Chemical reactions Study Guide

In chemistry, atoms and molecules are incredibly small, making it impractical to count them individually. To deal with the vast numbers of particles involved in chemical reactions, chemists use a unit called the <b>mole</b>. The mole is a fundamental SI unit that represents a specific quantity of a substance. One mole of any substance contains exactly the same number of particles. This number is known as the <b>Avogadro constant</b>, symbolised as N_A.

The value of the Avogadro constant is approximately 6.02 x 10²³ per mole. This means that if you have one mole of carbon atoms, you have 6.02 x 10²³ carbon atoms. Similarly, one mole of water molecules contains 6.02 x 10²³ water molecules. The 'particles' can be atoms, molecules, ions, or even formula units, depending on the substance.

The concept of the mole links microscopic particles to macroscopic quantities that can be measured in a lab. For example, the molar mass of an element or compound is the mass in grams of one mole of that substance. For elements, the molar mass in grams is numerically equal to its relative atomic mass (Ar). For compounds, the molar mass in grams is numerically equal to its relative formula mass (Mr). So, if the relative atomic mass of carbon is 12, then one mole of carbon atoms has a mass of 12 grams. This relationship allows chemists to convert between the mass of a substance and the number of moles, and subsequently, the number of particles present, which is crucial for stoichiometry and understanding chemical reactions. Being able to calculate using the Avogadro constant and the mole definition is essential for higher tier chemistry.

Neutralisation is a fundamental chemical reaction where an acid reacts with a base to form a salt and water. In the context of aqueous solutions, this reaction can be generalised to the specific interaction between hydrogen ions (H⁺) from the acid and hydroxide ions (OH⁻) from the base. Acids are substances that release hydrogen ions (H⁺) when dissolved in water, while bases (specifically alkalis, which are soluble bases) release hydroxide ions (OH⁻) in water. When these two ionic species encounter each other in an aqueous solution, they combine to form water molecules (H₂O). This is the essential ionic equation for all neutralisation reactions involving aqueous acids and bases: H⁺(aq) + OH⁻(aq) → H₂O(l). The remaining ions from the acid and the base then combine to form a salt. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the H⁺ ions from HCl react with the OH⁻ ions from NaOH to form water, and the remaining Na⁺ and Cl⁻ ions form sodium chloride (NaCl), which is the salt produced. This simplified ionic equation helps to highlight the core chemical event occurring during neutralisation, irrespective of the specific acid and base used, as long as they are in aqueous solutions.