Introduction to Chemical Engineering

Thermodynamics

Chapter 2

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Internal energy

It does not include energy due to macroscopic position ormovement

Addition of heat increases molecular activity and hence internalenergy

First Law of Thermodynamics

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Although energy assumes many forms, the total quantity

of energy is constant, and when energy disappears in one

form it appears simultaneously in other forms

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potential, kinetic and internal energy are

stored with matter

heat and work are energy in transit across

the boundary separating system and

surroundings

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Energy balance for closed system

no energy associated with matter is transportedacross the boundary

All energy exchange appears as heat and work

closed system

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Specific and molar internal energy are intensive

SI Units are Joule/kg and Joule/mole

No absolute value of the internal energy

But only changes in this property can be calculated

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Depend only on present conditions and not on how they

were reached

State functions

Differential of a state function = infinitesimal change

Differentials of heat and work = infinitesimal amounts

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W+Q is the same for all paths but the amount of

heat or work exchanged is dependent on the process

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Equilibrium

No change and no driving force or tendency for change

In a system not at equilibrium with an appreciable driving force:

change will not occur if the resistance to change is very large

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The phase rule (Gibbs, 1875)

Number of phases

Number of chemical species

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Reversible ProcessesFrictionless

Differentially removed from equilibrium

Traverses succession of equilibrium states

Driven by force imbalances which are differential inmagnitude

Can be reversed at any point by differential change inexternal conditions

When reversed it retraces the forward path and restores theinitial state of the system and the surroundings

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Reversible processes are said mechanically reversible

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Constant-volume process

Mechanically reversible closed-system process:

If in addition, constant-volume process:

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Constant-pressure process

Enthalpy

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Heat Capacity at Constant Volume

Mechanically reversible closed-system contant-volume process:

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Heat Capacity at Constant Pressure

Mechanically reversible closed-system contant-pressure process:

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Mass Balance for Open Systems

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Assume: (1) steady-state

(2) single entrance and single exit stream

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The General Energy Balance

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Shaft work

Expansion/ contraction work

Stirring work

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Energy Balance for steady-state processes

Steady-state: no changes with time

Thus consider only shaft work

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Flow calorimeter for enthalpy measurements

liquid water at 0°C