Chemical changes Study Guide

When copper(II) sulfate solution undergoes electrolysis using copper electrodes, distinct processes occur at the anode and cathode. At the cathode (negative electrode), copper(II) ions (Cu²⁺) dissolved in the solution are attracted due to their positive charge. Here, they gain two electrons each to become neutral copper atoms, which then deposit onto the cathode, increasing its mass. The half-equation for this process is: Cu²⁺(aq) + 2e⁻ → Cu(s).

At the anode (positive electrode), the copper electrode itself participates in the reaction. Instead of non-metal ions or hydroxide ions being discharged, the copper atoms from the anode lose two electrons each to form copper(II) ions, which then dissolve into the electrolyte. The half-equation for this is: Cu(s) → Cu²⁺(aq) + 2e⁻. This process causes the anode to decrease in mass over time. Due to the regeneration of copper(II) ions at the anode, the concentration of copper(II) sulfate in the solution remains relatively constant, and the blue colour of the solution persists.

This principle is crucial for the purification of copper. In this application, an impure copper block serves as the anode, and a thin sheet of pure copper acts as the cathode. As electrolysis proceeds, copper from the impure anode dissolves into the solution as Cu²⁺ ions. Less reactive metals present as impurities in the anode, such as silver or gold, fall to the bottom as an 'anode sludge' because they do not oxidise as easily as copper. More reactive metals, such as zinc or iron, also oxidise and dissolve into the solution as ions, but they are not deposited at the cathode because copper(II) ions are more readily reduced due to copper's position in the reactivity series. At the pure copper cathode, only the copper(II) ions from the solution are reduced and deposited, resulting in a build-up of highly pure copper. This method efficiently refines impure copper to typically over 99.9% purity.

When acids are dissolved in water, they release hydrogen ions (H⁺) into the solution. It is these hydrogen ions that are responsible for the characteristic acidic properties, such as a sour taste or reacting with metals to produce hydrogen gas. Conversely, alkalis are soluble bases that release hydroxide ions (OH⁻) when dissolved in water. These hydroxide ions are responsible for the characteristic alkaline properties, such as a slippery feel or reacting with acids in neutralisation reactions. The concentration of these ions determines the strength of the acid or alkali, and therefore its pH. Strong acids and alkalis fully dissociate in solution, releasing a high concentration of H⁺ or OH⁻ ions, respectively, while weak acids and alkalis only partially dissociate.