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Introduction to Chemical Engineering
Thermodynamics
Chapter 4
1KFUPM Housam Binous CHE 303
2KFUPM Housam Binous CHE 303
Sensible heat effects are characterized bytemperature changes
Experimental measurements provide heat effects ofchemical reactions, phase transitions, formation andseparation of solutions
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Sensible Heat Effects
Heat transfer without phase transitions, chemical
reactions or change in composition results in a change
of the temperature of the system
For a homogeneous substance: variance=2
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For a constant-volume process:
For an ideal gas:
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For mechanically reversible constant-volume process:
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For a constant pressure process:
For an ideal gas:
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For a mechanically reversible constant-pressure
closed-system process:
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Temperature Dependency of the Heat Capacity
Units of CP depends on choice of R
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We use the ideal-gas heat capacity to evaluate
thermodynamic properties such as enthalpy
Thermodynamic-property evaluations:
(1) ideal-gas-state values using ideal-gas heat capacities
(2) correction of the ideal-gas-state value to the real-gas values
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Ideal-gas heat capacity of a mixture:
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Evaluation of the Sensible-Heat Integral
Given T and T0 , one can compute Q and ∆H
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Given ∆H and T0 , one can compute T
depends on T
Calculation requires an iterative technique
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Latent Heats of Pure Substances
Latent heat of vaporization
Heat is transferred without pressure or temperature
changes in order to vaporize a pure substance.
Heat of vaporization may be calculated from vapor-
pressure and volumetric data using Clapeyron’s equation
saturation pressure
Volume change upon phase change
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Trouton’s rule:
Watson’s method:
at normal boiling point
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Standard Heat of Reaction
Reactants and products have different molecular
structures hence different energies
Reactants in combustions possess greater energy
than products thus energy is either transferred to
the surrounding as heat or produce products at
elevated temperatures
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We calculate heat effect for reactions carried out in
diverse ways from reaction carried out in standard way
where products and reactants are at the same
temperature
Combustion in a calorimeter: fuel and air flow at ambient
temperature into a combustion chamber. Products are
cooled to ambient temperature. Heat flowing from the
calorimeter and absorbed by a water jacketed section equals
the enthalpy change caused by reaction or heat of reaction
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Standard heat of reaction = enthalpy change withreactants and products are at their standard state andat temperature T.
Standard state: 1 bar pure species at Tsystem
physical state for gases = ideal gas
physical state for solids and liquids = real state
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Standard heat of reaction given for a particular
reaction apply for the stoichiometric
coefficients as written:
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Standard heat of reaction can be calculated from
standard heat of formation of the species taking part
in the reaction
formation reaction = reaction which forms a single
compound from its constituent elements
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Heat of reaction at any temperature can be computed
from heat-capacity data if a value at 298 K is known
One should just compile standard heat of formation at 298 K
One can form any desired equation from formation equations
Fictitious states are often assumed as convenient
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Standard Heat of Combustion
Many standard heat of formation come from standard heat of
combustion measured calorimetrically because formation
reaction not feasible
Combustion reaction = reaction between compound/element
and oxygen to form combustion products (generally CO2 and
H2O)
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Temperature dependence of ∆H°
How to calculate standard heat of reaction at temperature
other than 298K
Products and reactants are at standard-state pressure of 1 bar
Standard-state enthalpies are function of T only
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with and so on
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