. 186. In concentrated sodium chloride solution such as 1.0 mol dm⁻³, the chloride ion is discharged and chlorine gas is produced at the anode p. 187. In dilute sodium chloride solution such as 0.0001 mol dm⁻³, the hydroxide ion is discharged and oxygen gas is produced at the anode p. 187. Experiment 6.3 compares 1.0 mol dm⁻³ and 0.0001 mol dm⁻³ hydrochloric acid, and chlorine gas is poisonous p. 187. If moist blue litmus paper bleaches, the gas in the test tube is halogen gas p. 188.

If the anode is the same metal as the metal ion in the electrolyte, the anode ionises and dissolves, metal is deposited at the cathode, and the concentration of metal ions does not change p. 189. With silver electrodes in silver nitrate solution, the silver anode dissolves and silver is deposited at the cathode p. 189. With carbon electrodes in silver nitrate solution, oxygen gas is produced at the anode and the concentration of silver ions decreases p. 190. Experiment 6.4 compares carbon electrodes and copper electrodes in 0.1 mol dm⁻³ copper(II) sulphate solution p. 190.

Metals like potassium, sodium, calcium, magnesium and aluminium are extracted from their molten ores or salts through electrolysis p. 191. In the purification of metal, the impure metal is the anode and the pure metal is the cathode, where pure metal is deposited p. 191. In electroplating, gold, platinum and silver are electroplated on other metals to look more attractive and withstand corrosion p. 191. Electrocoagulation is an innovative technique to treat wastewater that applies electrolysis and coagulation p. 191. In electrocoagulation, an aluminium anode produces aluminium ions, Al³⁺, and hydrogen gas bubbles are released from the cathode p. 192. Aluminium ions, hydroxide ions and pollutants combine to produce floc, which is carried up by the hydrogen gas bubbles or sinks to the base p. 192.

6.2 Chemical Cell

A simple chemical cell is made up of two different metals immersed in an electrolyte and connected to an external circuit with connecting wires p. 192. In a magnesium and copper cell, magnesium becomes the negative terminal because magnesium is more electropositive than copper p. 193. Magnesium donates electrons, forms magnesium ions and dissolves in the copper(II) sulphate solution p. 193. The electrons flow through the external circuit from magnesium to copper, where copper(II) ions receive them and solid copper is deposited on the copper strip p. 193. The flow of electrons through the external circuit produces electrical energy, so the energy change is from chemical energy to electrical energy p. 193.

In Activity 6.2, a magnesium ribbon and a copper strip in 150 cm³ of 1.0 mol dm⁻³ sodium chloride solution are connected to a voltmeter p. 194. The voltmeter readings are compared for magnesium and copper, a pair of magnesium ribbons, and a pair of copper strips p. 194. In Activity 6.3, a STEM project builds a simple chemical cell that generates electrical energy from fruits or other plant parts and seawater p. 195.