Master Chemistry Unit 1: Formulae and Equations – WJEC AS

Common Compounds and Their Formulae

You'll need to memorise these essential chemical formulae - they appear in nearly every chemistry exam! Water (H₂O), oxygen (O₂), and carbon dioxide (CO₂) are the absolute basics you must know.

The acids are particularly important: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). These three pop up constantly in reactions and calculations.

Don't forget the common bases and compounds like sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), and ammonia (NH₃). Getting these right from the start will make everything else much easier.

Quick Tip: Make flashcards for these formulae - you'll thank yourself later when they become second nature!

Understanding Ions

Positive ions (cations) form when atoms lose electrons, whilst negative ions (anions) form when atoms gain electrons. It's that simple! Metals typically become cations, and non-metals become anions.

There's a brilliant shortcut for working out ion charges using the periodic table. Group 1 elements have a +1 charge, Group 2 have +2, Group 6 have -2, and Group 7 have -1 charge.

For example, sodium (Na) from Group 1 becomes Na⁺, whilst chlorine (Cl) from Group 7 becomes Cl⁻. This pattern works because atoms want to achieve a stable electron arrangement.

Memory Trick: Remember that metals "give away" electrons (become positive), whilst non-metals "take" electrons (become negative).

Types of Ions and Conservation of Mass

Simple ions contain just one element (like H⁺ or O²⁻), whilst complex ions contain multiple elements bonded together (like NH₄⁺ or SO₄²⁻). Complex ions act as single units in reactions.

The conservation of mass principle is crucial - the total mass of reactants always equals the total mass of products. In the reaction CH₄ + 2O₂ → CO₂ + 2H₂O, both sides add up to 80g.

State symbols show the physical state of each substance: $s$ for solid, $l$ for liquid, $g$ for gas, and (aq) for dissolved in water. Only include these if the question specifically asks for them.

Exam Tip: Always check your balanced equations by counting atoms on both sides - they must be equal!

Ionic Equations and Reactions

Ionic equations show only the ions that actually react, ignoring the "spectator ions" that don't change. Both sides must have the same overall charge - this is your key check.

In the reaction NaI(aq) + AgNO₃(aq) → AgI$s$ + NaNO₃(aq), the ionic equation becomes: I⁻(aq) + Ag⁺(aq) → AgI$s$. The Na⁺ and NO₃⁻ are spectators.

Spectator ions remain unchanged throughout the reaction and can be cancelled out from both sides. They're like bystanders watching the real action happen.

Pro Tip: Look for the precipitate (solid formed) - this usually shows you which ions are actually reacting!

Redox Reactions Basics

Redox reactions involve the transfer of electrons between substances. One substance gets oxidised (loses electrons) whilst another gets reduced (gains electrons) - they always happen together.

Oxidation means losing electrons, gaining oxygen, or losing hydrogen. Reduction means gaining electrons, losing oxygen, or gaining hydrogen. The classic memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

An oxidising agent causes oxidation in other substances by accepting their electrons, but gets reduced itself. Think of it as the electron "thief" in the reaction.

Memory Aid: Remember OIL RIG - it's the most reliable way to identify what's happening in redox reactions!

Oxidation Numbers and Common Ions

A reducing agent donates electrons to other substances, causing them to be reduced, but gets oxidised itself. It's the electron "donor" in reactions.

Oxidation numbers follow simple rules: elements alone are zero, simple ions equal their charge, and all oxidation numbers in a compound must add up to zero (or the overall charge for complex ions).

Key ions you must memorise include: H⁺ (hydrogen), Ag⁺ (silver), Zn²⁺ (zinc), OH⁻ (hydroxide), NO₃⁻ (nitrate), and SO₄²⁻ (sulfate). These appear constantly in equations.

Quick Check: In H₂O, hydrogen is +1 and oxygen is -2, giving 2$+1$ + $-2$ = 0. Perfect!

Essential Ion Table

The ammonium ion (NH₄⁺) and carbonate ion (CO₃²⁻) complete your essential ion collection. These complex ions behave as single units in all reactions.

Master these common ions and you'll find writing chemical equations becomes much more straightforward. They're the building blocks for countless reactions you'll encounter.

Final Tip: Practice writing formulae for compounds using these ions - it's the best way to make them stick in your memory!