CEE 680

Fall 2011

Homework Set #2

2 points total

 

1.  Thermodynamics #1

Based on only the following information, determine the standard free energy change DG° and K value at 25°C and 1 atm for the dissociation of carbonic acid,  H2CO3 = H+ + HCO3-.

Available information:

 

       Reaction 

  K  (@25°C, 1 atm)

 

H2CO3 = CO2(aq) + H2O

          630

 

 

 

DG°f  (@25°C, 1 atm)

 

Species

        (kJ/mol)         .

 

CO2(aq)

        -386.22

 

H2O

         -237.18

 

H+

              0.0

 

HCO3-

         -586.85

 

1 point

 

Solution to 1:

First we determine DG for the hydrolysis reaction:

 

Then using this information we can determine DG of formation for carbonic acid

 

Finally we use this to get the DG for the first proton loss reaction:

 

And the corresponding K:

or:

 

 

2.  Thermodynamics #2

Estimate the value of Kw at 20°C and 1 atm.  Assume that at 25°C and 1 atm, the following is true:

 

       Equation

Equilibrium Constant (Kw)

 

H2O = H+ + OH- 

     1.01 x 10-14

 

 

Species

   H°f  (kJ/mol) 

 

   H+ 

            0

 

  OH- 

       -230.0

 

 H2O

       -285.83

 

1 point

 

Solution to 2

First we calculate the DH at 20°C:

 

Then assuming DH remains constant with small changes in temperature, we use:

or: