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

3KFUPM Housam Binous CHE 303

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

4KFUPM Housam Binous CHE 303

For a constant-volume process:

For an ideal gas:

5KFUPM Housam Binous CHE 303

For mechanically reversible constant-volume process:

6KFUPM Housam Binous CHE 303

For a constant pressure process:

For an ideal gas:

7KFUPM Housam Binous CHE 303

For a mechanically reversible constant-pressure

closed-system process:

8KFUPM Housam Binous CHE 303

Temperature Dependency of the Heat Capacity

Units of CP depends on choice of R

9KFUPM Housam Binous CHE 303

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

10KFUPM Housam Binous CHE 303

11KFUPM Housam Binous CHE 303

Ideal-gas heat capacity of a mixture:

12KFUPM Housam Binous CHE 303

Evaluation of the Sensible-Heat Integral

Given T and T0 , one can compute Q and ∆H

13KFUPM Housam Binous CHE 303

Given ∆H and T0 , one can compute T

depends on T

Calculation requires an iterative technique

14KFUPM Housam Binous CHE 303

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

15KFUPM Housam Binous CHE 303

Trouton’s rule:

Watson’s method:

at normal boiling point

16KFUPM Housam Binous CHE 303

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

17KFUPM Housam Binous CHE 303

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

18KFUPM Housam Binous CHE 303

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

19KFUPM Housam Binous CHE 303

Standard heat of reaction given for a particular

reaction apply for the stoichiometric

coefficients as written:

20KFUPM Housam Binous CHE 303

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

21KFUPM Housam Binous CHE 303

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

22KFUPM Housam Binous CHE 303

23KFUPM Housam Binous CHE 303

24KFUPM Housam Binous CHE 303

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)

25KFUPM Housam Binous CHE 303

26KFUPM Housam Binous CHE 303

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

27KFUPM Housam Binous CHE 303

with and so on