Oxidation State Short Cuts

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Gerard Miller
5 years ago
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1 Oxidation State Short Cuts The oxidation state of an atom in its elemental state is zero The sum of atomic oxidation states = total charge on an ion or molecule The oxidation state of F is always -1 (except in F 2 ) The oxidation state of O is always -2 (except with F or O) The oxidation state of H is always +1 (except with metals or in H 2 ) Atoms always want to be like the Nobel Elements

2 In Class Example Find the oxidation state of Fe: Fe (s) + O 2 Fe 2 O 3 Al: Al (s) + O 2 AlO 2 Can this compound exist?

3 Balancing Redox Reactions 1. Assign oxidation states. 2. Identify reduction and oxidation half-reactions. 3. Balance the electrons, then balance atoms except H and O. 4. Combine half-reactions so no free e appear. 5. Add H 2 O to balance the O. 6. Add H + to balance H. 7. Double check charge balance.

4 Balancing Redox Reactions (in Basic Solutions) Same as acidic solution, but just a couple extra steps 1. Balance the equation using approach we used for the acid solution 2. Add OH - to each side to eliminate H + 3. Cancel any H 2 O that is both a product and a reactant

5 In Class Example Balance the reaction: Zn(s) + ClO (aq) Zn(OH) 4 2 (aq) + Cl (aq) in basic solution.

6 Chapter 18.3 Voltaic (Galvanic) Cells Objectives Identify the components of a voltaic cell Write half-cell reactions and the overall reaction from a diagram of a voltaic cell. Identify the direction of flow of electrons and ions through a salt bridge in a voltaic cell. November 22 nd, 2016

7 Voltaic Cell Definition A Voltaic cell, or galvanic cell, is an apparatus that produces electrical energy directly from a redox reaction.

8 Voltaic Cell e V e - e - Salt Bridge KNO 3 (aq) NO - 3 K + e - Cu NO 3 - K + Ag NO 3 - Cu 2 + Ag + NO - 3 Oxidation Cu Cu e - Reduction Ag + + e - Ag

9 Chapter 18.4 Potentials of Voltaic (Galvanic) Cells Objectives Relate cell potential to a spontaneous reaction Calculate the standard potential of a voltaic cell by combining two halfreactions Use reduction potentials to predict the spontaneity of chemical reactions under standard conditions December 1 st, 2015

10 Standard Potentials The overall potential of a cell depends on the concentrations of the components in the reaction. Standard potential, E o cell, is the cell potential when each component in the reaction is present in its standard state. Solids, liquids and gases are in their pure state at 1 atm pressure. Solutes have 1.0 M concentration.

Voltage of the Cell All half-cell potentials are referenced versus

11 Half-Cell Voltages All standard potentials are determined using the voltages of the half-cell reactions. Voltage of the Cell All half-cell potentials are referenced versus the standard hydrogen potential. = 2H + (aq) + 2e - H 2(g) Sum of ½-Cell Voltages by definition E = 0.00 V

12 Combining Half-Reactions and Half-Cell Voltages Zn(s) Zn 2+ (aq) + 2e - E o = 0.76 V Cu 2+ (aq) + 2e - Cu(s) E o = 0.34 V Zn(s) + Cu 2+ (aq) Cu(s) + Zn 2+ (aq) E o cell = 1.10 V A positive cell potential means that the reaction is spontaneous in the forward direction.

13 Standard Reduction Potentials Reduction Half-Reaction E o (V) F 2 (g) + 2e - 2F - (aq) 2.87 Ag + (aq) + e - Ag(s) 0.80 Fe 3+ (aq) + e - Fe 2+ (aq) 0.77 Sn 4+ (aq) + 2e - Sn 2+ (aq) H + (aq) + 2e - H 2 (g) 0.00 Co 2+ (aq) + 2e - Co(s) Fe 2+ (aq) + 2e - Fe(s) Zn 2+ (aq) + 2e - Zn(s) Mg 2+ (aq) + 2e - Mg(s) -2.37

14 Standard Reduction Potentials 2 High Reduction Potential Easy to Reduce Hard to Oxidize Low Reduction Potential Hard to Reduce Easy to Oxidize V V V V V V V

15 Calculating Cell Potential The standard potential of a cell reaction is given by E o cell = E o red + E o ox Both half-reactions MUST transfer the same number of electrons. Multiplying the coefficients of a half-reaction to does NOT change the potential. Fe 3+ (aq) + e - Fe 2+ (aq) 2Fe 3+ (aq) + 2e - 2Fe 2+ (aq) E o = 0.77 V E o = 0.77 V Reversing the reaction DOES change the sign of E o Ag + (aq) + e - Ag (s) Ag(s) Ag + (aq) + e - E o = 0.80 V E o = V

16 In Class Example Which elements are oxidized and which are reduced when Fe is added to a solution of Co 2+? Co 2+ (aq) + 2e - Co (s) E = V Fe 2+ (aq) + 2e - Fe (s) E = V

17 Gibbs Free Energy and Cell Potential G and E both predict spontaneity work available from reaction = electrical work done by cell G cell = - n F E cell

18 Student Example Calculate the Gibbs Free Energy of the spontaneous reaction when Fe is added to a solution of 1 M Zn(NO 3 ) 2? Zn 2+ (aq) + 2e - Zn (s) E = V Fe 2+ (aq) + 2e - Fe (s) E = V

ELECTROCHEMISTRY. Oxidation/Reduction

ELECTROCHEMISTRY. Oxidation/Reduction ELECTROCHEMISTRY Electrochemistry involves the relationship between electrical energy and chemical energy. OXIDATION-REDUCTION REACTIONS SPONTANEOUS REACTIONS Examples: voltaic cells, batteries. NON-SPONTANEOUS