ChemE 260 Internal Energy, Enthalpy &

The NIST Webbook

April 5, 2005

Dr. William BaratuciSenior Lecturer

Chemical Engineering Department

University of Washington

TCD 3: A & BCB 2: 9 – 11, Supplement

Internal Energy & Enthalpy• Internal Energy

– Non-nuclear energy stored within molecules– Sum of the vibrational, translational and rotational kinetic

energies– U = strong fxn of T and a weak fxn of P– U sharply as T but U slightly as P .– Ideal Gas, Incompressible Liquids, Solids

• U = fxn(T) only• U fxn(P)

• Enthalpy– H = U + P V dH = dH + d(PV) H = U +

(PV)– H = strong fxn(T)– H = moderate fxn(P)– Ideal Gas: H fxn(P)S

BaratuciChemE 260April 5, 2005

NIST Webbook

BaratuciChemE 260April 5, 2005

Reference State

• We cannot determine an absolute U or H in the way we can determine an absolute T.

• We must choose a reference state andassign = 0 or = 0 at that state.

• Calculate all other values of and relative to the reference state.

• You cannot use thermodynamic data from different sources that are based on different reference states without correcting for the difference in reference state !!

U H

U H

BaratuciChemE 260April 5, 2005

Generating a Saturated Temperature Table

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BaratuciChemE 260April 5, 2005

Saturated Liquid Properties

BaratuciChemE 260April 5, 2005

Saturated Vapor Properties

BaratuciChemE 260April 5, 2005

Extra Info from the NIST Webbook

BaratuciChemE 260April 5, 2005

Example #1

• Determine the Tsat, and of saturated

liquid ammonia at 300 kPa. (Default ref. state)

• Ans.: Tsat = -9.2243 oC

U = 300.25 kJ/kg

H = 300.71 kJ/kg

U H

BaratuciChemE 260April 5, 2005

Example #2

• Determine the , and of butane at

14.696 psia and 77oF in units of Btu, lbm and

ft3. (Default ref. state)

• Ans.: V = 6.5394 ft3/lbm

U = 251.92Btu/lbm

H = 269.71 Btu/lbm

VHU

BaratuciChemE 260April 5, 2005

Example #3

• Determine the , and of a saturated mixture of R-

123 at –40oC and x = 0.30. (Default ref. state, kJ, mole, m3)

• Ans.: Psat = 3.5752 Kpa

Usat liq = 24.660 kJ/mol , Usat vap = 52.800 kJ/mol , Ux=0.03 =

33.102 kJ/mol

Hsat liq = 24.660 kJ/mol, Hsat vap = 54.731 kJ/mol,

Hx=0.03 = 33.681 kJ/mol

Vsat liq = 9.4405 x 10-5 m3/mol, Vsat vap = 0.54014 m3/mol ,

Vx=0.03 = 0.16211 m3/mol

U HV

BaratuciChemE 260April 5, 2005

Next Class

• Heat Capacities– How much does the temperature of 1 mole or kg of a

substance change when 1 J is added ?

• Phase Changes– Latent heats of vaporization, fusion and sublimation

• Hypothetical Process Paths– HPP’s make it easier to calculate how much a property

changes during any real process.

BaratuciChemE 260April 5, 2005