Consider the hydrolysis of ethyl acetate (CH3CO2CH2CH3) to ethanol (CH3CH2OH) an
ID: 900017 • Letter: C
Question
Consider the hydrolysis of ethyl acetate (CH3CO2CH2CH3) to ethanol (CH3CH2OH) and acetic acid (CH3 CH2CO2H): CH3CO2CH2CH3 + H2O = CH3CH2OH + CH3 CH2CO2H Experiments in acid solution show a value of 14 for the equilibrium constant, written as Keq = [CH3CH2OH][CH3 CH2CO2H]/[CH3CO2CH2CH3] = 14
What is the value of rGº calculated from this equilibrium constant?
b. Now consider the acid dissociation reaction CH3 CH2CO2H = CH3 CH2CO2 - + H+ Given that the pKa for acetic acid is 4.60 at 25ºC, what is rGº for this reaction?
c. Finally, consider the reaction CH3CO2CH2CH3 + H2O = CH3CH2OH + CH3 CH2CO2 - + H+ This is the sum of the preceding 2 reactions. What is the overall rGº for this sum reaction?
d. What is rG for this reaction at pH=7, i.e. under non-standard-state conditions? (pH=7 is known as the biochemists’ standard state for [H+], as opposed to chemists’ standard state. Your answer to this part should be close to the value given by biochemistry textbooks of rG´=-20.1 kJ mol-1 ).
Explanation / Answer
a)
Take T =298 K
rGo = -R*T*ln Kc
= -8.314*298* ln (14)
= -6538.5 J
Answer: -6538.5 J
b)
T= 25oC = (273+25) K = 298 K
use:
pKa = -log Ka
5.6 = - log Ka
Ka = 2.512*10^-6
rGo = -R*T*ln Ka
= -8.314*298* ln (2.512*10^-6 )
= - 31947 J
Answer: - 31947 J
c)
rGo will add up
rGo = -6538.5 J - 31947 J
= -38485.5 J
d)
something missing here. Question is not clear. Initial concentration or Keq whould be given