REDUCTION: O 2 (g) + 4e - 2O 2- (s) Oxidant -is reduced OVERALL REDOX EQUATION: 2Mg (s) + O 2 (g) 2 Mg 2+ (s) + 2O 2- (s) 2Mg (s) + O 2 (g) 2 MgO (s)
OXIDANT / OXIDISING AGENT -are substances that cause oxidation and are themselves reduced. Eg; Oxygen in the previous example. REDUCTANT/ REDUCING AGENT -are substances that cause reduction and are themselves oxidised. Eg; Magnesium in the previous example Remember: An assassin kills but doesn’t get killed himself. Get it!
OTHER EXAMPLES OF REDOX REACTIONS • extraction of metals from their ores • the bleaching of hair • Respiration • Photosynthesis • corrosion of metals • reactions involved in photography
In class, answer Q1 & 2 on p51
CLASSIFYING REDOX REACTIONS USING OXIDATION NUMBERS Oxidation numbers are used to determine whether or not a redox reaction has occurred and they eliminate the need for multiple definitions for oxidation and reduction. OXIDATION: an increase in oxidation number. REDUCTION: a decrease in oxidation number.
RULES FOR ASSIGNING OXIDATION NUMBERS (1) Free elements have an oxidation number of zero. 0 0 0 Eg Na (s) C (s) Cl 2 (g) NOTE: The Oxidation number of an element is written above its symbol. The plus or minus precedes the number and so distinguishes it from the charge of an ion. -2 EgO 2- oxidation number = -2 & valence = 2 -
(2) The sum of the oxidation numbers in a neutral compound is zero. +1 -2 +1 +5 -2 Eg H 2 O HNO 3 The sum of the oxidation numbers in polyatomic ions is equal to the charge on the ion. Eg CO 3 2- (4 + 3x-2 = -2)
(3) In ionic compounds the oxidation number is equal to the charge on the ion. +1 -1 +2 -1 +3 -2 Eg NaCl Ca Cl 2 Al 2 O 3
(4) Some elements have oxidation numbers that are regarded as fixed except in a few circumstances. • Group I and II metal ions & Aluminium ions in ionic compounds always have an O.N. = to their valence. • Oxygen is usually –2 in compounds, whereas in peroxides it is -1 . -2 Eg H 2 O -1 -1 -1 Eg Peroxides: H 2 O 2 BaO 2 Na 2 O 2
(5) The most electronegative element in a compound has the negative oxidation number. Electronegative elements, eg; F, Cl and O have oxidation numbers equal to the charges on their simple ions when part of a compound, provided they are the most electronegative element present in the compound.
Electronegativity: F > O > N, Cl > other elements +1 -2 -1 +2 Eg H 2 O F 2 O Hydrogen is usually +1 in compounds, whereas in metal hydrides it is –1 +1 -1 Eg HCl -1 -1 -1 Eg Metal Hydrides: NaH CaH 2 LiH
EXAMPLE Find the oxidation number of all the elements in HNO 3 .
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