WJEC Chemistry for AS Level Student Book: 2nd Edition

You should have covered formulae and equations at GCSE level, but this section gives a recap on the minimum knowledge required at AS level. It also includes a section on oxidation numbers which will be new to you. Formulae of compounds and ions The formula of a compound is a set of symbols and numbers. The symbols say what elements are present and the numbers give the ratio of the numbers of atoms of the different elements in the compound. The compound carbon dioxide has the formula CO 2 . It contains two oxygen atoms for every carbon atom. The formula for ethanol is C 2 H 5 OH. It contains two carbon atoms and six hydrogen atoms for every oxygen atom. These compounds consist of molecules in which the atoms are bonded covalently. To show two molecules you write 2C 2 H 5 OH. The 2 in front of a formula multiplies everything after it. Therefore, in 2C 2 H 5 OH, there are 4C, 12H and 2O atoms, a total of 18 atoms. For advanced level chemistry you will need to know the formulae of a wide range of compounds. The table below gives a list of the formulae of some common compounds. Since many compounds do not consist of molecules but consist of ions and form through ionic bonding, the list contains both ionic and covalent compounds. Name Formula Name Formula Water H 2 O Sodium hydroxide NaOH Carbon dioxide CO 2 Sodium chloride NaCl Sulfur dioxide SO 2 Sodium carbonate Na 2 CO 3 Methane CH 4 Sodium hydrogencarbonate NaHCO 3 Hydrochloric acid HCl Sodium sulfate Na 2 SO 4 Sulfuric acid H 2 SO 4 Copper(II) oxide CuO Nitric acid HNO 3 Copper(II) sulfate CuSO 4 Ethanoic acid CH 3 COOH Calcium hydroxide Ca(OH) 2 Ammonia NH 3 Calcium carbonate CaCO 3 Ammonium chloride NH 4 Cl Calcium chloride CaCl 2 The compound calcium chloride is composed of calcium ions, Ca 2+ , and chloride ions, Cl − . There are twice as many chloride ions as calcium ions, so the formula is CaCl 2 . This is not a molecule of calcium chloride but a formula unit of calcium chloride. For an ionic compound the total number of positive charges must equal the total number of negative charges in one formula unit of the compound. The table below gives the formulae for common ions that you need to learn. Positive ions Negative ions Name Formula Name Formula Ammonium NH 4 + Bromide Br − Hydrogen H + Chloride Cl − Lithium Li + Fluoride F − Potassium K + Iodide I − Sodium Na + Hydrogencarbonate HCO 3 − Silver Ag + Hydroxide OH − Barium Ba 2+ Nitrate NO 3 − Calcium Ca 2+ Oxide O 2− Magnesium Mg 2+ Sul de S 2− Copper(II) Cu 2+ Carbonate CO 3 2− Iron(II) Fe 2+ Sulfate SO 4 2− Iron(III) Fe 3+ Phosphate PO 4 3− Aluminium Al 3+ Note that non-metals change to end in -ide, but if non-metals combine with oxygen to form negative ions, the negative ion starts with the non-metal and ends in -ate (apart from hydroxide). Link Covalent and ionic bonding pages 50–51 Knowledge check How many atoms of each element are present in: (a) P 4 O 10 (b) 2Al(OH) 3 ? 1 Knowledge check How many oxygen atoms are present in 3Fe(NO 3 ) 3 ? 2 Knowledge check Name the following compounds: (a) Na 2 SO 4 (b) Ca(HCO 3 ) 2 (c) CuCl 2 3 1.1 Formulae and equations 11