Ionic formula
Summary of most important ions to know How to work out the formula from the ionic charge What is the formula of Lithium Sulphate? 1. Identify the ionic charges of the two ions Lithium is in group 1 so has a +1 charge, Li+ sulphates have a -2 charge, SO4 2- 2. Combine the ions together to get a neutral compound. i.e. combine so that the total +ve charge cancels out the total –ve charge Li + SO4 Li+ 2- We need two lithium ions to cancel out the -2 charge on the sulphate The formula is therefore Li2SO4 What is the formula of Calcium phosphate? 1. Identify the ionic charges of the two ions Calcium is in group 2 so has a +2 charge, Ca2+ Phosphates have a -3 charge, PO4 3- 2. Combine the ions together to get a neutral compound. i.e. combine so that the total +ve charge cancels out the total –ve charge We need to multiply up to get the same charge. Three calcium ions would produce +6 charge to cancel out the -6 charge on two phosphate ions The formula is therefore Ca3 (PO4 )2 Only use brackets when there is more than one of the compound ion in the formula. e.g. Copper nitrate is Cu(NO3 )2 Calcium hydroxide is Ca(OH)2 Ammonium sulphate is (NH4 )2SO4 2. +1 +2 +3 -3 -2 -1 Group 1 Hydrogen Silver Gold Ammonium (NH4+ ) Group 2 Zinc Copper (II) Iron (II) Tin Lead Group 3 Iron (III) Group 5 Phosphate (PO4 3-) Group 6 Carbonate (CO3 2-) Sulphate (SO4 2-) Group 7 Nitrate (NO3 -) Hydroxide (OH-)
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Formulae and equations (a) the writing of formulae of ionic compounds from ionic charges, including: (i) prediction of ionic charge from the position of an element in the periodic table (ii) recall of the names and formulae for the following ions: NO3 –, CO3 2–, SO4 2–, OH–, NH4 +, Zn2+ and Ag+ Note that ‘nitrate’ and ‘sulfate’ should be assumed to be NO3 – and SO4 2–. Charges on ions other than in (i) and (ii) will be provided.
Full equations
There must be the same number of atoms belonging to each element on each side of the equation. For example,
C+O2 → CO2
On the left there is one atom of carbon (C) and two atoms in a molecule of oxygen (O2). This must balance with the right: CO2 is one atom of carbon combined with two atoms of oxygen. If the equation is not balanced, only big numbers in front of various chemicals (ie in the spaces) to balance the equation. For example,
__Fe2O3 +__C →__Fe + __CO2
Numbers in subscript (ie 2 and 3 in Fe2O3) cannot be changed because during so will change the identity of the chemical (ie Fe2O3 is different from FeO).
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2.1.2 Compounds, formulae and equations
(b) construction of balanced chemical equations (including ionic equations), including state symbols, for reactions studied and for unfamiliar reactions given appropriate information.
Ionic equations
Writing Ionic equations We usually write ionic equations to show the key changes in a reaction. Ionic equations only show the ions that are reacting and leave out spectator ions. Spectator ions are ions that are not • Not changing state • Not changing oxidation number Pb(NO3 )2 (aq) + 2NaCl (aq) PbCl2 (s) + 2 NaNO3 Take full equation (aq) Separate (aq) solutions into ions Pb2+ (aq) + 2NO3 – (aq) + 2Na+ (aq)+ 2Cl- (aq) PbCl2 (s) + 2 Na+ (aq)+ 2NO3 – (aq) Cancel out spectator ions leaving ionic equation Pb2+ (aq) + 2Cl- (aq) PbCl2 (s)
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2.1.2 Compounds, formulae and equations
(b) construction of balanced chemical equations (including ionic equations), including state symbols, for reactions studied and for unfamiliar reactions given appropriate information.
Credits: Neil Goalby