CHEM1003 Lecture Notes - Lecture 27: Oxidizing Agent, Magnesium Oxide, Oxidation State
Lecture 27
Monday, 16 May 2016
3:22 pm
Oxidation and Reduction (1)
Redox Reactions:
• Oxidation is the loss of electron(s)
• Reduction is the gain of electron(s)
• Electron transfer reactions
• Oxidation and reduction occur together
• So if a species is being oxidised, then the other species in the reaction is being
reduced
Oxidation Numbers:
• Help recognise what is oxidised and what is reduced
• Is the 'oxidation state' (o.s.) of an atom
• Oxidation increases oxidation number
• Reduction decreases oxidation number
General rules of oxidation numbers:
• Oxidation number of an element not combined with other elements = 0
o E.g. Mg = 0; Cl in Cl2 = 0
• Oxidation number of an ion equals its charge
o E.g. for Fe2+ = +2, Cl- = -1
• Specific oxidation numbers
o H is +1 with non-metals
o O is -2 (for most compounds - except H2O2)
• Sum of the oxidation numbers for all atoms in a neutral molecule equals 0
o E.g. for H2O = (2 x 1) + (1 x -2) = 0
• Sum of the oxidation numbers of all atoms in a polyatomic ion equals the total
charge
o E.g. oxidation state of Mn in MnO4-
• XMn + (4 x -2) = -1
• XMn = -1 + 8 = 7
Some Exceptions:
• F is always -1 in compounds with other elements
• Cl, Br and I are always -1 except when combined with oxygen
• H is +1 except when it forms a compound with a metal
• O is -2 in most compounds except H2O2
Hydrogen Peroxide, H2O2:
• Uses 1
o As an antiseptic (the killing of bacteria) 3% in water solution
• Uses 2
o Commonly used (in very low concentrations) to bleach human hair
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