Formulae and Equations Chemistry AS Level- CCEA

the subscripts tell us the number of atoms of each kind in a compond

Example: NaCl has one atom of sodium and one atom of chlorine

H2SO4 has two atoms of hydrogen, one atom of sulfur and four atoms of oxygen

Ionic formulas are formed from positive cations and negative anions

Positive ions are formed by metals that lost electrons eg Na1+ Mg2+ Al

Negative ions are formed by nonmetals that have gained electrons eg Cl1- O2- N3- Si4-

the overall charge of ionic compounds must equal zero eg Na1+Cl1- Ca2+(Br1-)2

when naming the compound

• the positive ion is name first- this is iusually the metal
• the negative ion is then named
• if it is a simple ion the name will end in ide

complex ions are groups of ions that behave as one unit.

these group ions are treated like single ions in formulas eg (S04)= 2-

the complex ions are named based on the atoms that they contain. those with oxygen and another nonmetal aoften end in 'ate' with the root of the other nonmetal at the start

some complex ions contain a regular complex ion and one or more hydrogen ions. each hydrogen that is present add one positiv charge to the total charge

eg (SO4)2- becomes H(SO4)1-

(So3)2- = H(SO3)1-

these ion groups are called hydrogen ______ or bi______ eg H(CO3)1- is called hydrogen carbonate or bicarbonate

H(SO4)1- is called hydrogen sulfate or bisulfate.

Those polyatomic atoms with one oxygen less than 'ate' ions are 'ite' ions (SO3)2- = sulfite

(NO3)1- = Nitrate ion

(SO4)2- = sulfate ion

(CO3)2- = carbonate ion

(HCO3)1- = hydrogencarbonate ion

(OH)1- = hydroxide ion

(NH4)1+ =n ammonium ion

Transition metals can have several different positive charges. The charge can be determined by the roman numeral written next to the metals name eg Cu1+ = copper I Pb2+ = Lead II Fe3+ = Iron III Sn4+ = tin IV

these transition metals are used in formulas just like other metals once the charge is determined from the roman numeral in the name

A few transition metal ions only have one charge and never change so they can be written without a roman numeral in their formula name eg Ag1+ Zn2+ Cd2+

1) identify the oxidarion number for the element making up each half (positive and negative) of the compound

2) Use the oxidation number (without the plus or minus) for each half as the subscript of the other half

3) do not write a subscript of 1. reduce the subscripts if needed

4) after doing this, if both subscripts are divisible by the same number then reduce them to have the formula in its proper form eg Ca2O2 must be reduced to CaO

USE A TABLE (writye aroms underneath reactants and products)

• if they look the same on both sides of the equation,treat polyatomic ions (such as SO4 2-) as a group and balance them as such
• if 'OH' and H2O are in the equation write water as HOH
• Balance one compound at a time and rewrite the number of atoms in the chart as things change
• only add coefficients, NEVER change subscripts
• if H and O appear in more than one place,attempt to balance them last

• Balance everything that isnt H or O 1st
• Balance the H's second to last
• Balance O's last

An ionic equation shows only the ions taking part in the reaction and leaves out the ions that do not react

ionic compounds when dissolved in water disassociate completely into ions

spectator ions i.e ions that do not take part in the reaction are omitted on both sides of the ionic equation

Step 1) write word equation and chemical formula

silver nitrate + sodium chloride --> silver chloride + sodium nitrate

AgNo3 (aq) + NaCl (aq) --> AgCl (s) + NaNO3 (aq)

Step 2) write formula of ions formed  AgNo3 (aq) + NaCl (aq) ---> AgCl (s) + NaNO3 (aq)

Ag+ (aq) + NO3- (aq) + Na+ (aq) + Cl- (aq) --> AgCl (s) + Na+ (aq) + NO3- (aq)

Step 3) Omit spectator ions Na+ and Cl- appear both sides. Do not take part in the reaction. Can be cancelled

Na+ (aq) + OH- (aq) + H+ (aq) + Cl- (aq) --> Na+ (aq) + Cl- (aq) + H2O (l)

H+(aq) + OH- (aq) --> H2O (l)