Chemistry 1023 Final E xam Fall 2013 Show your work where appropriate. Data on l

Question

Chemistry 1023 Final E xam Fall 2013 Show your work where appropriate. Data on last page. 1. Name the compound and identify the oxidation number of the underlined element in CrNOsh. Questions 2-4 use the following data: One beaker contains 0.1 M zinc nitrate and a zinc electrode. The other beaker contains 0.05 M aluminum nitrate and an aluminum 55 C. electrode. They are connected to a salt bridge and a voltmeter. The system is at 2. Write a balanced equation for this reaction, and the balanced halfreactions. 3. The standard voltage of this cell is 0.90 V with Al as the anode. What is the voltage of this cell? 4. What is the free energy ofthe system?

Explanation / Answer

(1) Cr(NO3)3 Chromium(III) nitrate

     +3 + 3 x + 9 ( - 2 ) = 0

+3 + 3x - 18 = 0

3x - 15 = 0

3x = + 15

x = + 15/3

x = oxidation state of N = + 5

(2)

Anode Half Cell reaction: Al (s) ----------> Al3+ (aq.) + 3 e-

Cathode Half Cell Reaction: Zn2+ (aq.) + 2e ----> Zn (s)

Cell Reaction: 2 Al (s) + 3 Zn2+ (aq.) ---------> 2 Al3+ (aq.) + 3 Zn (s)

(3) E0 = 0.90 V

T = 273 + 55 = 328 K

Q = [Al3+]2 / [Zn2+]3

Q = (0.05)2 / (0.1)3

Q = 2.5

Applying Nernst equation for the cell reaction,

E = E0 - (RT/n)lnQ

E = 0.90 - (8.314 * 328 / (6 * 96500)) * ln2.5

E = 0.896 V

Change in Gibbs free energy, deltaG = - n F Ecell = - 6 * 96500 * 0.896 = - 518784 J = - 519 kJ

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