Chemical reactions Study Guide

Electrolysis is a process that uses electrical energy to drive non-spontaneous chemical reactions. When an electric current is passed through an ionic compound in a molten state or in solution, the ions move towards the oppositely charged electrodes. The electrodes are typically made of inert materials, meaning they do not react themselves, but simply provide a surface for the reactions to occur. The cathode is the negatively charged electrode, and the anode is the positively charged electrode. At the cathode, positively charged ions (cations) gain electrons and are reduced. If the electrolyte contains metal ions, these will be deposited as pure metal at the cathode. If the electrolyte is an aqueous solution and contains hydrogen ions from water, or if the metal ions are more reactive than hydrogen, then hydrogen gas will be produced at the cathode instead. At the anode, negatively charged ions (anions) lose electrons and are oxidised. Non-metals, such as chlorine, oxygen, or iodine, are typically formed at the anode. For example, in the electrolysis of molten lead bromide, lead metal forms at the cathode and bromine gas forms at the anode. In an aqueous solution of sodium chloride using inert electrodes, hydrogen gas forms at the cathode and chlorine gas forms at the anode, while sodium ions remain in solution.

When a chemical reaction occurs in a non-enclosed system, the total mass of the substances might appear to change. This apparent change in mass is due to substances entering or leaving the system, rather than a violation of the law of conservation of mass. The particle model helps to explain this. In any chemical reaction, atoms are rearranged, but they are never created or destroyed. Therefore, the total mass of the reactants must equal the total mass of the products if all substances are accounted for within the system.

In a non-enclosed system, if a gaseous product is formed, it can escape into the surroundings, leading to a decrease in the measured mass of the system. For example, when calcium carbonate reacts with hydrochloric acid, carbon dioxide gas is produced. If this reaction takes place in an open beaker, the carbon dioxide gas will escape, causing the mass of the beaker and its contents to decrease over time. Conversely, if a reactant is a gas from the atmosphere and it is consumed in the reaction, the mass of the system could appear to increase. For instance, when iron rusts, it reacts with oxygen from the air. If a lump of iron is left uncovered and allowed to rust, its mass will increase because oxygen atoms from the air become incorporated into the solid iron oxide product. Similarly, during combustion, if a solid fuel burns in air, the system's mass may decrease as gaseous products like carbon dioxide and water vapour escape. However, if the combustion involves a gas from the air, like oxygen, reacting with a solid, the mass of the solid product could increase if the gaseous reactants are absorbed.

It is crucial to remember that the total mass of all reactants, including any gases from the atmosphere, always equals the total mass of all products, including any gases that escape. The observed change in mass in a non-enclosed system merely reflects the movement of particles (atoms or molecules) into or out of the system being weighed.