8/8/2019 ChE 122 Lecture 1

1/53

Thermodynamics is a funny subject. The first

time you go through it, you don't understand it

at all. The second time you go through it, you

think you understand it, except for one or two

small points. The third time you go through it,

you know you don't understand it, but by thattime you are so used to it, it doesn't bother you

any more.

Arnold Sommerfeld

8/8/2019 ChE 122 Lecture 1

2/53

Laws of Thermodynamics

1) You cannot win, you can only break even.

2) You can only break even at absolute zero.

3) You cannot reach absolute zero.

8/8/2019 ChE 122 Lecture 1

3/53

The 1st law and other

basic concepts

CHEMICAL ENGINEERING 122:

Chemical Engineering Thermodynamics 1

8/8/2019 ChE 122 Lecture 1

4/53

Thermodynamics

the science that deals with heat and workand the properties and work of substancesthat bear a relation to heat

the principles of thermodynamics governthe conversion of thermal energy to other,

more useful forms

the science of energy and entropy

8/8/2019 ChE 122 Lecture 1

5/53

Lets talk about goose bumps

The body looses heat faster

than it can internally replace it.

The body instinctively starts to

shiver

Your skin develops goosebumps to increase it's ability

as an insulator

Shivering produces

mechanical energy that is

converted into heat

energy

8/8/2019 ChE 122 Lecture 1

6/53

Thermodynamicschemical energy stored in fuel

and oxygen molecules

thermal energy by combustion

extracts part of that thermal

energy to perform the work

necessary to propel the car

forward, overcoming friction

8/8/2019 ChE 122 Lecture 1

7/53

Energy and Entropy

Energy is conserved: it can neither be producednor destroyed, although it is possible to change

its form or move it around.

Entropy has a different character: it can't be

destroyed, but it's easy to produce more entropy

(and almost everything that happens actuallydoes). Like energy, entropy too can appear in

different forms and be moved around.

8/8/2019 ChE 122 Lecture 1

8/53

Thermodynamics

Gives information on what direction theprocess proceeds

Predicts the extent at which processes goes

Do not establish the rates of chemical andphysical processes

8/8/2019 ChE 122 Lecture 1

9/53

PROBLEM

PROTOTYPE

MODEL

DESIGN AND

IMPLEMENTATION

DESIGN

PROCESS

MODEL

Energy / Heat Transport

Mass Transport/ Diffusion

Momentum Transport/ Fluid

Dynamics

Thermodynamics/ Kinetics

Economics

8/8/2019 ChE 122 Lecture 1

10/53

Thermodynamics

Useful equations Calculation of heat and work

requirements for physical and chemical

processes

Determination of equilibrium conditionsfor chemical reactions

8/8/2019 ChE 122 Lecture 1

11/53

Thermodynamics

Thermodynamics allowsus to understand why

matter appears in different

phases (solid, liquid, or

gaseous), and under whatconditions one phase will

transform to another

8/8/2019 ChE 122 Lecture 1

12/53

Classical thermodynamics

developed in the 19th century, before the atomicnature of matter was accepted, and it makes no

reference to atoms

theory which is based on a set of postulates abouthow macroscopic matter behaves

postulates (the most important of which are energy

conservation and the impossibility of completeconversion of heat to useful work) can't be derived

within the context of classical, macroscopic

physics

8/8/2019 ChE 122 Lecture 1

13/53

Thermodynamics

Their validity lies in the absence of contrary

experience.

Based on primitive law.

8/8/2019 ChE 122 Lecture 1

14/53

ThermodynamicsA theory is the more impressive the greaterthe simplicity of its premises, the more

different kinds of things it relates, and themore extended its area of applicability.Therefore the deep impression that classical

thermodynamics made upon me. It is theonly physical theory of universal contentwhich I am convinced will never beoverthrown, within the framework of

applicability of its basic concepts.A. Einstein

8/8/2019 ChE 122 Lecture 1

15/53

Statistical Thermodynamics

The average values of variables whichdescribe all particles under considerationare taken to represent or describe the system

used in kinetic theory and statisticalmechanics

Reduces the number of equations and

variables to a few that can be computedeasily

8/8/2019 ChE 122 Lecture 1

16/53

System and Environment

focus our attention on

only a small part of the

world at a time

Both energy and matter

can be exchanged withthe environment

open system closed system isolated system

Energy, but not matter,

can be exchanged withthe environment

Neither energy nor matter

can be exchanged with theenvironment

{ in fact, no interactions

with the environment are

possible at all.

