Chemical reactions Study Guide

Electrolysis is the process of using electrical energy to drive non-spontaneous chemical reactions. When an ionic compound is in a molten state (melted), its ions are free to move and therefore can conduct electricity. In a molten binary ionic compound, there are only two types of ions: one positive ion (cation) and one negative ion (anion). These compounds contain a metal ion and a non-metal ion. When electric current is passed through the molten ionic compound, the positively charged cations are attracted to the negative electrode (cathode), where they gain electrons and are reduced to form a neutral metal element. For example, if the cation is Na+, it will gain one electron to become Na. The reaction at the cathode is reduction. Conversely, the negatively charged anions are attracted to the positive electrode (anode), where they lose electrons and are oxidised to form a neutral non-metal element. For example, if the anion is Cl-, it will lose one electron to become half a molecule of Cl2. The reaction at the anode is oxidation. The products of molten binary ionic electrolysis are always the pure metal at the cathode and the pure non-metal (or its gaseous form, if applicable) at the anode. This process is crucial for extracting reactive metals from their ores and for purifying certain substances. It is important to remember which electrode attracts which type of ion and the type of reaction that occurs at each electrode (reduction at cathode, oxidation at anode).

In chemistry, it's crucial to distinguish between the terms 'dilute' and 'concentrated' when describing acid solutions, and 'weak' and 'strong' when referring to the nature of the acid itself. These terms are often confused but describe fundamentally different properties.

Concentration refers to the amount of solute (the acid) dissolved in a given volume of solvent (usually water). A concentrated acid solution has a large amount of acid particles per unit volume, irrespective of its strength. Conversely, a dilute acid solution has a small amount of acid particles per unit volume. For instance, you can have a dilute solution of a strong acid or a concentrated solution of a weak acid.

Strength of an acid relates to its degree of ionisation (dissociation) in water. When an acid dissolves in water, it releases hydrogen ions (H⁺). A strong acid is one that fully ionises in an aqueous solution, meaning nearly all its molecules dissociate to release H⁺ ions. Examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). The ionisation of a strong acid is represented with a single arrow, for example: HCl(aq) → H⁺(aq) + Cl⁻(aq).

In contrast, a weak acid only partially ionises in an aqueous solution. This means that only a small fraction of its molecules dissociate to release H⁺ ions, while most remain as undissociated molecules. The ionisation of a weak acid is an equilibrium reaction, represented by a reversible arrow (⇌), for example: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). Examples of weak acids include ethanoic acid (acetic acid), carbonic acid, and citric acid.

It is important to remember that concentration and strength are independent properties. A concentrated solution of a weak acid might still have a higher overall concentration of H⁺ ions than a very dilute solution of a strong acid, but the fundamental nature of the acid (how much it ionises) remains. For example, a 1 M solution of ethanoic acid (weak) will have fewer H⁺ ions than a 1 M solution of hydrochloric acid (strong), because ethanoic acid only partially ionises.