Chemical reactions Study Guide

Electrolysis is the process of using electrical energy to drive non-spontaneous chemical reactions. When an ionic compound is dissolved in water, an aqueous solution is formed. This solution contains not only the ions from the dissolved compound, but also hydrogen ions (H+) and hydroxide ions (OH-) from the dissociation of water. This presence of multiple types of ions leads to 'competing reactions' at the electrodes, where more than one species could potentially gain or lose electrons.

At the cathode (negative electrode), positive ions are attracted. These include the metal cations from the ionic compound and hydrogen ions from water. A competition occurs: the species that is more easily reduced (gains electrons) will be preferentially discharged. For metals less reactive than hydrogen, like copper or silver, the metal ions will be reduced to form the pure metal. If the metal is more reactive than hydrogen (e.g., sodium, potassium, calcium), then hydrogen ions will be reduced to form hydrogen gas, as it is easier to reduce H+ than these reactive metal ions.

At the anode (positive electrode), negative ions are attracted. These include the non-metal anions from the ionic compound and hydroxide ions from water. Again, a competition takes place: the species that is more easily oxidised (loses electrons) will be preferentially discharged. If halide ions (chloride, bromide, iodide) are present in high concentration, they tend to be oxidised to their respective halogen gases. However, if other anions like sulfate (SO4^2-) or nitrate (NO3^-) are present, or if the halide concentration is low, then hydroxide ions will be oxidised to form oxygen gas and water. This is because sulfate and nitrate ions are very stable and difficult to oxidise compared to hydroxide ions.

Neutralisation is a fundamental chemical reaction that occurs when an acid and a base react to form a salt and water. In the context of aqueous solutions, acids produce hydrogen ions (H⁺) when dissolved in water, while bases (specifically alkalis) produce hydroxide ions (OH⁻). The core of any aqueous neutralisation reaction is the combination of these two ions to form water. This can be represented by the ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l). The 'aq' subscript indicates that the ions are dissolved in an aqueous (water) solution, and 'l' indicates liquid water. This general ionic equation highlights that, regardless of the specific acid or base used (e.g., hydrochloric acid and sodium hydroxide, or sulfuric acid and potassium hydroxide), the essential chemical event is the reaction between hydrogen ions and hydroxide ions. The other ions present (from the acid and base) remain in the solution as spectator ions and form the dissolved salt. For example, in the reaction between HCl and NaOH, the Na⁺ and Cl⁻ ions are spectator ions, forming sodium chloride in solution.