WJEC Chemistry for AS Level Student Book: 2nd Edition

numbers in accordance with certain rules, given in the following table. Rule Example The oxidation number of an uncombined element is zero. Metallic copper, Cu: oxidation number 0 Oxygen gas, O 2 : oxidation number 0. The sum of the oxidation numbers in a compound is zero. In an ion the sum equals the overall charge. In CO 2 the sum of the oxidation numbers of carbon and oxygen is 0. In NO 3 − the sum of the oxidation numbers of nitrogen and oxygen is −1. In compounds the oxidation numbers of Group 1 metals is +1 and Group 2 metals is +2. In MgBr 2 the oxidation number of magnesium is +2 (oxidation number of each bromine is −1). The oxidation number of oxygen is −2 in compounds except with uorine or in peroxides (and superoxides). In SO 2 the oxidation number of each oxygen is −2 (oxidation number of sulfur is +4). In H 2 O 2 the oxidation number of oxygen is −1 (oxidation number of hydrogen is +1). The oxidation number of hydrogen is +1 in compounds except in metal hydrides. In HCl the oxidation number of hydrogen is +1 (oxidation number of chlorine is −1). In NaH the oxidation number of hydrogen is −1 (oxidation number of sodium is +1). In chemical species with atoms of more than one element, the most electronegative element is given the negative oxidation number. In CCl 4 , chlorine is more electronegative than carbon, so the oxidation number of each chlorine is −1 (oxidation number of carbon is +4). Oxidation numbers are used in redox reactions (reactions where both red uction and ox idation take place) to show which species is oxidised and which one is reduced. If the oxidation number of a species increases, it is oxidised; if the oxidation number decreases, it is reduced. Oxidation numbers are used to name compounds unambiguously, e.g. potassium, nitrogen and oxygen can combine to give two different compounds, KNO 3 and KNO 2 . Since the oxidation number of potassium is +1 and that of oxygen is −2, the oxidation number of nitrogen must be +5 in KNO 3 and +3 in KNO 2 . Therefore KNO 3 is called potassium nitrate(V) and KNO 2 is called potassium nitrate(III). Chemical and ionic equations Chemical equations are written to sum up what happens in a chemical reaction. Since atoms are neither created nor destroyed in a chemical reaction, there must be the same number of atoms of each element on each side of the chemical equation. The steps in writing a balanced chemical equation are: 1. Write a word equation for the reaction (optional). 2. Write the symbols and formulae for the reactants and products (make sure that all formulae are correct). 3. Balance the equation by multiplying formulae if necessary (never change a formula). 4. Check your answer. 5. Add state symbols (if required). The state symbols used are: (s) for solid, (l) for liquid, (g) for gas. A solution in water is described as aqueous, so (aq) is used for a solution. Exam tip Always write an oxidation number with the sign of the charge rst, followed by the number, e.g. the oxidation number of sulfur in SO 4 2– is +6. Writing 6+ is incorrect since an S 6+ ion does not exist. Exam tip All the elements in the list HOFBr INCl are diatomic molecules. You could create your own mnemonic to remember them in the exam. Link Electronegativity page 52 Oxidation and reduction page 67 Knowledge check What is the oxidation number of: (a) nitrogen in NH 3 (b) phosphorus in P 4 (c) manganese in MnO 4 – (d) chromium in K 2 Cr 2 O 7 ? 5 Knowledge check Balance the following equations (a) SO 2 + O 2 A SO 3 (b) Fe 2 O 3 + CO A Fe + CO 2 (c) Al + HCl A AlCl 3 + H 2 (d) HI + H 2 SO 4 A I 2 + H 2 O + H 2 S 6 1.1 Formulae and equations 13