Chemical reactions Study Guide

Chemical reactions involve the rearrangement of atoms, not their creation or destruction. This fundamental principle is known as the Law of Conservation of Mass. It states that in any closed system, the mass of the reactants before a chemical reaction must be equal to the mass of the products after the reaction. This means that during a chemical change, atoms are conserved: the number of atoms of each element remains the same on both sides of a chemical equation. Ensuring a chemical equation reflects this conservation is called 'balancing' the equation.

To write a balanced chemical equation, you first need to know the correct chemical formulae for all the reactants (the starting materials) and products (the substances formed). For example, water is H₂O, not HO. Once you have the correct formulae, you write the unbalanced equation, placing reactants on the left side of an arrow and products on the right. Then, you adjust the coefficients (the numbers placed in front of chemical formulae) to make the number of atoms of each element equal on both sides. You must never change the small numbers within a chemical formula (subscripts), as this would change the identity of the compound. For instance, changing H₂O to H₃O would mean it is no longer water. Balancing an equation is like solving a puzzle, where you modify the quantities of entire molecules to achieve atomic equality. For example, to balance H₂ + O₂ → H₂O, you would write 2H₂ + O₂ → 2H₂O, ensuring four hydrogen atoms and two oxygen atoms on both sides.

Acids are a fundamental class of chemical compounds that exhibit specific properties when dissolved in water. The defining characteristic of an acid is its ability to release hydrogen ions (H⁺) into the solution. For instance, when hydrochloric acid (HCl) dissolves in water, it dissociates into hydrogen ions and chloride ions (HCl → H⁺ + Cl⁻). These hydrogen ions are responsible for the acidic properties we observe, such as tasting sour, corroding certain materials, and turning blue litmus paper red. The concentration of hydrogen ions in a solution dictates its acidity, with a higher concentration indicating a stronger acid and a lower pH value. It's important to remember that in aqueous solutions, hydrogen ions often associate with water molecules to form hydronium ions (H₃O⁺), but for simplicity, we generally refer to them as H⁺ ions. Alkalis, on the other hand, are soluble bases. When an alkali dissolves in water, it produces hydroxide ions (OH⁻). For example, sodium hydroxide (NaOH), a common alkali, dissociates into sodium ions and hydroxide ions in water (NaOH → Na⁺ + OH⁻). These hydroxide ions are responsible for the characteristic alkaline properties, such as a slippery feel, a bitter taste, and turning red litmus paper blue. The concentration of hydroxide ions is inversely related to the hydrogen ion concentration; as hydroxide ion concentration increases, hydrogen ion concentration decreases, leading to a higher pH and a more alkaline solution. Understanding the role of H⁺ and OH⁻ ions is crucial for comprehending acid-base reactions, such as neutralisation.