WJEC Chemistry A-level 1.1: Formulae and Equations Detailed Notes

Elements and Compounds

• An element is a substance consisting of one single type of atom that cannot be broken down into simpler substances in any chemical reaction.
• Elements are represented using chemical symbols.

Simple Compounds

• A compound is a substance that is made up of two or more types of atom that are chemically bound together.
• Simple compounds are expressed using formulae and can also be represented using a diagram and key, showing how the electrons are distributed and shared between the different atoms.

Ionic Compounds

• Ionic compounds are expressed using formulae that contain the ions that make up the compound.
• The overall compound will have a neutral charge (zero), meaning the charges of the separate ions must be balanced when writing the formula.

Oxidation Numbers

• Oxidation number gives the oxidation state of an element or ionic substance.
• Rules for allocating oxidation numbers:
  • Oxidation number of an element is zero.
  • Oxidation numbers in a neutral compound add up to zero.
  • Oxidation numbers in a charged compound add up to the total charge.
  • Hydrogen has an oxidation number of +1 (except in metal hydrides where it is -1).
  • Oxygen has an oxidation number of -2 (except in peroxides and F2O where it is -1).
  • All halogens have an oxidation number of -1.
  • Group I metals have an oxidation number of +1.

Chemical Reactions and Equations

• Reactions can be seen as a re-arrangement of the reactant atoms to produce various different products.
• The number of atoms must remain the same throughout, as physical matter cannot be created in the chemical reaction.
• Chemical equations must be balanced to be complete and can be represented using word equations or balanced chemical equations.
• State symbols can be included to help predict what observations might be made from a reaction.

Ionic Equations

• Ionic equations show chemical species as dissociated ions with individual charges.
• The total charge must remain the same during the reaction, so the total charge of the reactants equals the total charge of the products.
• State symbols should be used, as all reactants need to be in an aqueous state to be able to dissociate to ions.
• The best method for writing an ionic equation is to start with the full chemical equation for the reaction, then separate it into the dissociated ions before cancelling the ‘spectator ions’ and re-writing it as an ionic equation.