Topic 3 - Redox
Oxidation is loss of electrons, reduction is gain of electrons
An oxidising agent is reduced, a reducing agent is oxidised
Oxidation numbers increase when electrons are lost and decrease when electrons are gained.
Redox reactions occur between metals and acids
Metal + Acid → Salt + Hydrogen
Metal atoms lose electrons and hydrogen atoms gain electrons
Rules for oxidation numbers
Uncombined elements have oxidation state of 0
Atoms in molecules bonded to identical atoms (e.g. O2) have an oxidation state of 0
Monatomic ions oxidation state is equal to its charge
Molecular ions sum of oxidation numbers equals overall charge
Neutral compound the sum of oxidation numbers is 0
Oxygen is usually -2 apart from on peroxides where it is +1 and OF2 where it is +2
Hydrogen is +1 apart from when a metal hydride when its -1
Fluorine is always -1
“ate”
Ions ending in ate are made of oxygen and another element. If element bonded to oxygen has variable oxygen numbers then roman numerals are used to differentiate.
Half equations
Ionic half equations show electron transfer
E.G. 1. O2 +4e- → 2O2-
Mg → Mg2+ +2e-
2Mg → 2Mg2+ + 4e-
O2 +4e- + 2 Mg → 2MgO + 4e-
O2 + 2Mg → 2MgO
A disproportionation reaction is when a single element is both oxidised and reduced