Chemical reactions Study Guide

The pH scale is a numerical scale used to specify how acidic or alkaline (basic) an aqueous solution is. It typically ranges from 0 to 14. A pH value of 7 is considered neutral, meaning the substance is neither acidic nor alkaline. Pure water at 25 °C has a pH of 7.

Solutions with a pH less than 7 are acidic. The lower the pH value (e.g., pH 1, 2, 3), the stronger the acid. Strong acids like hydrochloric acid or sulfuric acid have very low pH values, close to 0 or 1. These solutions have a high concentration of hydrogen ions (H⁺).

Solutions with a pH greater than 7 are alkaline (also known as basic). The higher the pH value (e.g., pH 11, 12, 13, 14), the stronger the alkali. Strong alkalis such as sodium hydroxide or potassium hydroxide have very high pH values, close to 13 or 14. These solutions have a high concentration of hydroxide ions (OH⁻).

The pH scale is logarithmic, which means that each whole pH value below 7 is ten times more acidic than the next higher whole value. For example, a solution with a pH of 4 is ten times more acidic than a solution with a pH of 5, and a hundred times more acidic than a solution with a pH of 6. Similarly, each whole pH value above 7 is ten times more alkaline than the next lower whole value.

pH can be measured using indicators, which are substances that change colour depending on the pH of the solution. Examples include litmus paper (red in acid, blue in alkali) and universal indicator (shows a range of colours across the entire pH scale). pH meters provide a more precise numerical reading.

Chemical reactions involve changes in energy. These energy changes often manifest as changes in temperature of the surroundings. We classify reactions into two main types based on whether they release or absorb energy: exothermic and endothermic reactions.

An exothermic reaction is a reaction that releases energy, usually in the form of heat, into its surroundings. This release of energy causes the temperature of the surroundings to increase. Common examples include combustion (burning fuels like wood or natural gas), neutralization reactions (acid + alkali), and the setting of concrete. When you light a candle, the heat you feel is a result of an exothermic reaction. The chemical potential energy stored in the reactants is converted into thermal energy, which is transferred to the surroundings.

An endothermic reaction is a reaction that absorbs energy, usually in the form of heat, from its surroundings. This absorption of energy causes the temperature of the surroundings to decrease. Examples include the thermal decomposition of calcium carbonate (limestone), dissolving ammonium nitrate in water (used in instant cold packs), and photosynthesis. When an instant cold pack is activated, it gets cold because the reaction inside is taking heat energy from your body and the immediate environment. The thermal energy from the surroundings is absorbed and converted into chemical potential energy stored in the products.

To distinguish between these two types of reactions, we simply need to measure the temperature change of the surroundings. If the temperature rises, it's exothermic. If the temperature falls, it's endothermic. This can be done using a thermometer and a suitable container for the reaction, such as a beaker or a polystyrene cup to minimise heat loss or gain to the external environment.