Introduction to Chemical Engineering Thermodynamics Chapter 4 1
Sensible heat effects are characterized by temperature changes Experimental measurements provide heat effects of chemical reactions, phase transitions, formation and separation of solutions 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 3
For a constant-volume process: For an ideal gas: 4
For mechanically reversible constant-volume process: 5
For a constant pressure process: For an ideal gas: 6
For a mechanically reversible constant-pressure closed-system process: 7
Temperature Dependency of the Heat Capacity Units of C P depends on choice of R 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 9
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Ideal-gas heat capacity of a mixture: 11
Evaluation of the Sensible-Heat Integral Given T and T 0, one can compute Q and H 12
Given H and T 0, one can compute T depends on T Calculation requires an iterative technique 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 vaporpressure and volumetric data using Clapeyron s equation saturation pressure Volume change upon phase change 14
Trouton s rule: at normal boiling point Watson s method: 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 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 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 T system physical state for gases = ideal gas physical state for solids and liquids = real state 18
Standard heat of reaction given for a particular reaction apply for the stoichiometric coefficients as written: 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 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 21
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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 CO 2 and H 2 O) 24
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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 26
with and so on 27