redoxshow.ppt

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 Electrochemistry : Oxidation and Reduction
 Electrochemical Reaction - Chemical reaction that involves the flow of electrons.
 Redox Reaction (oxidation-reduction reaction) - A reaction in which at
 least one atom changes in oxidation state. 
 Reduction - Any process in which the oxidation number of an atom 
 decreases (becomes more negative). 
 Oxidation - Any process in which the oxidation number of an atom 
 increases (becomes more positive). 
 Oxidation Number - The charge that an atom would have if the compound
 in which it were found were ionic. (Next page is a refresher on “How to”.)
To help remember oxidation and reduction, remember the following: OILRIG: Oxidation Is Loss Reduction Is Gain
Types of Redox Reactions 
Corrosion - A type of redox reaction in which a metal is destroyed. 
 4 Fe(s) + 3 O2(g) 2 Fe2O3 • 3 H2O 
 Metathesis Reaction - A reaction in which atoms are interchanged
 and there is no change in oxidation number. 
 Disproportionation Reaction - A reaction in which a single reactant
 undergoes both oxidation and reduction. Disproportionation
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Before we balance a Redox equation lets first refresh our memory on how to calculate oxidation numbers. 
			Assigning Oxidation Numbers 
 Category Oxidation # Example
 1) Neutral substances containing only a single element 	 0 		N2, He
 2) Monatomic ions 		 same as the charge 	Na+ = +1
 3) Hydrogen combined with a nonmetal 		+1 		HBr, CH4, OH-
 4) Hydrogen combined with a metal			 -1		NaH, CaH2
 5) Metals in Group IA				+1		Li3N, Na2S
 6) Metals in Group IIA 				+2		Mg3N2
 7) Oxygen 					 -2		H2O, NO
	(Exceptions: H2O2, O22-)			 -1 
 8) Halogens					 -1		AlF3, HCl
Oxidation Number - The charge that an atom would have if the compound in which it were found were ionic. 
The rules:									 	1) The sum of the oxidation numbers of the atoms in a molecule must be equal to the overall charge on the 	molecule. 									2) To assign a number to a transition metal ion (not listed in the table below) start with the overall charge, add 	the total number of negative charges for oxygen (if there were four as in the 	case of MnO4- then you would 	add 8 for a total of +7 for Mn), continue until all other species listed in the table below are considered (subtract 	if it is a positive value.) The result is the oxidation number of the transition metal ion. 			3)The most electronegative element will have a negative oxidation number. 
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1. What are the reduction and oxidation pairs?
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 a) Al(s) and Al(OH)-4(aq) (oxidized)
 b) ? and H2(g) (reduced) 
Hint: 1. The reaction is taking place in a basic, aqueous media.
2. Look for a reduction potential for H2(g) in a table.
2. Calculate the Oxidation numbers
3. Mass Balance
4. Charge Balance
Balancing Redox equations using the Oxidation number method
 (Basic solution is demonstrated)
 and transfer to the redox partner 
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1. What are the reduction and oxidation pairs?
2. Mass Balance both equations
+ H2O
OCl-(aq)
+ H+
+ H2O
2 H+ +
3. Charge balance both equations (add extra e-)
+ 2e-
2e- +
4. Cancel any common terms.
OH- +
+ OH-
6. Add the two half reactions and cancel any extra water.
5. Is the reaction taking place in a basic solution? 
 H2O + 2e- +
H2O +
Pb(OH)-3(aq) +
PbO2(s)
+ Cl-(aq)
+ 2H2O
+ OH-
Balancing Redox equations using the Half Reaction method (Basic solution is demonstrated)
(add H2O to balance extra oxygens 
then add extra H+ to balance extra hydrogens from the added H2O)
 Are there are any H+ left? add OH- to both sides. 
H+ and OH- will make H2O on one side.