Oxidation – Reduction Reactions ● Redox reactions ● Why oxidation-reduction? – Originally oxidation referred to a reaction in which a substance reacts with oxygen – Reduction described reactions where oxygen was removed – Don't let the name fool you, oxygen is not required! ● Actually: – Oxidation occurs when a substance LOSES electrons – Reduction occurs when a substance GAINS electrons – LEO goes GER ● Use oxidation numbers to determine if a substance is oxidized or reduced

Oxidation Numbers ● Elemental form of an atom = 0 – H 2, Zn, P 4 all have an oxidation number of 0 ● Monatomic ions = ionic charge (periodic table) – Na +1, S -2 ● Some oxidation numbers never change when in a compound: – O is usually -2 ( Except peroxides: O 2 -2, so O is -1) – H is +1 when bonded to nonmetals, -1 when bonded to metals – F is always -1 ● Sum of oxidation numbers-- – Zero for a compound – Equal to the charge of a polyatomic ion

Oxidation Number Practice ● MgCl 2 ● Mg(NO 3 ) 2 ● Al 2 (SO 4 ) 3 ● Ba(NO 3 ) 2 ● FeCl 3 ● OF 2 ● H 2 O 2 ● Al 2 S 3 ● Fe ● O 2