Introduction to Chemical Engineering Thermodynamics

Chapter 4

KFUPM Housam Binous CHE 303 1 Sensible heat effects are characterized by temperature changes

Experimental measurements provide heat effects of chemical reactions, phase transitions, formation and separation of solutions

KFUPM Housam Binous CHE 303 2 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

KFUPM Housam Binous CHE 303 3 For a constant-volume process:

For an ideal gas:

KFUPM Housam Binous CHE 303 4 For mechanically reversible constant-volume process:

KFUPM Housam Binous CHE 303 5 For a constant pressure process:

For an ideal gas:

KFUPM Housam Binous CHE 303 6 For a mechanically reversible constant-pressure closed-system process:

KFUPM Housam Binous CHE 303 7 Temperature Dependency of the Heat Capacity

Units of CP depends on choice of R

KFUPM Housam Binous CHE 303 8 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

KFUPM Housam Binous CHE 303 9 KFUPM Housam Binous CHE 303 10 Ideal-gas heat capacity of a mixture:

KFUPM Housam Binous CHE 303 11 Evaluation of the Sensible-Heat Integral

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

KFUPM Housam Binous CHE 303 12 Given ∆H and T0 , one can compute T

depends on T

Calculation requires an iterative technique

KFUPM Housam Binous CHE 303 13 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

KFUPM Housam Binous CHE 303 14 Trouton’s rule: at normal boiling point

Watson’s method:

KFUPM Housam Binous CHE 303 15 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

KFUPM Housam Binous CHE 303 16 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

KFUPM Housam Binous CHE 303 17 Standard heat of reaction = enthalpy change with reactants and products are at their standard state and at temperature T.

Standard state: 1 bar pure species at Tsystem physical state for gases = ideal gas physical state for solids and liquids = real state

KFUPM Housam Binous CHE 303 18 Standard heat of reaction given for a particular reaction apply for the stoichiometric coefficients as written:

KFUPM Housam Binous CHE 303 19 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

KFUPM Housam Binous CHE 303 20 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

KFUPM Housam Binous CHE 303 21 KFUPM Housam Binous CHE 303 22 KFUPM Housam Binous CHE 303 23 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)

KFUPM Housam Binous CHE 303 24 KFUPM Housam Binous CHE 303 25 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

KFUPM Housam Binous CHE 303 26 with and so on

KFUPM Housam Binous CHE 303 27