Chemical Reactions SCH3u/4C Ms. Richardson Electrochemistry Chemical Reactions SCH3u/4C Ms. Richardson

Reduction-Oxidation Reactions Reduction and oxidation reactions (redox reactions) involve the transfer of electrons. This either is caused by, or produces an electric current.

Reduction Reactions According to modern theory, a reduction reaction involves a gain of electrons. Ag+(aq) + e-  Ag(s) We demonstrate this gain of electrons by placing the electrons on the reactant side of the equation

Oxidation Reactions According to modern theory, an oxidation reaction involves a loss of electrons. Ag(s)  Ag(aq) + e- We demonstrate this loss of electrons by placing the electrons on the product side of the equation

Redox Reactions Oxidation and reduction reactions are separate processes that occur simultaneously. The total number of electrons gained must be equal to the total number of electrons lost. The overall equation includes both processes and must be balanced!

Assigning Oxidation Numbers Free elements: each atom has an oxidation # of zero Monatomic ions: oxidation # is equal to the ion’s charge Oxygen: oxidation # is -2, unless in peroxides it is -1 Hydrogen: when bonded to non-metal is +1, and when bonded to a metal is -1 Fluorine: always oxidation # of -1 In neutral compounds: Sum of all oxidation #’s is zero Oxidation States An oxidation number (or state) indicates how many electrons may be lost by an atom in a compound. We can follow this set of rules to predict oxidation numbers:

0 = oxidation of H + oxidation of S Examples: What is the oxidation number of sodium in sodium chloride? NaCl Na = +1 Cl = -1 What is the oxidation number of sulfur in H2S? H = +1 S = -2 Make sure its balanced! 0 = 2(+1) + (-2) 0 = oxidation of H + oxidation of S

Oxidation of Ca + Oxidation of S + Oxidation of O = 0 Example What is the oxidation number for each element in CaSO4 Ca = +2 S = -2 O = -2 Make sure its balanced! Oxidation of Ca + Oxidation of S + Oxidation of O = 0 +2 + (?) + 4(-2) = 0 ? = 6

Predicting Reduction and Oxidation In redox reactions, oxidation numbers for particular elements change from reactants to products. When oxidation #’s increase, oxidation has occurred. When oxidation #’s decrease, reduction has occurred

Examples Which of the following are Redox reactions? 2K + Cl2  2KCl N2 + 3H2  2NH3 S + 3Cl2  SCl6 Al + Cu2+ Cu + Al3+ Al(OH)3 + 3NaCl  3NaOH + AlCl3

Half Reactions Half reactions are used to show which components of the overall reaction are being oxidized and which are being reduced in separate, smaller equations. Example: FeO + Al  Al2O3 + Fe Oxidation half reaction: Fe  Fe2+ + 2e- Reduction half reaction: Al3+ + 3e-  Al