chemical equilibrium questions

[匿名]

2011-03-29 2:05 am

Questions about chemical equilibrium:

All are mutiple choice

1) Which of the following values corresponds to the equilibrium constant K for the dissociation of acetic acid? The standard free energy of formation, G0f of CH3COOH (aq) is -396.46 kJ mol-1 and that of CH3COO- (aq) is – 369.31 kJ mol-1.
(a) 1.75 x 10-5
(b) 3.4 x 10-5
(c) 3.4 x 10-4
(d) 1.75 x 105
(e) None of these is correct
PLZ SHOW WORKING So I can easily understand

2)The standard free energy of formation of the aqueous complex ion Cd(NH3)42+ is -226.1 kJ mol-1. The free energy of formation of NH3 is -26.50 kJ mol-1 and that of the Cd2+ ion is -77.61 kJ mol-1. The value of the stability constant for the complex ion Cd(NH3)42+ is:
(a) 2.14 x 105
(b) 2.84 x 105
(c) 5.28 x 10-5
(d) 2.78 x 107
(e) None of these is correct
PLZ SHOW WORKING So I can easily understand

3)Select all of the following statements which are true about the Gibbs Free Energy
(a) If the equilibrium constant K is doubled, the Go for the reaction is also doubled
(b) If a reaction increases the disorder in the system, then the Gibbs Free Energy in the system must have increased.
(c) If a reaction is exothermic and the entropy increases, the G for this reaction is negative for all temperatures
(d) If the G of a reaction equals 0 then the reaction is at equilibrium
(e) None of these is true

thank you so much!

回答 (1)

己式庚辛

2011-04-06 10:52 am

✔ 最佳答案

please refer to Van't Hoff equation, which relates the free energy change and the equilibrium constant.

for a reversible reaction dG = -RT(lnK)
where dG is the change in free energy, R is gas constant, T is temperature (usually taken as 298K), and lnK is natural-log of eqm. constant.

1.
since energy of formation of H(+) is also needed to calculate the free energy change for reaction, K could not be found.

2.
change in free energy of reaction
= (-226.1) - (-77.61 + -26.50x4) = -42.49 kJ/mol
thus, -42.49x1000 = -(8.314)(298)(lnK)
K = 2.81E7

3.
a is false. for K doubled, free energy is only (ln2)-times greater.
b is false. also consider the enthalpy change.
c is true.
dG = dH -TdS
when reaction is exothermic, dH is negative; when entropy increases, dS is positive, thus -TdS is always negative.

d is false. equilibrium constant only describes a particular reaction, which doesn't imply that any system is in equilibrium.
imagine a reaction A<===> B ; when dG = 0, K = 1, [A]=[B].
what if i add only A to the reaction vessel? it is in equilibrium?

(don't mix up with Arrhenius equation, which relates the activation energy and reaction rate constant)

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