Naming Compounds and Writing Chemical Equations

Atomic structure and the periodic table • Atomic models and isotopes

Flashcards

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What state symbols may appear in symbol equations?

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State symbols include (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous solution.

Key concepts

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Chemical symbols and simple formulae

Element symbols consist of one or two letters derived from the element name. Chemical formulae indicate the number and type of atoms in a particle. Subscripts show the number of atoms of an element in a molecule or formula unit (for example, H2O, CO2). The absence of a subscript implies one atom of that element. Ionic compounds use formulae that reflect the simplest ratio of ions necessary to balance overall charge.

Formation and naming of ionic compounds

Metals lose electrons to form positive ions (cations); non-metals gain electrons to form negative ions (anions). Opposite charges attract and produce ionic compounds in ratios that make the total charge zero. The name of an ionic compound places the metal (cation) name first and the non-metal (anion) name second, with the anion ending changed to -ide for simple anions (for example, sodium chloride from Na+ and Cl-). Transition metals that form more than one ion use Roman numerals in names to indicate their charge (for example, iron(II) chloride for FeCl2).

Naming covalent molecules

Covalent molecules form when non-metal atoms share electrons. Prefixes indicate the number of atoms of each element in simple covalent molecules: mono- (1), di- (2), tri- (3), tetra- (4), etc. The more electronegative element appears last and uses the -ide ending. The prefix mono- is usually omitted for the first element (for example, carbon dioxide for CO2 and sulfur dioxide for SO2).

Word equations, formula equations and symbol equations

Word equations describe reactions using names of substances (for example, magnesium + oxygen → magnesium oxide). Formula equations replace names with chemical formulae (for example, 2Mg + O2 → 2MgO). Symbol equations include formulae and, where appropriate, state symbols (s, l, g, aq). Balanced symbol equations use coefficients to ensure the same number of atoms of each element appear on both sides of the arrow, which preserves mass and follows the conservation of atoms.

Steps to write and balance equations

Identify reactants and products and write their correct formulae according to bonding types and ion charges. Count atoms of each element on both sides of the tentative equation. Adjust coefficients (whole numbers placed before formulae) to balance atom counts. Recheck each element and simplify coefficients if possible. Never change subscripts in chemical formulae to balance an equation, because subscripts change the identity of the compounds.

Common reaction types and example equations

Combination reactions join elements or simple compounds to form a single product (for example, 2Mg + O2 → 2MgO). Combustion of hydrocarbons produces carbon dioxide and water (for example, CH4 + 2O2 → CO2 + 2H2O). Acid–base neutralisation produces a salt and water (for example, HCl + NaOH → NaCl + H2O). Displacement reactions occur when a more reactive metal displaces a less reactive metal from a compound (for example, Zn + CuSO4 → ZnSO4 + Cu).

Key notes

Important points to keep in mind

Always identify whether bonding is ionic or covalent before writing formulae.

Balance equations using coefficients, not by changing subscripts.

Write ions with correct charges to deduce ionic formulae by charge cancellation.

Use -ide for simple anions and use established names for common polyatomic ions.

Include state symbols when required to show the physical state of reagents and products.

Check that total atoms of each element match on both sides after balancing.

Oxygen normally appears as O2 in its elemental gaseous form when written in equations.

Simplify coefficients to the smallest whole-number ratio once the equation is balanced.