Chapter 20 – Oxidation and Reduction Oxidation-Reduction reactions called redox reactions

O 2 + Fe → Fe 2 O 3 ► Oxidation – is the loss of electrons through transfer or unequal sharing ► Reduction – the gain of electrons through transfer or unequal sharing ► Iron was oxidized. Oxygen is the oxidizing agent (it causes the iron to be oxidized) ► Oxygen was reduced. Iron is the reducing agent (it causes oxygen to be reduced) ► We cannot have oxidization without reduction If something gains electrons something must lose electrons

Assigning Oxidation Numbers ► The oxidation number of any uncombined element is zero. Examples: Fe H 2 O 3 He ► The oxidation # of any ion is the charge on that ion. Examples Cl - Ca +2 OH - SO 4 -2 ► We assign elements oxidation numbers based on their oxidation number given on the periodic table. Many elements have only one oxidation number. Examples: HF NaClAlBr 3 Fe 2 O 3 ► Hydrogen is assigned a +1 and oxygen is assigned a -2 when combined to form compounds. ► The algebraic sum of the oxidation numbers of each atom in a neutral compound is zero or the charge on the polyatomic ion.

► Assign those oxidation numbers you can be sure of, and then solve for the oxidation number you are not sure of. ► Do several examples ► Demonstrate sample problem page 661 ► Assignment: page 661 practice problems 1 and 2 ► Demonstrate sample problem page 664 ► Assignment: page 665 practice problems 3 and 4

► Where on the periodic table do you think strong oxidizing agents are found? ► Where are strong reducing agents? Why?

► Synthesis, decomposition, and single replacement are usually redox reactions. ► Double replacement reactions are usually not redox reactions.

See metal activity series on page 668 ► K is a reactive metal (is easily oxidized) ► Au is not a reactive metal (not easily oxidized) ► Au 3+ ion is easily reduced (very reactive) ► K + ion is not easily reduced. (not reactive) ► Active metals have stable ions. ► Stable metals have reactive ions.

How does Zn prevent the rusting of iron? ► The O 2 takes electrons from Zn in preference to Fe because Zn gives up electrons easier. ► Other applications of redox reactions are bleaching, fuels, explosives and photography.

Balancing Redox Reactions ► Difficult equations can be balanced by using the change in oxidation numbers as your starting point. ► You then balance the change in charge (which is really balancing electrons). ► See example on page 676 ► Assignment: practice problems 5 and 6 page 677

► Complete chapter questions on page 680 and 681 (1-16, 18, 19, 20, 23, 26, 28)