unit 8: redox and electrochemistry - oak park … 20, 2014 • electrochemical cell potential:...
TRANSCRIPT
May 20, 2014
Unit 8: Redox and Electrochemistry
http://www.firefly.org/firefly-pictures.html
May 20, 2014
Rule #1: Oxidation number of any uncombined atom is zero.
Example: C, H2, Al, Cl2...etc.
Oxidation Number• numbers assigned to atoms that allow us to keep
track of electrons.
May 20, 2014
Rule #2: The oxidation number of a monatomic ion is equal to its charge.
Example: Na+, Mg2+, Cl-, S2-
May 20, 2014
Rule #3: The oxidation number of the more electronegative atom in a molecule or complex ion is the same as the charge it would have if it were an ion.
Example: NH3 Nitrogen has oxidation number of -3.
Rule #4: The oxidation number of fluorine in a compound always -1.
May 20, 2014
Rule #5: Oxygen has an oxidation number of -2 in most compounds.
Exception 1: In peroxides (like H2O2), oxidation number = -1.
Exception 2: When bonded to fluorine, oxidation number = +2
May 20, 2014
Rule #6: The oxidation number of hydrogen in most of its compounds is +1 except when bonded to metals, where it is -1.
Example: H2O, MgH2
Rule #7: In compounds, the elements of groups 1 and 2, and aluminum have oxidation numbers +1, +2, and +3 respectively.
May 20, 2014
Rule #8: The sum of the oxidation numbers in a neutral compound is zero.
Example: NaCl, CaBr2, CCl4
Rule #9: The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.
Example: SO32-, OH-
May 20, 2014
*side note:
Charges are written with signs after the number:
2-, 3-, 2+, 3+
Oxidation numbers are written with signs before the number
-3, -2, -1, +1, +2, +3,
May 20, 2014
Example 1: Assign oxidation numbers
a) F2
b) Na2O
c) F-
d) BH3
e) NaOH
f) PO43-
May 20, 2014
Oxidation = losing electrons (oxidation number increases)
Reduction = gaining electrons (oxidation number decreases)
Oxidation-Reduction Reactions• also called Redox reactions• reaction in which one or more electrons are
transferred from one atom to another• Oxidation and reduction always happens together
May 20, 2014
LEO the lion says GERoxidation
electrons
losing
reduction
electrons
gaining
May 20, 2014
Why does redox happen?• Atoms transfer electrons to another atom.• The more electronegative atom attracts electrons
more strongly, resulting in a transfer of electrons.
May 20, 2014
Example 2: Identify the following as oxidation or reduction
a) I2 + 2e- 2I-
b) K K+ + e-
c) Fe2+ Fe3+ + e-
May 20, 2014
Oxidation numbers in Redox Reaction1. Assign oxidation numbers to all elements2. When an atom is oxidized, its oxidation #
increases3. When an atom is reduced, its oxidation #
decreases
2K + Br2 2KBr
May 20, 2014
Example 3:
Cu + AgNO3 Ag + CuNO3
Assign oxidation numbers.• Which atom is oxidized?• Which atom is reduced?
May 20, 2014
Example 4:
2KBr + Cl2 2KCl + Br2
Assign oxidation numbers.• Which atom is oxidized?• Which atom is reduced?
May 20, 2014
Example 5:
CH4 + 2O2 CO2 + 2H2O
Assign oxidation numbers.• Which atom is oxidized?• Which atom is reduced?
May 20, 2014
# of electrons lost = # electrons gained
Balancing Redox Reactions• oxidation = reduction
We will learn to balance redox reactions using the half-reaction method.
May 20, 2014
Half-reactions• Equations that have electrons as reactants or
products• One half reaction represents oxidation• One half reaction represents reduction
Example 6:
SnCl4 + Fe SnCl2 + FeCl3
Example 7:
Fe + CuSO4 Cu + Fe2(SO4)3
May 20, 2014
Using Half-Reactions to Balance Redox Equations1. Identify the species oxidized and the species
reduced2. Write the half-reaction3. Multiply the half-reaction by the smallest
coefficient possible so that the # of e- is the same.
4. Rewrite as a complete balanced equation (add 2 half-reactions together).
May 20, 2014
Example 8:
Rewrite half reactions from example 6.
May 20, 2014
Example 9:
Rewrite half reactions from example 7.
May 20, 2014
Example 10: Balance the following using the half-reaction method.
H2S + Cl2 HCl + S
a. write half reactions
b. balance oxidation/reduction
c. rewrite balanced equation
May 20, 2014
Electrochemistry• Study of how chemical energy is converted to
electrical energy or vice versa.
Electrochemistry
Redox
Electrochemical Cell
Electrolytic CellVoltaic Cell
May 20, 2014
Voltaic Cells• converts chemical energy to electrical energy• spontaneous redox reaction• generates a current
May 20, 2014
Voltaic Cells• consists of two half-cells• Separate oxidation and reduction reaction• Each half-cell contains:
> electrode> solution
• Anode: oxidation• Cathode: reduction
May 20, 2014
Voltaic Cells• Half-cells are connected by a salt bridge
> allows ions to pass from one side to another> prevents build up of ions that prevent redox
reactions
May 20, 2014
Example 11: Sketch the diagram from the animation and identify• anode and half-reaction• cathode and half-reaction• write the overall balanced cell reaction
May 20, 2014
Cell Notation• shows you the oxidation and reduction half-cells in
a voltaic cell
Zn Zn2+ Cu2+ CuOxidation Reduction
Salt Bridge
Anode Cathode
Example 12: Write the cell notation for the voltaic cell in example 11.
May 20, 2014
Electrochemical Cell Potential• reduction potential: tendency of a substance to
gain electrons> reduction potential of an electrode is measured
in volts> standard reduction potential (E0)
http://wpscms.pearsoncmg.com/wps/media/objects/3662/3750317/Aus_content_18/Table18-01.jpg
May 20, 2014
• Electrochemical cell potential: difference in potential between the half-reactions> potential must be > 0 in order for the redox
reaction to be spontaneous> In a voltaic cell, the half-reaction with the lower
reduction potential will be the oxidation reaction (opposite reaction given on chart)
E0cell = E0reduction - E0oxidation
May 20, 2014
Example 12: Write the cell notation and calculate the cell potential for the following redox reaction.
Sn(s) + 2Cu+(aq) Sn2+ + 2Cu(s)
May 20, 2014
Example 13: Write the cell notation and calculate the cell potential for the following redox reaction.
Mg(s) + Pb2+(aq) Pb(s) + Mg2+(aq)
May 20, 2014
Example 14: Given the pair of half-reactions, • write the balanced equation for the overall cell
reaction• calculate the cell potential• write the cell notation
Co2+(aq) + 2e- Co(s)
Cr3+(aq) + 3e- Cr(s)
May 20, 2014
Example 15: Given the pair of half-reactions, • write the balanced equation for the overall cell
reaction• calculate the cell potential• write the cell notation
Fe2+(aq) + 2e- Fe(s)
I2(s) + 2e- 2I-(s)
May 20, 2014