A detailed approach

Presented by
 Ahmed Asad – CIIT/SP10-BEC-003/LHR
 Muhammad Usama – CIIT/SP10-BEC-017/LHR
 Mohammad Abubakar – CIIT/SP10-BEC-022/LHR
 Noaman Ahmed – CIIT/SP10-BEC-037/LHR
 Saad Wazir – CIIT/SP10-BEC-043/LHR
 Saim Khan – CIIT/SP10-BEC-044/LHR
 Waqar Farooq – CIIT/SP10-BEC-050/LHR

Presentation Outline
 Restatement of first law of thermodynamics
 Definition of enthalpy
 Some common enthalpy changes
 Enthalpy of vaporization
 Characteristics of enthalpy of vaporization
 Physical model for vaporization
 Experimental determination
 Sample readings and calculations
 Applications

First Law of Thermodynamics
 Energy conservation law
 Describeschange in internal energy of a
thermodynamic system
 Clausius’ statement: In a thermodynamic process, the
increment in the internal energy of a system is equal to
the difference between the increment of heat
accumulated by the system and the increment of work
done by it.

First Law of Thermodynamics (contd.)
 In any incremental process, the change in the internal
energy is considered due to,
 Heat added to the system
 Work done by the system
dU = dQ - dW

First Law of Thermodynamics (contd.)
 For a quasistatic process (infinitely slow process),
dU = dQ – PdV
 NO real process is quasistatic
 A quasistatic process ensures that the system will go
through a sequence of states that are infinitesimally close
to equilibrium (so the system remains in quasistatic
equilibrium), in which case the process is typically
reversible

Quasistatic and Reversibility
 Any reversible process is a quasistatic process
 Any quasistatic process may not be reversible
 Due to heat flow
 Due to entropy generation
 Example of an irreversible quasistatic process
 Compression against a system with a piston subject to
friction

Enthalpy
 Measure of total energy of a thermodynamic system
 A state function
 Includes
 Internal energy (energy required to create a system)
 Amount of energy required to establish system’s pressure and
volume
ΔH = ΔU + Δ(PV)
 SI Unit – Joule
 Other conventional units – Btu and Calories

Why Enthalpy is measured?
 Total enthalpy of a system can’t be measured directly
 Enthalpy change of a system is measured instead
 It is measured to,
 Calculate “useful work” obtainable from a closed
thermodynamic system under constant pressure
 Determine nature of reaction e.g., exothermic or
endothermic

Enthalpy is not necessarily heat !!
 Enthalpy is sometimes described as heat content of a
system
 Heat is defined as thermal energy in transit
 For the description that enthalpy is in-fact heat to be
valid, no energy exchange must occur with
environment other than heat or expansion work

Common Enthalpy Changes
 Enthalpy of reaction
 Enthalpy of formation
 Enthalpy of combustion
 Enthalpy of neutralization
 Enthalpy of solution
 Enthalpy of vaporization
 Enthalpy of sublimation

Vaporization
 Phase transition from liquid phase to gas phase
 Two types
 Evaporation
 Occurs at temperatures below boiling temperature
 Usually occurs on surface
 Boiling
 Occurs at or above boiling temperature
 Occurs below the surface

Enthalpy of Vaporization (EOV)
 Enthalpy change required to completely change the
state of one mole of substance between liquid and
gaseous states
 Energy required to transform a given quantity of a
substance from a liquid into a gas at a given pressure
 Usually measured at boiling point of a substance

Characteristics of Enthalpy of
Vaporization
 It is temperature dependent
 EOV decreases with increase in temperature
 EOV diminishes completely at critical temperature
beyond which liquid and vapor phase no longer co-
exist
 Units – J/mol or kJ/mol, kJ/kg, Btu/lb, kcal/mol

Characteristics of Enthalpy of
Vaporization (contd.)
 Enthalpy of condensation is same as enthalpy of
vaporization but with opposite sign
 Enthalpy change of vaporization is always positive
 Enthalpy change of condensation is always negative

Temperature dependence of EOV

Physical model for vaporization
 Proposed by professor Jozsef Garai, Florida International
University, USA
 Energy required to free an atom from liquid is equivalent to
energy required to overcome surface resistance of liquid
 This model states,
Latent heat = (Max. surface area) x (Surface tension) x (No. of
atoms in liquid)

Physical model for vaporization
(Diagrammatic representation)

