Calculation of enthalpy and entropy of fusion of an unknown solid..?
The molar volume of a certain solid is 142.0 cm3/mol at 1.00 atm and 427.15 K, its melting temperature.
The molar volume of the liquid at this temperature and pressure is 152.6 cm3/mol.
At 1.2 MPa the melting temperature changes to 429.26 K. Calculate the enthalpy and entropy of fusion of the solid.
The molar volume of a certain solid is 142.0 cm3/mol at 1.00 atm and 427.15 K, its melting temperature.
The molar volume of the liquid at this temperature and pressure is 152.6 cm3/mol.
At 1.2 MPa the melting temperature changes to 429.26 K. Calculate the enthalpy and entropy of fusion of the solid.
1 Answer
Explanation:
Your tool of choice for this problem will be the Clapeyron equation used in the form
color(blue)((dp)/(dT) = (DeltaS_"fus")/(DeltaT_"fus"))" " " "color(purple)((1))
Now, you know that the following relationship exists between the enthalpy change of fusion,
color(blue)(DeltaS_"fus" = (DeltaH_"fus")/T)" " " " color(purple)((2))
This is derived from the Gibbds free energy change at equilibrium
DeltaG = DeltaH - T DeltaS
Since at equilibrium
DeltaH = T * DeltaS implies DeltaS = (DeltaH)/T
Now, in your case,
T_"average" = ("427.15 K" + "429.26 K")/2 = "428.21 K"
Now, you should rearrange equation
You can get away with such an approximation because you're operating on the solid - liquid phase line, so you're bound to have small changes in temperature in such cases.
Now, if you take this route, you can say that
(dp)/(dT) ~~ (DeltaP)/(DeltaT) = (DeltaS_"fus")/(DeltaV_"fus")
At this point, you have everything you need to solve for
DeltaT = T_2 - T_1 = "2.11 K"
Here comes the tricky part - you need to convert
DeltaV_"fus" = V_2 - V_1 = "152.6 cm"^3 - "142.0 cm"^3 = "10.6 cm"^3
This means that you have
10.6color(red)(cancel(color(black)("cm"^3)))/"mol" * "1 m"^3/(10^6color(red)(cancel(color(black)("cm"^3)))) = 10.6 * 10^(-6)"m"^3"/mol"
Finaly, you have
DeltaP = 1.2 * 10^6"Pa" - 1.01325 * 10^5"Pa" = 10.987 * 10^5"Pa"
So, plug in these values and solve for
DeltaS_"fus" = (DeltaP)/(DeltaT) * DeltaV_"fus"
DeltaS_"fus" = (10.987 * 10^5"Pa")/"2.11 K" * 10.6 * 10^(-6)"m"^3/"mol"
This will be equal to
DeltaS_"fus" = 55.195 * 10^(-1)"Pa m"^3/"K mol"
But
DeltaS_"fus" = color(green)(5.52"J"/"mol K")
Now use equation
DeltaH_"fus" = DeltaS_"fus" * T_"average"
DeltaH_"fus" = 5.52"J"/("mol" * color(red)(cancel(color(black)("K")))) * 428.21color(red)(cancel(color(black)("K"))) = 2363.72 "J"/"mol"
I'll leave this value rounded to three sig figs as well, but expressed in kilojoules per mole
DeltaH_"fus" = color(green)("2.36 kJ/mol")