oxidation is the process of electron loss: Zn Zn2+ + 2eIt involves an increase in oxidation number reduction is the process of electron gain: Cl2 + 2e- 2ClIt involves a decrease in oxidation number Rules for assigning oxidation numbers 1. All uncombined elements have an oxidation number of zero eg . Zn, Cl2, O2, Ar all have oxidation numbers of zero 2. The oxidation numbers of the elements in a compound add up to zero In NaCl Na= +1 Cl= -1 Sum = +1 -1 = 0 3. The oxidation number of a monoatomic ion is equal to the ionic charge e.g. Zn2+ = +2 Cl- = -1 4. In a polyatomic ion (CO3 2-) the sum of the individual oxidation numbers of the elements adds up to the charge on the ion e.g. in CO3 2- C = +4 and O = -2 sum = +4 + (3 x -2) = -2 5. Several elements have invariable oxidation numbers in their common compounds. Group 1 metals = +1 Group 2 metals = +2 Al = +3 H = +1 (except in metal hydrides where it is –1 eg NaH) F = -1 Cl, Br, I = –1 except in compounds with oxygen and fluorine O = -2 except in peroxides (H2O2 ) where it is –1 and in compounds with fluorine. We use these rules to identify the oxidation numbers of elements that have variable oxidation numbers. Note the oxidation number of Cl in CaCl2 = -1 and not -2 because there are two Cl’s Always work out the oxidation for one atom of the element What is the oxidation number of Fe in FeCl3 Using rule 5, Cl has an O.N. of –1 Using rule 2, the O.N. of the elements must add up to 0 Fe must have an O.N. of +3 in order to cancel out 3 x –1 = -3 of the Cl’s N Goalby chemrevise.org Naming using Roman Numerals In IUPAC convention the various forms of sulphur, nitrogen and chlorine compounds where oxygen is combined are all called sulfates, nitrates and chlorates with relevant oxidation number given in roman numerals. If asked to name these compounds remember to add the oxidation number. NaClO: sodium chlorate(I) NaClO3 : sodium chlorate(V) K2SO4 potassium sulfate(VI) K2SO3 potassium sulfate(IV) Use a Roman numeral to indicate the magnitude of the oxidation state of an element, when a name may be ambiguous. NaNO2 : Sodium nitrate(III) NaNO3 : Sodium nitrate(V) FeCl2 Iron(II) Chloride FeCl3 Iron(III) Chloride
2.1.5 Redox
Oxidation number
(a) rules for assigning and calculating oxidation number for atoms in elements, compounds and ions Learners will be expected to know oxidation numbers of O in peroxides and H in metal hydrides.
(b) writing formulae using oxidation numbers HSW8 Appropriate use of oxidation numbers in written communication.
(c) use of a Roman numeral to indicate the magnitude of the oxidation number when an element may have compounds/ions with different oxidation numbers Examples should include, but not be limited to, iron(II) and iron(III). Learners will be expected to write formulae from names such as chlorate(I) and chlorate(III) and vice versa. Note that ‘nitrate’ and ‘sulfate’, with no shown oxidation number, are assumed to be NO3 – and SO4 2–. HSW8 Systematic and unambiguous nomenclature
Redox Reactions metals generally form ions by losing electrons with an increase in oxidation number to form positive ions: Zn Zn2+ + 2enon-metals generally react by gaining electrons with a decrease in oxidation number to form negative ions Cl2 + 2e- 2Cl- 4Li + O2 2Li2O 0 0 +1 -2 Lithium is oxidising because its oxidation number is increasing from 0 to +1 Oxygen is reducing because its oxidation number is decreasing from 0 to -2 WO3 + 3H2 W + 3H2O 0 0 +1 +6 Hydrogen is oxidising because its oxidation number is increasing from 0 to +1 Tungsten is reducing because its oxidation number is decreasing from +6 to 0 2Sr(NO3 )2 2SrO + 4NO2 + O2 -2 +4 0 +5 Oxygen is oxidising because its oxidation number is increasing from -2 to 0 Nitrogen is reducing because its oxidation number is decreasing from +5 to+4 Redox Reactions of Metals and acid 2HCl + Mg MgCl2 +H2 ACID + METAL SALT + HYDROGEN 0 0 +2 +1 Magnesium is oxidising because its oxidation number is increasing from 0 to +2 Hydrogen is reducing because its oxidation number is decreasing from +1 to 0 Fe + H2 SO4 FeSO4 +H2 Be able to write equations for reactions of metals with hydrochloric acid and sulphuric acid Observations: These reaction will effervesce because H2 gas is evolved and the metal will dissolve Note that not all the oxygen atoms are changing oxidation number in this reaction 2 NH3 + NaClO N2H4 + NaCl + H2O +1 -2 -1 -3 Nitrogen is oxidising because its oxidation number is increasing from -3 to -2 Chlorine is reducing because its oxidation number is decreasing from +1 to -1
2.1.5 Redox
Redox reactions
(d) oxidation and reduction in terms of:
(i) electron transfer
(ii) changes in oxidation number
Should include examples of s-, p- and d-block elements.
(e) redox reactions of metals with acids to form salts, including full equations (see also 2.1.4 c) Metals should be from s-, p- and d-blocks e.g. Mg, Al, Fe, Zn. Ionic equations not required. In (e), reactions with acids will be limited to those producing a salt and hydrogen. Reactions involving nitric acid or concentrated sulfuric acid could be assessed in the context of (f).
(f) interpretation of redox equations in (e), and unfamiliar redox reactions, to make predictions in terms of oxidation numbers and electron loss/ gain.
