Chemical reactions Study Guide

Electrolysis is a chemical process that uses electrical energy to drive non-spontaneous chemical reactions. It is used to decompose ionic compounds, either molten or dissolved in solution, by passing an electric current through them. The setup for electrolysis typically involves an electrolyte (the molten or dissolved ionic compound), a power supply, and two electrodes. The electrodes are electrical conductors that dip into the electrolyte. The positive electrode is called the anode, and the negative electrode is called the cathode. Cations (positively charged ions) are attracted to the cathode, where they gain electrons and are reduced. Anions (negatively charged ions) are attracted to the anode, where they lose electrons and are oxidised. The type of electrode used significantly impacts the products formed during electrolysis. Inert electrodes, such as graphite or platinum, are unreactive and do not participate in the chemical reactions during electrolysis. They merely act as surfaces for electron transfer. In contrast, non-inert (or active) electrodes, such as copper, are reactive and can participate in the electrochemical reactions, often dissolving themselves at the anode. The choice of electrode dictates whether the electrode itself reacts or if only the ions from the electrolyte react.

The pH scale is a measure of the acidity or alkalinity of a solution. It ranges from 0 to 14. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are alkaline. This scale is logarithmic, meaning that a change of one pH unit represents a tenfold change in the concentration of hydrogen ions (H⁺).

Acids are substances that release hydrogen ions in solution. The higher the concentration of hydrogen ions, the more acidic the solution, and the lower its pH value. For example, a solution with a pH of 3 has ten times the concentration of H⁺ ions compared to a solution with a pH of 4. Conversely, a solution with a pH of 2 has one hundred times more H⁺ ions than a solution with a pH of 4.

Alkalies (or bases) are substances that accept hydrogen ions or release hydroxide ions (OH⁻) in solution. While the pH scale directly measures H⁺ concentration, in alkaline solutions, the H⁺ concentration is very low. As the concentration of H⁺ ions decreases, the pH value increases, indicating a more alkaline solution. For example, a solution with a pH of 10 has ten times fewer H⁺ ions than a solution with a pH of 9.

Neutral solutions, such as pure water, have an equal concentration of hydrogen ions and hydroxide ions, resulting in a pH of 7. The relationship between pH and H⁺ concentration is crucial for understanding the relative strengths of acids and bases.