Download Reactions in Aqueous Solutions

Electrochemistry
Is the study of the interchange of chemical and
electrical energy
HW: Read CH 8 and 18
3 min https://www.youtube.com/watch?v=GhbuhT1GDpI
Oxidation and Reduction
reactions (REDOX)
•
Assigning Oxidation Number rules (you probably have most of this
figured out). The point is to keep track of electrons.
1)
Free element is 0
2)
Oxygen is -2; Aluminum is +3
3)
Group 1 is +1; Group 2 is +2
4)
More electronegative element in a compound is assigned charge if it
were an ion (Halogens are -1)
5)
Neutral compound all numbers add to 0
6)
Polyatomic ion all numbers add to ion charge
*Assign charge where known and then solve for unknown*
*NOTE: there are some exceptions. You will learn about them in college 
Oxidation and Reduction
reactions (REDOX)
• What is the oxidation number of Se in HSeO3-?
HSeO3+1 + x + (-2)3 = -1
x = +4
You could think of it like an algebra problem (subscript = multiply
by)
1(+1) + 1(x) + 3(-2) = -1
Oxidation and Reduction
reactions (REDOX)
• YOU TRY! What is the oxidation number?
• A) Sulfur in SO3
• B) Manganese in MnO4-
• C) Cobalt in (CoCl6)3-
Oxidation and Reduction
reactions (REDOX)
• Think of ionic compounds between metals and nonmetals
• Oxidation Is a Loss of electrons (so charge is positive)
• K  K+ + e- (potassium is oxidized)
• Reduction Is a Gain of electrons (so charge is negative)
• Cl2 + 2e-  2Cl- (Chlorine is reduced)
OIL RIG
• Movement of electrons = energy
• Which is getting oxidized? Reduced? (not balanced
equations)
• Zn + Cu2+  Zn2+ + Cu
• H2 + O2  H2O
• Ag(NO3) + Cu  Cu(NO3)2 + Ag
(most electronegative = ionic charge)
Oxidation and Reduction
reactions (REDOX)
• Write 2 half-reactions (oxidation and reduction).
• 1) write equation for reaction
• 2) assign oxidation numbers to all atoms
• 3) identify who is getting oxidized/reduced
• 4) divide reaction into 2 half reactions and balance
separately (ADD electrons)
Oxidation and Reduction
reactions (REDOX)
Write 2 half reactions for
Zn(s)+ Cu(SO4)(aq) → Zn(SO4)(aq) + Cu(s)
0
+2 -2
+2 -2
0
• Oxidation: Zn0  Zn2+ + 2e• Reduction: Cu2+ + 2e-  Cu0
Oxidation and Reduction
reactions (REDOX)
Write 2 half reactions for
Na + Cl2  NaCl
0
0
+1 -1
• Oxidation: Na0  Na+ + e• Reduction: Cl20 + e-  Cl-
Oxidation and Reduction
reactions (REDOX)
• YOU TRY! Identify what is getting oxidized and reduced
and write their half-reactions
Mg + O2  MgO
• Reduction:
• Oxidation:
MnO4-1 + Br -1  MnO2 + BrO3-1
• Reduction:
• Oxidation:
Electrochemistry Involves TWO MAIN
TYPES Of Electrochemical Cells:
• Galvanic (voltaic) cells – generates useful electrical
energy (battery; frequently used in power plants)
• Electrolytic cells –require external energy source (a
direct current or DC power source like a flashlight
battery)
• * you can vary the materials to produce different
batteries (alkaline, lithium, mercury) that in turn
produce different energy (lots energy that slowly
decreases vs. less energy but longer life span)
•
How its made 5 min https://www.youtube.com/watch?v=ksxSOwA933M
Galvanic Cell “anatomy”
• Anode – Electrode where oxidation occurs. Negative end.
(anode may appear to become smaller as it falls into solution)
• Cathode – Electrode where reduction occurs. Positive end.
(it may appear larger, due to ions from solution plating onto it)
• Salt bridge – used to maintain electrical neutrality in a
galvanic cell
• Electron flow – ALWAYS through the wire from anode to
cathode
• Voltmeter – measures the cell potential in volts
Galvanic Cell pneumonic
devices
• AN OX – oxidation occurs at the anode (may show
mass decrease)
• RED CAT – reduction occurs at the cathode (may
show mass increase)
• FAT CAT – The electrons in a voltaic or galvanic
cell ALWAYS flow
From the Anode To the CAThode
• ca+hode (cathode is + end)
Galvanic Cell
• Write 2 half reactions for the following reaction. Then fill in the
Galvanic cell. (answer on next slide)
• Zn + Cu2+ → Zn2+ + Cu
•
https://www.youtube.com/watch?v=SO7VLy2BoU8 (5 min)
And the answer is…
• RED: Cu2+ + 2e- → Cu
• OX: Zn → Zn2+ + 2e-
Can you try on your own??
• Write 2 half reactions for the following reaction. Then fill
in the Galvanic cell. HINT: NO3 is not in the reaction.
• Cu + H(NO3) --> Cu(NO3) + H2
Electrolytic Cell
• Electrolytic cells are forced to occur by using an
electron pump or battery or any type of DC source
• A galvanic cell is separated into two half cells to
generate electricity; an electrolytic cell occurs in a
single container.
• A galvanic cell IS a battery, an electrolytic cell
NEEDS a battery
Electrolytic Cell