8/8/2019 ChE 122 Lecture 1

17/53

Control masses and control volumes

A control mass is a system which is

defined to consist of a specified piece or

pieces of matter.

By definition, no matter can enter or

leave a control mass.If the matter of the control mass is

moving, then the system boundary moves

with it to keep it inside (and matter in the

environment outside).

A control volume is a system which

is defined to be a particular region

of space.

Matter and energy may freely

enter or leave a control volume, andthus it is an open system.

8/8/2019 ChE 122 Lecture 1

18/53

8/8/2019 ChE 122 Lecture 1

19/53

State of a Substance/System

The condition of a substance as describedby certain observable macroscopicparameters calledproperties; (examples of

properties are pressure, temperature,specific volume, etc.)

Each of the properties of a substance in a

given state has only one definite value Conversely, these properties always have

the same (or unique) value for the same

state

8/8/2019 ChE 122 Lecture 1

20/53

Property Any quantity that depends only on the state of the

system

Independent of the path by which the state is arrivedat.

Intensive Property independent of the mass;e.g. pressure, temperature, density

Extensive Property varies directly with themass; e.g. mass, total volume; extensiveproperties per unit mass are intensive properties,e.g., specific volume = total volume / mass

A property of a system has significance forthe entire system only when the system is inEQUILIBRIUM

8/8/2019 ChE 122 Lecture 1

21/53

A Note on Units

SI system is Simple Energy, no matter what its form, is

measured in Joules (1 J = 1 kg-m2/s2)

unit of force is identical to the unit of (mass

x acceleration)

8/8/2019 ChE 122 Lecture 1

22/53

8/8/2019 ChE 122 Lecture 1

23/53

English Units

Unit of force: 1 lbf= force that accelerates 1 poundmass 32.1740 ft/s2

2

1 2

1

11 x 1(lb ) x 32.1740 (ft)(s)

32.1740 (lb )( )( ) ( )

c

f m

c

c m f

F mag

lbg

g ft lb s

=

=

=

8/8/2019 ChE 122 Lecture 1

24/53

Temperature1. The degree of hotness or coldness of a

body or environment.

2. A measure of the average kinetic

energy of the particles in a sample of

matter, expressed in terms of units or

degrees designated on a standard

scale.

8/8/2019 ChE 122 Lecture 1

25/53

8/8/2019 ChE 122 Lecture 1

26/53

8/8/2019 ChE 122 Lecture 1

27/53

Heat Heat always flows from a higher temperature to a

lower one.

Temperature as the driving force for the transfer ofenergy as heat.

Heat is never regarded as being stored within a body.

Unit: Calorie- quantity of heat which when transferred toone gram of water raised its temperature 1oC,

British thermal unit (BTU) - quantity of heat whichwhen transferred to one pound mass of water raised its

temperature 1o

FJoule SI Unit equal to 1 N-m

8/8/2019 ChE 122 Lecture 1

28/53

Pressure

Amg

AFP ==

Gauge PressureAbsolute Pressure

Some terms

8/8/2019 ChE 122 Lecture 1

29/53

Pressure

P = Po +gh

Pressure is expressed as aheadof a fluid

(e.g., 33.9 ft H2O or 76

cm Hg )

8/8/2019 ChE 122 Lecture 1

30/53

Work

Is performed whenever a force acts througha distance.

