Question 4: Balancing Redox Equations (4.5 points)
a. The following reaction takes place in an acidic solution.
MnO4–(aq) + Cl–(aq) Mn2+ + Cl2(g) (unbalanced)
i. Write the reduction and oxidation half-reactions (without electrons). (.5 point)
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ii. Balance the equations for atoms (except O and H). (.5 point)
iii. Balance the equations for atoms O and H using H2O and H+. (.5 point)
iv. Balance the charge in the half-reactions. (.5 point)
v. Multiply each half-reaction by the proper number to balance charges in the reaction. (.5 point)
vi. Add the equations and simplify to get a balanced equation. (.5 point)
b. Assume a reaction takes place in a basic solution to form the given products:
MnO4–(aq) + Cl–(aq) MnO2(s) + Cl2(g) (unbalanced)
i. Balance the given half-reactions for atoms and charge. (.5 point)
MnO4– + H2O MnO2 + OH–
Cl– Cl2
ii. Multiply to balance the charges in the reaction. (.5 point)
iii. Add the equations and simplify to get a balanced equation. (.5 point)
Question 5: Galvanic Cell (3 points)
a. Give an example of a galvanic cell. What kind of reaction occurs in a galvanic cell? (1 point)
b. If one electrode in a galvanic cell is made of zinc (Zn) and one is made of silver (Ag), which metal would be the cathode and which would be the anode? Explain your answer. (1 point)
c. Write the standard cell notation for a galvanic cell with a silver cathode and a zinc anode. (1 point)
Question 6: Electrolytic Cell (3 points)
a. What kind of reaction occurs in an electrolytic cell? What characterizes its voltage? (1 point)
b. What makes an electrolytic cell work? Name two applications of electrolytic cells. (1 point)
c. If a piece of jewelry were electroplated with gold for 25 seconds at 1.5 A, how many grams of gold would be plated?
(Note: 1 A = 6.241 1018 e–/s. The reaction for the reduction of gold ions to gold metal is Au+ + 1e– Au(s).) (1 point)
Question 7: Stoichiometry (3 points)
a. How is stoichiometry used to calculate amounts of substances in a chemical reaction? (1 point)
b. Use the balanced equation to answer the following questions. (2 points)
CuSO4(aq) + 2NaOH(aq) Cu(OH)2(s) + Na2SO4(aq)
i. What is the ratio of moles of CuSO4 to moles of NaOH? (.5 point)
ii. If 638.44 g CuSO4 reacts with 240.0 NaOH, which is the limiting reagent? (.5 point)
iii. Use the limiting reagent to determine how many grams of Cu(OH)2 should precipitate out in the reaction. (.5 point)
iv. If only 174.6 g of Cu(OH)2 precipitate were actually collected from the reaction, what would the percent yield be? (.5 point)
Question 8: Scientific Themes (3 points)
a. What does matter have to do with chemistry? How are forces involved? (.5 point)
b. How do chemists use systems and order in their work? (.5 point)
c. How do atoms demonstrate unity and diversity at the same time? (.5 point)
d. How are models used in chemistry? How does evidence change these models? (.5 point)
e. How do structure and function affect how molecules work? (.5 point)
f. How are both stability and change seen in properties of elements? (.5 point)
