Question

Pure phospholipid membranes composed of the lipid dipalmitoyl phosphatidyl choline (abbreviated as DPPC) undergo a melting phase transition at T = 41˚C (T = 314 K). In a reversible melting process, the enthalpy of melting is found to be ∆H_m = 9 kcal/mol. Under constant pressure conditions, the enthalpy of reversible melting is equal to the reversible heat change (q_rev) for the process. The entire melting process occurs at the melting temperature. Under these conditions, the change in entropy upon melting is just given by q_rev/T.

a) Calculate the entropy of melting, ∆S_m, in units of Joules/Kelvin-mole.

b) Calculate the Gibbs free energy of melting, ∆G_m, in units of Joules/mole.

c) Explain why the answer to part b should have been expected.

Answer 1

a)

according to the given conditions,

               ∆S_m = q_rev/T.

q_rev   = 9 kcal/mole

T= 41 degree celcius= 314 kelvin

delta S= 0.02867 kcal/mole-kelvin

conversion of kcal to joules:

1 kcal= 4184 J

0.002867 kcal= 11.995 J

answer= 11.995 joules/mole-kelvin

b)

delta G = delta H - T delta S\

plugging in values;

delta H in joules= 9*4184= 37656 Joules

delta G= 33889.57 Joules/moles

c)

the answer in part b) is expected because delta H value is high but delta S value is not so high to make overall delta G value negative.