Hunter College of the City University of New York Department of Chemistry & Bioc

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Hunter College of the City University of New York Department of Chemistry & Biochemistry For question 24-25 consider that: The student was measuring the pH at various temperatures, the Kb for that chemical was found to be temperature dependent, yielding the values in the table. Temperature (°C) Kb 10 30 1.34 x 10 1.5 x 10 3 24. Using the data provided estimate H° for this chemical a. -4.0 kJ/mol b. 4.0 kJ/mol c. 140 k.J/mol d. -140 kJ/mol 25. Using the data provided estimate S° for this chemical a. +79 J b. -79 J c. -107J d. 107J Creditpts) a ofbiochemicalules, we can mode dprotein ttemperatures and Extra CreditCopts) 26. A very active area of biochemical research is the study of how proteins fold. Although proteins are complex molecules, we can model the folding of a protein in an aqueous solution as a simple equilibrium problem: Folded protein -> Unfolded Protein The equilibrium constant for this reaction was measured at different temperatures and the following data were collected 0.983 1.07 1.15 1.24 141 1.59 10 temperature Which of the following statements about this reaction is correct? 20 30 40 60 80 CHEM104_Midas Spring 2016 Exam 2_A

Explanation / Answer

24) we know that

Delta H = 2.303 R (logK1- logK2 / [1/T2-1/T1])

Delta H = 2.303 X 8.314 X (log 1.34 X 10^-5 - 1.5 X 10^-5) / (1/303 - 1/ 283)

Delta H = 2.303 X 8.314 X (-4.873 +4.823) / (0.0033-0.0035)

Delta H = -0.96 /- 0.0002 = 4800 J / mole = 4 KJ / mole

25) DeltaG = -RTln Kb = -8.314 X 303 X 2.303 X log 1.5 X 10^-5 = 27981 J / mole =

Delta G = Delta H - T Delta S

27981 = 4800 - 303 X DeltS

Delta S = -79.1 J

26) we can solve the problem graphically

Let us plot a graph between 1/T and lnK

Linear equation is

y = -683.79x + 2.3991

slope = -Delta H / R

So Delta H = -Slope X R = 683.79 / R = positive (Delta H > 0)

Intercept = Delta S / R

2.3991 = Delta S / R

Delta S = 2.3991 X R= positive (Delta S >0)

K T T (K) 1/T lnK 0.983 10 283 0.003534 -0.01715 1.07 20 293 0.003413 0.067659 1.15 30 303 0.0033 0.139762 1.24 40 313 0.003195 0.215111 1.41 60 333 0.003003 0.34359 1.59 80 353 0.002833 0.463734

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