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Oxidation and Reduction.

1. Define the following concepts:
a) oxidizing agent
b) oxidized substance
c) reducing agent
d) reduced substance

2. Determine the oxidation numbers of the underlined elements in the following formulas:
a) KClO3
b) NH4+
c) Cr2O72-
d) MnO42-

3. Balance the following equations by the half-cell method. Show both half-cell reactions and identify them as oxidation or reduction.
a) SO32- + MnO4- + H+ ←→ Mn2+ + SO42- + H2O(l)
b) Cl2(g) + OH- ←→ Cl- + ClO3- + H2O(l)
c) SO42- + I- + H+ ←→ S2- + I2(s) + H2O(l)

4. Observations
i) A reacts spontaneously with 1 mol/L BNO3, 1 mol/L D(NO3)2 and dilute sulfuric acid. A does not react with 1 mol/L C(NO3)2.
ii) B does not react spontaneously with any of the 1 mol/L solutions above or with dilute sulfuric acid.
iii) C reacts spontaneously with dilute sulfuric acid and with 1 mol/L solutions of all the other metallic salts.
iv) D reacts spontaneously with 1 mol/L BNO3. It does not react with dilute sulfuric acid.

a) Use the observations and arrange the following five reduction half-cell reactions in order, the one with the largest positive reduction potential listed first.
A2+ + 2e- → A(s)
B+ + e- → B(s)
C2+ + 2e- → C(s)
D2+ + 2e- → D(s)
2H+ + 2e- → H2(g)
b) Which metal is the best reducing agent?
c) Which ion is the best oxidizing agent?

5. Calculate the voltage of a standard cell with the following half-cell reactions:
Ni(s) → Ni2+ + 2e-
2e- + Cl2(g) → 2Cl-

6. a) What metal might you fasten to the hull of an aluminum boat to give it cathodic protection in sea water?
b) Name two metals you would not use. Explain the choices.

7. By the electrolysis of water, 11.2 L of oxygen at STP was prepared.
a) What charge was required?
b) If a current of 0.5 A was used, how long did it take?

8. Aluminum is one of the most widely used metals. Aluminum is made by electrolyzing aluminum oxide in a solvent called cryolite.
a)Identify the products of the anode and cathode half-reactions that occur during the electrolysis of aluminum oxide.
b) Why is it important to recycle as much aluminum as possible?
c) Identify one environmental concern and one health concern associated with the product of aluminum.

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Solution Preview

1) Oxidizing agent: the substance in an oxidation-reduction reaction that gains electrons and whose oxidation number is reduced

In a reaction, the reductant or reducing agent loses electrons and is the oxidized substance

A reducing agent (also called a reductant or reducer) is the element or a compound in a redox (reduction-oxidation) reaction (see electrochemistry) that reduces another species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox. For example consider the following reaction:
2Mg(s) + O2 → 2Mg2+(s) + 2O2-
The reducing agent in this reaction is magnesium. Magnesium donated its two valence electrons, has become an ion, and allows itself as well as oxygen to become stable.

In a reaction the oxidant or oxidizing agent gains electrons and is the reduced substance.

2) a) KClO3 => 1+ + x + 3 x 2-
1+ + x + 6-
1+ + x + 6- = 0
Cl = x = 5+

b) NH4+ => x + 4 x 1+
x + 4+
x + 4+ = 0
N = x = 4-

c) Cr2O72- => x + 7 x 2-
x + 14-
x + 14- = 0
Cr = x = 14+

d) MnO42- => x + 4 x 2-
x + 8-
x + 8- = 0
Mn = x = 8+

3) Step 1 Divide the skeleton equation into half-reactions.

Step 2 Balance atoms other than H and O.

Step 3 Balance oxygen atoms by adding H2O to the side that needs O.

Step 4 Balance hydrogen by adding H+ to the side that needs H.

Step 5 Balance the charge by adding electrons.

Step 6 Make the electrons gained equal to the ...

Solution Summary

The solution looks at oxidation and reduction reactions.