REDUCTION O 2 g 4e 2O 2 s Oxidant is reduced OVERALL REDOX EQUATION 2Mg s O 2 g

# Reduction o 2 g 4e 2o 2 s oxidant is reduced overall

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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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