ANSWERS TO PRACTICE PROBLEMS
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5. Calculate DH for each of the following reactions. Be sure
to show sign (positive or negative) and per mole of which chemical.
a. When 0.070 mole Al is oxidized 28 kcal of heat is released.
DH = -400 kcal/mole of Al
b. When 20.0 g of MgCl2 is converted to Mg and Cl2, 32.2 kcal of
heat is required.
DH = +153 kcal/mole of MgCl2
c. When .335 g of K reacts with excess Cl2 to make KCl, enough heat is
released to change the temperature of 360 g of water from 18.0EC
to 19.3EC.
DH = -54.6 kcal/mole of K
6. Use the information in Example 2a to answer the following questions.
a. How much heat is released when 6.0 mole CH4 is burned?
1300 kcal (1278 kcal)
b. How much heat is released when 4.2 g CH4 is burned?
56 kcal
c. How much CH4 must be burned to provide 55 kcal of heat?
0.26 moles of CH4
d. A person wants to heat the water in a hot tub (320 kg) from 70°F
(21°C) to 100°F (38°C) using a natural gas (mostly CH4) burner. Assuming all
the heat from the reaction goes into the water, how much CO2 will be put into
the atmosphere?
26 moles of CO2
11. Equilibrium Constant Expressions
a. Equilibrium constant expression for the reaction
N2(g) + 3 H2(g) Û 2 NH3(g).
|
[NH3]2 |
| Keq = |
)))))))))))) |
|
[N2] x [H2]3 |
Where [ ] represents equilibrium concentrations. Notice that substances on right of
equation go on top of the expression and coefficients in equations become exponents.
Concentrations are multiplied together.
b. Write the equilibrium constant expression for:
2 A + B2 Û A2B + B
|
[A2B] x [B] |
| Keq = |
)))))))))))) |
|
[A]2 x [B2] |
12. Calculating Equilibrium Constant Values
a. Calculation of Keq for reaction 2 A + B Û A2B
if at equilibrium:
[A] = 0.50 M [B] = 1.0 M
[A2B] = 2.0 M
| Keq = |
[A2B]
)))))))
[A]2[B] |
= |
(2.0)
)))))))))))
(.50)2(1.0) |
= 8.0 |
b. Calculate the numerical value of Keq for the reaction
X + 3 Y Û XY3
if at equilibrium [X]=0.1 M, [Y]=2.0 M, and [XY3]=0.4 M.
| Keq = |
[XY3]
)))))))))
[X] x [Y]3 |
= |
(0.4)
)))))))))))
(0.1)(2.0)3 |
= 0.5 |
14. Practice with equilibrium constants:
a. Write the equilibrium constant equation and calculate the value of the equilibrium
constant for this reaction:
Ag+ + 2 NH3 Û Ag(NH3)2+
[Ag+] = 3.0 x 10-11 M
[NH3] = 1.0 M
[Ag(NH3)2+] = 5.0 x 10-4 M
| Keq = |
[Ag(NH3)2+]
)))))))))
[Ag+] x [NH3]2 |
= |
(5.0x10-4)
)))))))))))
(3x10-11)(1.0)2 |
= 1.7x107 |
b. For the reaction A + B Û C, which of the following cases
is not at equilibrium (given that only one is not)?
Case I: [A] = 5 [B] = 2 [C] = 1
Case II: [A] = 2 [B] = 5 [C] = 1
Case III: [A] = 4 [B] = 5 [C] = 2
*Case IV: [A] = 1 [B] = 5 [C] = 2
15. Practice problems: Equilibrium concentrations
a. For the reaction A(g) + B(g) Û AB(g), Keq =
3.5.
If at equilibrium [A] = .1 M and [B] = 2 M, calculate [AB].
[AB] = 0.7 M
b. The Ksp for AgCl is 1.6 x 10)10.
If Ag+ and Cl) are both in
solution and in equilibrium with AgCl. What is [Ag+] if [Cl)] = 0.0050 M?
[Ag+] = 3.2x10-8 M
c. For the reaction PCl5 Û Cl2 + PCl3,
the equilibrium constant value at a particular temperature is 9.0 x 10-2. What
is the concentration of PCl5 if [Cl2]=0.020 M and [PCl3]=0.30
M?
[PCl5] = 0.067 M
d. In the same reaction, and at the same temperature, calculate what concentration PCl5
would have to be if the concentrations of both Cl2 and PCl3 were
doubled.
[PCl5] = 0.27 M
