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

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

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

Determine which element is oxidized and which is re duced in the following equations:

2Fe2O3(s) → 4Fe(s) + 3O2(g)

2NH3 → N2 + 3H2

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

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?

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?

Calculate the voltage produced by Al (s) + Cu+2 → Al+3 + Cu(s)

Is this reaction spontaneous?

↔

→

Corrosion – redox reaction of metals.

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,…