Chemistry 223 Summer 2002

Worksheet 3 July 17, 2002 Dr. Nafshun

Course ID Number ______Score ______

1. Write a balanced chemical equation for an (use a text as a source of possible reactions):

(A) Exothermic process in which entropy increases.

Combustion of octane: 2 C8H18 (l) + 25 O2 (g) → 16 CO2 (g) + 18 H2O (g)

(B) Exothermic process in which entropy decreases.

Combustion of methane: CH4 (g) + 2 O2 (g) → CO2 (g) + 2 H2O (l)

Ice melting

(D) Endothermic process in which entropy decreases.

The reverse of 2 C8H18 (l) + 25 O2 (g) → 16 CO2 (g) + 18 H2O (g)

2. Balance C4H10 (g) + O2 (g) → CO2 (g) + H2O (l). Without using thermodynamic data from the appendix, comment on spontaneity.

3. Identify ΔH, ΔS, and ΔG as being (-), (+), or (?) for:

(A) H2O(s) → H2O(l); H2O(g) → H2O(l); NaCl(s) → NaCl(aq)

(B) 2 C8H18(l) + 25 O2(g) → 16 CO2(g) + 18 H2O(g)

4. Balance CH4(g) + O2(g) → CO2(g) + H2O(g). Calculate (see Appendix for thermodynamic values) ΔHºreaction, ΔSºreaction, ΔGºreaction, and comment on spontaneity.

ΔHºreaction, -802.3 kJ

ΔSºreaction, -5.2 J/K

ΔGºreaction, -804 kJ (spontaneous)

5. What is the relationship between K and ΔG? Explain for cases when K is really large, really small, and 1.00.

ΔG = -RT lnK

Large K ΔG = (-)

Small K ΔG = (+)

K = 1.00 ΔG = 0 (equailibrium)

6. Write the Ksp expression for magnesium phosphate.

Ksp = [Mg2+]3[PO43-]2

7. Determine the solubility of calcium fluoride (mol/L).

Ksp = 5.3 x 10-9 = [Ca2+][F-]2 = (x)(2x)2 = 4x3

x = [CaF2] = 1.1 x 10-3 M

8. A solution was made 3.22 x 10-5 M in [Mg2+] and

7.30 x 10-3 M in [CO32-]. Will a precipitate form?

Q = 2.35 x 10-7 (greater than Ksp of 3.5 x 10-8) so a ppt will form.

9. The solubility of PbF2 is 2.09 x 10-3 mol/L. What is the concentration of Pb2+? What is the concentration of F-? Calculate Ksp. 3.7 x 10-8

10. Consider the production of water from the elements. Do you expect ΔH to be positive or negative? Do you expect ΔS to be positive or negative? Do you expect ΔG to be positive or negative? Does a decrease or increase in T change spontaneity? Explain.

11. Discuss the four cases involving the different combinations of enthalpy and entropy. Give an example for each case. See question 1.