More Problems in Chemical
Equilibrium
True/False
The change in free energy for a reaction:
-
is the driving force for Chemical Change
-
is an extensive property
-
is called DG
-
predicts whether the reaction will proceed forward (DG<
0), backward (DG>0) or is equilibrium (DG=0)
-
DG = DH - TDS
-
depends on the concentration of the reactants and products
The standard free energy change for a reaction
-
is called DG0
-
predicts the equilibrium constant for a reaction through DG0=
-RT ln Keq
-
DGrxn0 = DHrxn0
- TDSrxn0
-
DGrxn0 = DG0formation(Products)
- DG0formation(Reactants)
-
is usually quoted as a per mole (intensive)
All spontaneous reactions that are endothermic must have a positive
change in entropy to be spontaneous
Consider the Entropy of the different Phases of the Same Substance:
-
DSsub0 > 0, DSvaporization0
> 0 , DSfusion0 > 0
-
DSsub0 > DSvaporization0
>> DSfusion0
-
At equilibrium between phases DG =0 , so DHfusion
/
Tmelt = DSfusion ,
for example
The standard free energy change for a reaction is -298.0
kJ/mol and the actual free energy change for the reaction conditions is
zero. The reaction is therefore
-
at equilibrium
-
has a large equilibrium constant
-
will be shifted from equilibrium if the temperature is changed
Consider the following reaction:
C(g) + 2H2(g) <--> CH4(g)
at a temperature where
S0 [C(g)] = 5.69 J/mol.K
S0 [H2(g)] = 130.58 J/mol.K
S0 [CH4(g)] = 186.19 J/mol.K
What is DS0 for the above reaction
at the same temperature in J/mol.K?
ANSWER: DS0
= -80.7 J/K.mol
Consider the reaction
CO(g) + H (g) -> H2CO(g) ,
where:
DHrxn0 = -5.36 kJ/mol
DSrxn0 = -109.8 J/mol.K
What is DG0 in kJ/mol at 300 K?
ANSWER:
27.6 kJ/mol
What is the equilibrium constant for the reaction?
ANSWER:
1.58 x 10-5
Of the following materials, predict the ordering of the molar entropy
-
salt water
-
pure water
-
powdered sugar
-
salt
Predict whether DSrxn is positive
or negative for each of the following processes.
H2 (g) + 1/2O2 (g) -> H2O(g)
2Cl2O7(g) -> 2Cl2(g)
+ 7O2(g)
H2O(g) -> H2O(s)
Sugar(s) -> Sugar(aq)
For the reaction
CO(g) + 2H2(g) -> CH3OH(g)
Kp = 91.4 at 350 K and Kp = 2.05 x 10-4
at 298 K.
What is the value of DHrxn0
?
ANSWER:
216.9 kJ/mol
using: ln(K2/K1) = -DH/R
(1/T2 - 1/T1)
Consider the reaction:
AB(g) <--> A(g) + B(g)
Keq = 0.729 and DH0
= 45.9 kJ/mol at 20 oC
What is Keq for this reaction at 377 K?
ANSWER:
Keq = 48.5
using: ln(K2/K1) = -DH/R
(1/T2 - 1/T1)
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