A Level Chemistry WJEC

Formulae and Equations

General Concepts

• Chemical formulas are a shorthand way of describing a chemical compound using atomic symbols and numerical subscripts.
• The molecular formula gives the exact number of atoms of each element in a molecule, while the empirical formula gives the simplest ratio of atoms in a compound.
• Structural formulas show the arrangement of atoms in a molecule.

Balancing Equations

• Chemical equations represent the changes that occur during a chemical reaction.
• A balanced chemical equation obeys the law of conservation of mass, showing that the number of atoms of each element are the same on both sides of the equation.
• Balancing equations involves adjusting the coefficients of the reactants and products until the number of atoms of each element is balanced.
• Reactants are the substances that react in a chemical reaction and appear on the left side of the equation, while products are the substances that are formed and appear on the right side.

State Symbols in Equations

• State symbols are used in chemical equations to indicate the physical state of each reactant and product: (s) for solid, (l) for liquid, (g) for gas, and (aq) for substances dissolved in water (aqueous).

Ions and Ionic Equations

• An ion is an atom or group of atoms that has lost or gained electrons and so has an electric charge.
• The charge on an ion is shown after the atomic symbol: e.g., Na⁺, Cl^-, OH^-.
• Ionic equations show only the ions that change in a reaction. Substances not changed by the reaction, known as spectator ions, are not included.

Moles and Stoichiometry

• The mole is a unit for amount of substance. One mole of a substance contains the same number of entities (such as atoms, molecules, or ions) as there are in 12 grams of carbon-12.
• Stoichiometry is the study of the quantitative relationships in a chemical reaction, determining the proportions in which elements or compounds react.

Chemical Reactions

• Chemical reactions may be categorized as combination, decomposition, displacement, or exchange reactions.
• In combination reactions, two or more substances react to form one product.
• In decomposition reactions, one substance breaks down into two or more substances.
• In displacement reactions, one element is replaced by another in an compound.
• In exchange reactions, the positive ions in the reacting compounds switch places.