Chemical reactions Study Guide

Neutralisation is a fundamental chemical reaction where an acid reacts with an alkali (a soluble base) or a base (an insoluble metal oxide or hydroxide) to form a salt and water. This reaction essentially 'neutralises' the acidic and alkaline properties of the reactants. The defining characteristic of neutralisation is the combination of hydrogen ions (H⁺) from the acid with hydroxide ions (OH⁻) from the alkali or base to form water (H₂O). For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the products are sodium chloride (NaCl), which is a salt, and water (H₂O). The overall ionic equation for this process is H⁺(aq) + OH⁻(aq) → H₂O(l).

Acids are substances that produce H⁺ ions in solution, making the pH value less than 7. Alkalis and bases accept H⁺ ions; alkalis produce OH⁻ ions in solution, making the pH value greater than 7. When these react, the pH of the resulting solution moves towards 7, indicating a neutral solution. The specific salt formed depends on the acid and alkali/base used. For instance, sulfuric acid reacting with potassium hydroxide produces potassium sulfate and water. This concept is crucial in many chemical processes, including waste treatment, agriculture (adjusting soil pH), and in everyday life, such as using indigestion tablets (which contain bases) to neutralise excess stomach acid.

Chemical reactions involve changes in energy, specifically the breaking and forming of chemical bonds. When bonds are broken, energy is absorbed from the surroundings. When new bonds are formed, energy is released to the surroundings. The overall energy change determines if a reaction is exothermic or endothermic. Reaction profiles (also known as energy diagrams) are graphical representations that illustrate these energy changes during a chemical reaction.

An exothermic reaction is one that releases energy to the surroundings, usually in the form of heat, leading to a temperature increase in the surroundings. In an exothermic reaction profile, the energy of the products is lower than the energy of the reactants. This difference in energy is the overall energy released. The activation energy is the minimum amount of energy required to start the reaction, represented as a hump on the profile, from the reactant energy level to the peak of the curve.

An endothermic reaction is one that absorbs energy from the surroundings, usually as heat, causing the surroundings to cool down. For an endothermic reaction, the energy of the products is higher than the energy of the reactants. The difference is the overall energy absorbed. Like exothermic reactions, endothermic reactions also have an activation energy, which is the energy needed to initiate the reaction, represented by the hump from the reactant energy level to the peak.

Key features to label on a reaction profile include:

• Reactants: The starting molecules in the reaction.
• Products: The molecules formed at the end of the reaction.
• Energy/Enthalpy: The vertical axis representing the energy content.
• Reaction Pathway/Progress of Reaction: The horizontal axis showing the progression from reactants to products.
• Activation Energy (Ea): The energy difference between the reactants and the transition state (the peak of the hump). This is the energy barrier that must be overcome for the reaction to occur.
• Overall Energy Change (ΔH): The energy difference between the reactants and the products. For exothermic reactions, ΔH is negative (energy released); for endothermic reactions, ΔH is positive (energy absorbed).