chapter 18 – electrochemistry – study of reactions th at
TRANSCRIPT
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Chapter 18 – Electrochemistry – study of reactions th at involve electron transfer.
Oxidation Number – the real or apparent charge of an atom or ion.
What is the Oxidation number of each element in:
Fe
O2
H2
Na
Cl2
Rules for determining oxidation numbers:1) The oxidation number of an uncharged particle m ade of only one element = 0
2) The oxidation number of elements in a compound (2 or more elements) is calculated by adding the charges, so t hat their sum = charge of molecule or ion.
Fe2O3
Na3N
Cr2O7-2
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Cu+2(aq) + Al (s) →
1. Straighten a paper clip, dip it in a test tube of Cu+2(aq), and burn the
solution using a bunsen burner. Note the flame colo r.
2. Cu+2(aq) is a blue color. Add a small piece of aluminum foi l to the test
tube, so that the aluminum is the limiting reactant . Note what happens to the aluminum foil. Roll the aluminum foil up in to a long cylinder, and add this to the copper solution. Note the cha nge of color in the blue solution. Note the formation of brown stuff.
3. Fold a paper towel into 4 layers. Pour just th e clear solution down the sink & place what is left of the aluminum cylinder and the brown stuff on the paper towel. Burn the brown stuff. Note th e flame color.
4. Isolate a clump of the brown stuff. Using a c hemical scoop, push on the brown stuff as hard as you can. Note the appe arance.
5. Clean up
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Oxidation: Loss of electrons OiL RiGReduction: Gain of electrons
Reducing Agent – Reactant that is oxidized.Oxidizing Agent – Reactant that is reduced.
Cr+3(aq) + Mn(s) → Cr(s) + Mn+2
(aq)
Write 2 half-reactions for this equation.
Identify the reaction that is oxidation, and the re action that is reduction.
Identify the oxidizing agent and reducing agent.
Balance the reaction
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Determine which element is oxidized and which is re duced in the following equations:
2Fe2O3(s) → 4Fe(s) + 3O2(g)
2NH3 → N2 + 3H2
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Balancing Redox reactions in acidic solutions:
1. Write ½ reactions of anything oxidized or reduced. Exclude electrons at this time.
2. Balance all elements except H & O
3. Balance Oxygen by adding H 2O
4. Balance H by adding H+
5. Add electrons to each half reaction. Be carefu l of coeffecients.
6. Equalize electrons in both ½ reactions so that t hey cancel.
7. Add up both ½ reactions and check charges.
H+ + Zn(s) + VO3- � VO+2 + Zn+2 + H2O
P4 + IO3- � H2PO4
- + I-
BrO3- + Br - � Br2
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Electrochemical Cell or BatteryAnode: site where oxidation occurs
Cathode: site where reduction occurs
Salt Bridge: Allows half reactions to maintain ele ctrical neutrality. Made up of inert ions such as KCl (aq)
How is this battery recharged?
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1) Eo refers to forward rxn. If you reverse the reaction , change the sign of the voltage.
2) The more positive the voltage, the higher the t endency of the reaction to occur.3) A change in stoichiometric coeffecients does not change the E o value.
Calc. the voltage of the Zn/Cu Battery
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Calculate the voltage produced by : 2Li + F 2 → 2Li+ + 2F-
Is this reaction spontaneous?
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Calculate the voltage produced by Al (s) + Cu+2 → Al+3 + Cu(s)
Is this reaction spontaneous?
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↔
→
Corrosion – redox reaction of metals.
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To protect metals from corrosion:
1. Paint it. This keeps water and air from contac ting the metal2. Alloy (stainless steel)3. Cathodic protection (galvanization) with a sacri ficial anode.
This is used with gasoline tanks buried in the ground, coffins, underground pipes,…