Experimental determination

Apparatus
 Round bottom boiling flask
 Distillation condenser or multiple condensers
 Heat source (a burner or a heating mantle)
 A vacuum gauge (Bourdon type gauge)
 Aspirator or trapped vacuum pump
 Pressure-regulating device (a needle valve that is part of a Bunsen
burner base)
 Thermometer

Basic Goal
 To determine boiling point of the liquid (water) under
study at different pressure values
 To determine enthalpy of vaporization using the
Clausius-Clapeyron relation

Clausius-Clapeyron relation
 A relation used to characterize a discontinuous phase
transition between two phases of a single constituent
 On a P-T diagram, line separating two phases is known
as coexistence curve
 This relation gives the slope of the tangents to this
curve

Clausius-Clapeyron relation (contd.)
 General form
dP/dT = L/TΔV
 Where
 dP/dT is slope of tangent to coexistence curve at any point
 L is latent specific heat
 T is temperature
 ΔV is specific volume change of phase transition
 For transitions between a gas and condensed
phase, the expression may be rewritten as,
ln(P) = (-L/R) x (1/T) + C

Procedure
 Maintain lowest possible pressure by closing bleed
valve
 Set water flow to the aspirator at maximum level to
provide highest vacuum
 Place few boiling stones in round bottom flask to
minimize bumping

Procedure (contd.)
 Temperature increases until boiling starts
 When boiling occurs, allow the thermometer reading
to stabilize for 1 to 2 minutes and note the temperature
 Note the pressure reading on the manometer at this
temperature

Procedure (contd.)
 Increase the pressure of the vessel by slightly opening
the bleed valve
 Repeat the same procedure as described previously
and then increase pressure again
 Take at least five readings and plot a graph between
reciprocal of temperature and log of pressure
difference

Sample Readings
Temp (°C) h (mm Hg) Temp (K) (1/T) x 103 P (mm Hg) Log P
(1/K) (P = 768 –h)
41.5 710 314.5 3.18 58.0 1.77
62.5 610 335.2 2.98 158 2.20
75.0 500 348.0 2.87 268 2.43
86.5 300 359.5 2.78 468 2.67
92.2 220 365.2 2.74 548 2.74
101.0 0 374.0 2.67 768 2.89

Graph between temperature and
pressure
Temperature and Log P
3.5
3
2.5
2
Log P
1.5
1
0.5
0
2.6 2.7 2.8 2.9 3 3.1 3.2 3.3
1/T x 103

Calculations
 Molar latent heat / enthalpy of vaporization can be calculated
from Clausius-Clapeyron relation as follows,
ΔHv = -Rx[d(ln(P))/d(1/T)]]
ln(P) = 2.303 log(P)
Slope = m = d(ln(P))/d(1/T)
ΔHv = -2.303(R)(m) - - - - Eq. (1)
 Where R is ideal gas constant = 1.987 cal/K
 m is slope of line obtained from graph

Calculations (contd.)
 Slope (m) can be obtained from linear regression
 A convenient method is to draw a trend-line on the graph
and select the option to display an equation of line
 The equation of line of the sample experiment graph is,
y = -2.233x+8.859
 From equation, value of slope (m) = -2.233

Calculations (contd.)
 Applying values in equation 1 from previous slide
ΔHv = 10.21 cal/mol
 Accepted value for water is 9.72 cal/mol
 Deviation is 4.79 %
 Results obtained from this experiment seldom
increase 5% deviation from expected value

Applications
 Major application in conversion of water into steam
 Steam is used in
 Power generation (steam turbines)
 Agriculture
 Energy storage
 Wood treatment
 Cleaning purposes
 Sterilization

Applications (contd.)
 Distillation
 Vapor Pressure Calculations

References
 http://en.wikipedia.org/wiki/First_law_of_thermodyn
amics
 http://en.wikipedia.org/wiki/Quasistatic_process
 http://en.wikipedia.org/wiki/Enthalpy
 http://en.wikipedia.org/wiki/Clausius-
Clapeyron_relation

References
 http://en.wikipedia.org/wiki/Enthalpy_of_vaporizatio
n
 http://www.sciencedirect.com/science/article/pii/S03
78381209002180
 http://www2.selu.edu/Academics/Faculty/delbers/He
at%20of%20vaporization.htm

QUESTIONS
ARE
WELCOMED !!