Chemical reactions Study Guide

Chemical reactions involve changes in energy. These energy changes can be visualised using a reaction profile diagram. A reaction profile diagram shows the energy of the reactants, products, and the activation energy for a chemical reaction. The y-axis represents the energy content of the substances, typically labelled 'Energy' or 'Enthalpy (H)', while the x-axis represents the progress of the reaction, often labelled 'Reaction pathway' or 'Progress of reaction'.

There are two main types of energy changes in chemical reactions: exothermic and endothermic.

An <b>exothermic reaction</b> releases energy to the surroundings, usually as heat. This means the products have lower energy than the reactants. On an exothermic reaction profile diagram, the energy level of the products will be drawn below the energy level of the reactants. The difference in energy between reactants and products is the overall energy change, often denoted as ΔH (delta H), and it will be negative for an exothermic reaction. The activation energy is the minimum energy required for the reaction to occur, represented by the peak of the curve above the reactants' energy level.

An <b>endothermic reaction</b> absorbs energy from the surroundings, usually as heat. This means the products have higher energy than the reactants. On an endothermic reaction profile diagram, the energy level of the products will be drawn above the energy level of the reactants. The overall energy change (ΔH) for an endothermic reaction will be positive. Similar to exothermic reactions, activation energy is the energy required to initiate the reaction, shown as the peak above the reactants.

Key features to label on a reaction profile diagram include: 'Energy' (y-axis), 'Reaction pathway' (x-axis), 'Reactants' energy level, 'Products' energy level, 'Activation energy', and 'Overall energy change (ΔH)'.

In chemistry, liquids can be classified as acidic, alkaline (basic), or neutral. This property is measured using a scale called pH. The pH scale ranges from 0 to 14. A pH value of 7 indicates that a solution is neutral. Pure water, for example, has a pH of 7. Solutions with a pH value less than 7 are considered acidic; the lower the pH, the stronger the acid. For instance, stomach acid has a pH of about 1-2, making it very acidic.

Conversely, solutions with a pH value greater than 7 are considered alkaline (or basic); the higher the pH, the stronger the alkali. Common examples of alkaline solutions include soap (pH ~9-10) and oven cleaner (pH ~13-14). The pH scale is logarithmic, meaning that a change of one pH unit represents a tenfold change in acidity or alkalinity. For example, a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4.

To determine the pH of a solution, indicators or a pH meter can be used. Indicators are substances that change colour depending on the pH of the solution. Litmus paper, for instance, turns red in acidic solutions and blue in alkaline solutions. Universal indicator provides a range of colours across the entire pH scale, allowing for a more precise estimation of pH. A pH meter, however, offers the most accurate digital reading of the pH value. Understanding pH is fundamental for various applications, from maintaining proper bodily functions to industrial processes and environmental monitoring.