Chapter 4 KFUPM Housam Binous CHE 303faculty.kfupm.edu.sa/che/binoushousam/files/CHE 303 Chapter...

Introduction to Chemical Engineering

Thermodynamics

Chapter 4

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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