chemistry unit 1.1 - formulae and equations

Element

A substance consisting of one single type of atom

Simple compounds

A substance made up of two or more types of atoms chemically bound together

simple compounds expression

expressed using formulae, for example: CaCO₃

ionic compounds overall charge

overall, the compound will have a neutral charge (zero), meaning the charges of the separate ion must be balanced.

ionic compounds expression

expressed using formulae that contain the ions that make up the compound

oxidation numbers

gives the oxidation state of element/ionic substance.

rules for assigning oxidation states - elements

oxidation number of an element is zero

rules for assigning oxidation states - neutral compound

oxidation numbers in a neutral compound add up to zero

rules for assigning oxidation states 3 - hydrogen

hydrogen is nearly always +1 except in metal hydrides (e.g. NaH) where it is -1

rules for assigning oxidation states 4 - oxygen

oxygen is nearly always -2 except in peroxides (e.g. H₂O₂) and F₂O where it is -1

rules for assigning oxidation states 5 - halogens

all halogens have an oxidation number of -1

rules for assigning oxidation states 6 - group 1 metals

all group 1 metals have an oxidation number of +1

rules for assigning oxidation states 7 - simple ions

on simple ions, the oxidation state is the charge on the ion

rules for assigning oxidation states 8 - molecules and complex ions

in molecules and complex ions, the more electronegative element is assumed to be the negative ion

EXAMPLE: work out the oxidation number of sulfur in: Na₂SO₄

2-8+X = 0

-6+X=0

X=+6

EXAMPLE: balance this equation:

NaOH+H₂SO₄—> Na₂SO₄+H₂O

2NaOH+H₂SO₄—> Na₂SO₄+2H₂O

writing ionic equations rules:

• the total charge must remain during the reaction

• use state symbols

• the number of species must balance throughout

• all reactants need to be in aqueous state to dissociate ions

EXAMPLE:

Na₂CO_3(aq)+2HCl_(aq)—> 2NaCl_(aq)+H₂O_(l)+CO_2(g)

Na₂CO_3(aq)+2HCl_(aq)—> 2NaCl_(aq)+H₂O_(l)+CO_2(g)

rewrite the equation as individual ions:

2Na⁺_(aq)+CO₃^2-_(aq)+2H⁺_(aq)+2Cl^-_(aq) —> 2Na⁺_(aq)+2Cl^-_(aq)+H₂O_(l)+CO_2(g)

this turns into:

CO₃^2-_(aq)+2H⁺_(aq)—>H₂O_(l)+CO_2(g)

this is the overall ionic equation