Assume heptane gas is an ideal gas at the relevant temperatures and pressures.
Thursday, April 05, 2007
HW #3, P6 - Hypothetical Process Paths and the Latent Heat of Vaporization - 8 pts
Use the hypothetical process path shown here to help you determine the change in enthalpy in Joules for 20.0 g of heptane (C7H16) as it changes from a saturated liquid at 300 K to a temperature of 370 K and a pressure of 58.7 kPa. Calculate the DH for each step in the path. Do not use tables of thermodynamic properties, except to check your answers. Instead, use the Antoine Equation to estimate the heat of vaporization of heptane at 300 K. Use the average heat capacity of heptane gas over the temperature range of interest.
Assume heptane gas is an ideal gas at the relevant temperatures and pressures.
Assume heptane gas is an ideal gas at the relevant temperatures and pressures.
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6 comments:
How do we find the constants D & E for the Cp equation if the NIST webbook only gives A-C?
wondering:
It is VERY important to note that the constants A, B & C for ANTOINE's Eqn are NOT the same as the constants A, B & C in the Shomate Eqn (Cp polynomial) !!
So, not only can you NOT find D & E for Shomate, the A, B & C that you did find are NOT useful in Shomate !
When you click on the link
"Search for Species Data by Chemical Name"
from the Moodle course page, you can search for heptane and get a variety of properties, but no Shomate coefficients. Instead you will find TWO values for the Cp of heptane vapor in the temperature range that we are interested in. Just take the average of those two values and treat Cp as a constant (kind of like you did in problems 3 & 4, parts b & c).
when you are taking the slope of the ln (pressure) vs 1/T plot, what units is that slope in? Are the units of ln(Pa) still Pa?
Hey boss, where do we get the initial pressure?
Questioning:
The slope has units of Kelvins. Why ? Well, Ln{P2} - Ln{P1} = Ln{P2/P1}
SO:
1- The units of pressure cancel
2- The units you use for pressure do not matter (as long as you use the SAME units for both pressures :)
Parwiz,
The vapor and liquid are saturated at 300 K. So, you can use 300 K in Antoine to determine P1 = Psat(300K).
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