WRITING CHEMICAL FORMULA
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1 WRITING CHEMICAL FORMULA For ionic compounds, the chemical formula must be worked out. You will no longer have the list of ions in the exam (like at GCSE). Instead you must learn some and work out others. Working out charges on ions Ionic compounds contain metals and non-metals. Metal atoms (under staircase) lose electrons during a reaction and will ALWAYS form positive ions. Non-metal atoms (above the stair case) gain electrons during a reaction and will ALWAYS form negative ions. Group number in the periodic table can therefore be used to determine the charge on single ions: Group 1 elements (1 st column) = 1+ ions Group 2 elements (2 nd column) = 2+ ions Metals Group 6 elements (6 th column) = 1- ions Group 7 elements (7 th column) = 2- ions Non-metals Ions to LEARN Some ions exist as covalent or polyatomic ions (bonding between atoms within these ions is covalent and the charge is overall) and these just have to be learnt! Below is a list of some of the most common polyatomic ions: ammonium - NH4 + hydroxide - OH - chlorate - ClO3 - nitrate - NO3 - cyanide - CN - nitrite - NO2 - carbonate - CO3 2- manganate - MnO4 - chromate - CrO4 2- peroxide - O2 2- dichromate - Cr2O7 2- phosphate - PO4 3- hydrogen carbonate - HCO3 - sulfate - SO4 2- hydrogen phosphate - HPO4 2- sulfite - SO3 2- hydrogen sulfate - HSO4 - thiosulfate - S2O3 2-
2 Working out chemical formula Charges must cancel out when working out the overall formula. For example: 1. Sodium Chloride: Sodium = group 1 metal = Na + Chloride = group 7 non-metal = Cl - Formula: Na + + Cl - = NaCl Charges are equal and opposite so cancel out (ratio = 1 Na + : 1 Cl - ) 2. Magnesium Bromide: Magnesium = group 2 metal = Mg 2+ Bromide = group 7 non-metal = Br - Formula: Mg Br - = MgCl2 Charges DO NOT cancel out with one of each need 2 x Br to cancel out Mg 2+ (ratio = 1 Mg 2+ : 2Br - ) 3. Calcium Carbonate: Calcium = group 2 metal = Ca 2+ Carbonate = polyatomic ion (from list) = CO3 2- Formula: Ca 2+ + CO3 2- = CaCO3 Charges are equal and opposite so cancel out (ratio = 1 Ca 2+ : 1 CO3 2- ) 4. Ammonium sulphate: Ammonium = polyatomic ion (from list) = NH4 + Sulphate = polyatomic ion (from list) = SO4 2- Formula: 2NH4 + + SO4 2- = (NH4 + )2SO4 2- Charges DO NOT cancel out with one of each need 2 x NH4 + to cancel out SO4 2- (ratio = 2 NH4 + : 1 SO4 2- ) NOTE: when you need more than one polyatomic ion you need to use brackets.
3 Substance STRUCTURE, FORMULA AND EQUATIONS Monatomic Simple molecular Giant covalent Ionic Metallic Elements: Compounds: Elements: one metal Group 0 metal and non-metal Elements: H 2, O 2, N 2, F 2, Cl 2, Br 2, I 2, At 2, S 8, P 4 Compounds: Different non-metals bonded Elements: Silicon, carbon (diamond and graphite) Compounds: SiO 2 Structure Individual atoms with very weak forces between them Individual molecules with weak forces between them (atoms within molecules are joined by covalent bonds) Lattice structure in which all atoms are joined to other atoms by covalent bonds Lattice structure of positive and negatively charged ions (held together by attraction between the oppositely charged ions - an Lattice structure of metal ions with outer shell electrons free to move (delocalised) through the structure ionic bond) Particles Atoms Molecules Atoms Ions Ions & delocalised electrons Formula Just the symbol e.g. Ar Find from the periodic table e.g. H 2O, NH 3, CO 2, CH 4 example Elements: just the symbol Compound: ratio of atoms e.g. SiO 2, C, Si example e.g. MgCl 2 ratio of Mg 2+ : Cl - ions is 1:2 through the structure ions (see below) and work out the formula Just the symbol e.g. Fe Find from the periodic table There are some reactions from GCSE that you use at A-level. Some of these include: 1. Metal + oxygen Metal oxide 2. Hydrocarbon + oxygen carbon dioxide + water 3. Metal + acid salt + hydrogen 4. Base + acid salt + water 5. Alkali + acid salt + water 6. Metal carbonate + acid salt + water + carbon dioxide
4 TASKS 1: Give the formula of the following substances, showing working out: a) aluminium nitrate d) Potassium manganate b) vanadium(v) oxide e) Magnesium bromide c) copper(ii) hydroxide f) potassium dichromate TASK 2: For each of the following substances, identify the structure type of particles it contains and write the formula. Substance Formula Structure Type Particles sodium hydroxide ammonia ammonium chloride calcium bromide silicon dioxide silver nitrate magnesium oxide propane copper sulphate zinc phosphate EXTENSION: write down some properties of each type of structure.
5 TASK 3: Complete the following word equations and write balanced equations for each reaction. a) Aluminium + oxygen b) Nitric acid + potassium carbonate c) Ethane + oxygen d) Hydrochloric acid + magnesium e) Copper oxide + sulphuric acid f) Potassium hydroxide + hydrochloric acid EXTENSION: write ionic equations for the above reaction
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