Comments re Chapter 6 Problems:
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6-2 |
Some T/F questions about chemical potentials. |
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6-3 |
An exercise in calculating equilibrium constants in several forms. Please remember that the only form of the equilibium constant that can be used in the formula for DGo and referred to values for DfGo in standard tables is Kp°. The value Kc° refers to the equilibrium constant as calculated from concentrations with each concentration being divided by a standard concentration of 1 mol/L. The concentrations in mol/L of perfect gases are just c = n/V = P/RT. (Some of this is a review of material from General Chemistry). |
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6-6 |
An exercise in determining whether or not a system is at equilibium with respect to a specific chemical reaction, i.e., the calculation and interpretation of the quantity Qp (same form as the equilibium constant but the gas pressures do not necessarily correspond to a set of equilibrium pressures). If Qp is equal to Kp then the reaction is at equilibrium; if Qp is less than Kp then the reaction must go forward (more reactants must be converted to products) for the reaction to come to equilibrium. If Qp is greater than Kp then the reaction must go backwards for the reaction to come to equilibrium. (Review of material from General Chemistry). |
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6-9 |
A T/F question about equilibrium constants. (Most of this material was covered in General Chemistry). |
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6-12 |
Use data in Tables to find Kp° at 298 K and then to estimate the value at 400 K. |
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6-14 |
The value of Kpo is given as a mathematical function of T. Find DGo using the standard equation DGo = -RTln Kop. Find DHo by using the relationship d(ln Kpo)/dT = DHo/RT2, and DCpo from d(DHo)/dT. Finally, get DSo from DS = (DH - DG)/T. At the end calculate DHo at two temperatures. The two values differ, but only by less than 6%, so the assumption implicit in the usual equation for the variation in Kpo with T that DHo does not vary is reasonable. (The variation in DHo is small because DCpo is usually much smaller than DHo). Levine's notation "(T/K)", where K means units of Kelvin, can be confusing; it is probably easiest to just ignore all the factors "/K" and to remember that temperature must be given in degrees Kelvin. |
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6-15 |
(Calculaton only). Shows how the temperature dependence of DHorxn affects the equation for the variation of the equilibrium constant with temperature. This derivation assumes that the Cpo values for all substances are independent of T, but remember that this assumption is less important than the assumption that DHorxn is independent of T. In any event DCpo changes only very slowly with T. This problem uses the same chemical reaction and the same temperature as problem 6-14, so comparisons are possible. Note that Kp values seldom have more than 2 significant digits; uncertainties of 20% or more are common. Please do not memorize this equation! |
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6-21 |
Short T/F exercise. |
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6-32 |
An exercise that relates the magnitude of the equilibrium constant to the form of the balanced reaction. If the relationship is not obvious write out Kp° for each of the reactions and compare their forms. (Review of material from General Chemistry). |
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6-35 |
A demonstration of how a modest error in the DrxnGo value can make a very significant difference in the value of the corresponding Kpo. Write Kp° as DrxtG°±error and remember that exp(a+b) = (exp(a))(exp(b)). |
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6-46 |
A question that can be answered if Le Chatelier's principle is understood. This principle says that if an equilibrium system is “stressed” the system will adjust to (partially) relieve that stress. So adding a reactant results in production of more product, raising the pressure favors the side of the reaction with the fewer number of gas molecules, and raising the temperature favors the direction of the reaction that is endothermic. (Review of material from General Chemistry). |
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6-53 |
Very short question. (Review of material from General Chemistry). |
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6-59 |
For what kind of reaction is the equilibrium position independent of pressure? (For all other reactions the equilibrium position does depend on the sum of the Pi's). For what kind of reaction is Kpo independent of temperature? |
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6-25 |
Use data in a Table to calculate DrxnGo, then compute Kpo and finally do the equilibrium problem by the method of successive approximations. The reaction itself is interesting. |
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6-26 |
Two more equilibrium problems, both based on the same chemical reaction, to be solved by the method of successive approximations. |
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