Unit 5: Oxidation- Reduction Reactions By Brittany Behrens & Joanna McConnell

What is Redox? An oxidation-reduction (redox) reaction is a chemical reaction in which electrons are transferred between reactants. LEO says GER: Loss of Electrons is Oxidation Gain of Electrons is Reduction Oxidation and reduction cause a change in the oxidation number of each element involved in the redox reaction.

Rules for Redox: Oxidation Numbers 1. The oxidation number of an atom in an uncombined element is zero. 2. The oxidation number of a monatomic ion is its ionic charge. 3. The algebraic sum of the oxidation numbers of all of the atoms in a neutral compound is zero. 4. The algebraic sum of the oxidation numbers of all of the atoms in a polyatomic ion is the net ionic charge. 5. Fluorine = -1 Elements that form monatomic ions often have their charge = oxidation number Group 1 = +1Group 2 = +2Group 7 = -1 H is usually +1O is usually –2 Peroxides are a notable exception: in H 2 O 2, H=+1 and O=-1.

Oxidation Number Example Step 1: Use oxidation number rules to assign numbers to as many elements as possible. Consider this compound:K 2 Cr 2 O 7 Group 1 elements, including K, have oxidation number +1 Oxygen has oxidation number -2 The neutral compound’s sum of oxidation numbers = 0 Step 2: Solve for the oxidation number of Cr. 2(+1) + 2(x) + 7(-2) = 0  2x = 12  x = 6 Thus, the oxidation number of K=+1, O=-2, and Cr=+6.

Balancing Redox Equations Step 1: Write a half-reaction for each of the reactants. Cr 2 O 7 2-  Cr 3+ Consider this redox reaction: Dichromate ions and Iron (II) ions react in acid. Fe 2+  Fe 3+

Balance for Mass Step 2: Since the reaction is being balanced in acid, add H + and H 2 O as needed to make sure that the same number of each element is present on each side of each half-reaction. Cr 2 O 7 2-  Cr 3+ 14H H 2 O Fe 2+  Fe 3+ 2

Balance for Charge Step 3: Since this is a redox reaction, add electrons (e - ) as needed to make sure that the net charge of each half-reaction is ZERO. 14H + + Cr 2 O 7 2-  2Cr H 2 O6e - + Fe 2+  Fe 3+ + e -

Add the Half-Reactions Step 4: Multiply the half-reactions by coefficients to cancel out the electrons. Add the two half-reactions to determine the equation for the net reaction. 6e H + + Cr 2 O 7 2-  2Cr H 2 O Fe 2+  Fe 3+ + e H + + Cr 2 O Fe 2+  2Cr Fe H 2 O

Check your Work Dichromate ions and Iron (II) ions react in acid. Balanced Equation: Word Equation: 14H + + Cr 2 O Fe 2+  2Cr Fe H 2 O Is mass conserved? Hydrogen: 14 atoms on the left, 7(2)=14 atoms on the right Chromium: 2 atoms on the left, 2 atoms on the right Oxygen: 7 atoms on the left, 7 atoms on the right Iron: 6 atoms on the left, 6 atoms on the right Is charge conserved? 14(+1) + 1(-2) + 6(+2) = 2(+3) + 6(+3) + 7(0) = = 24 

Basic Balancing Step 5: If the reaction is to be balanced in a basic solution, follow the steps for balancing in acid, then add OH - as needed. 14H + + Cr 2 O Fe 2+  2Cr Fe H 2 O +14OH- 14H 2 O + Cr 2 O Fe 2+  2Cr Fe H 2 O + 14OH - 7 7H 2 O + Cr 2 O Fe 2+  2Cr Fe OH -

Voltmeter AlAg Salt Bridge (KNO 3 ) Applications: Electrochemistry e - flow Anode Cathode Oxidation Reduction Loses MassGains Mass NO 3 - K+K+ Ag + Al 3+ AnOx RedCat LEO GER Al (s)  Al 3+ (aq) + 3e - E o = V Ag + (aq) + e -  Ag (s) E o = V Al (s) + 3Ag + (aq)  3Ag (s) + Al 3+ (aq) E o rxn = V If E o rxn is positive, the reaction is spontaneous Look up E o values in a table of standard reduction potentials.

The Nernst Equation E=EoEo – RT nF lnQ Nonstandard Reduction Potential (V) Standard Reduction Potential (V) 8.31 J/Kmol Temperature (K) Reaction Quotient: [products] [reactants] 96,500 C/mol e - Moles of e - transferred

Remember… For Oxidation-Reduction Reactions, A redox reaction is a chemical reaction in which electrons are transferred between reactants. LEO says GER AnOx, RedCat The Nernst Equation (given on the AP exam)

Remember to Study for the AP Exam ~End~