FdldW = FdldW =

8/8/2019 ChE 122 Lecture 1

31/53

l

lF x l

Fdl

dl

8/8/2019 ChE 122 Lecture 1

32/53

8/8/2019 ChE 122 Lecture 1

33/53

8/8/2019 ChE 122 Lecture 1

34/53

U

u

8/8/2019 ChE 122 Lecture 1

35/53

Kinetic Energy (Ek)

ui

=0 uf

uf/t

uf

muf2

8/8/2019 ChE 122 Lecture 1

36/53

Potential Energy (EP)

When a body of a certainweight is lifted from z1to z2

W = F x d

F = ma = mg

W = mg (z2-z1) =(mzg)EP = mzg

8/8/2019 ChE 122 Lecture 1

37/53

Mechanical Energy Balance

EK = -EP

EK + EP = 0

2 2

2 12 1 0

2 2

mv mvmz g mz g + =

8/8/2019 ChE 122 Lecture 1

38/53

Joules ExperimentsPlaced known amounts of

water, oil and mercury

Agitated fluid with a stirrerWork done on fluid by stirrer

is accurately measured

Temperature changes offluid were carefully noted

Fluid is brought back to its

original temperature by

contact with colder object

Conclusion: Heat is a form of energy

8/8/2019 ChE 122 Lecture 1

39/53

Internal Energy(U)

Ekand Pkare external energy

Absolute values are unknown

Only changes can be determined

8/8/2019 ChE 122 Lecture 1

40/53

The 1st law of thermodynamics

It was first a postulate. However, theoverwhelming evidence accumulated over time

has elevated it to the stature of a law of nature.

Although energy assumes many forms, the totalquantity of energy is constant, and when energy

disappears in one form it appears simultaneously

in other forms.

0)()( =+ gssurroundinofenergysystemtheofenergy

8/8/2019 ChE 122 Lecture 1

41/53

First Law of Thermodynamics

( )Energy of the sys(Energy of the surr)= -Q-W

-Q-W 0Q+W

t t t

k P

t t t

k P

t t t

k P

E E U

E E U E E U

= + +

+ + = + + =

8/8/2019 ChE 122 Lecture 1

42/53

SYSTEM

Boundary(1) adiabatic or diathermal(2) permeable or impermeable

(3) rigid or movable

Work

Heat effectEnvironment/Surroundings

An isolated system: impermeable, rigid, adiabatic and independent

of events in the environment

8/8/2019 ChE 122 Lecture 1

43/53

Example

The driver of a 1350 kg car, coasting downa hill sees a red light at the bottom for

which he must stop. His speed at the time

the brakes are applied is 28 m/s and he is 30m vertically above the bottom of the hill.

How much energy as heat must be

dissipated by the brakes if wind and otherfrictional effects are neglected.

8/8/2019 ChE 122 Lecture 1

44/53

Thermodynamics For most chemical system: no change in

position and stationary

for a closed system of n moles

( )

( )

n=1,

and

t

t

U Q W

dU dQ dW

nU n U Q W

d nU ndU dQ dW

for

U Q W dU dQ dW

= +

= +

= = +

= = +

= + = +

8/8/2019 ChE 122 Lecture 1

45/53

WQE +=

WQUt +=

Total internal energy of the system,depends on the quantity of material

in a system, i.e., the extensive

property.

c.f. intensive property,e.g. temperature and pressure.

e.g. specific or molar properties

If the gas is heated or cooled, compressed or expanded, and

then returned to its initial temperature and pressure, its

intensive properties are restored to their initial values.

Such properties do not depend on the past history of the

substance nor on the means by which it reaches a given state.Such quantities are known as state functions.

A state function may therefore be expressed mathematically

as a function of other thermodynamic properties. Its values

may be identified with points on a graph.

8/8/2019 ChE 122 Lecture 1

46/53

State and Path Functions

State functions depend only on presentconditions reached

Path functions depend on the path of theprocess

8/8/2019 ChE 122 Lecture 1

47/53

When a system is taken from state a to state b along path acb 100 J of heat

8/8/2019 ChE 122 Lecture 1

48/53

When a system is taken from state a to state b along path acb, 100 J of heat

flows into the system and the system does 40 J of work. (1) How much heat

flows into the system along path aeb if the work done by the system is 20J? (2)The system returns from b to a along path bda. If the work done on the system

is 30J, does the system absorb or liberate heat? How much?

P

V

a

bc

d

e20

6040100

=

+=

==+=

aeb

aebaeb

t

ab

Q

WQ

JWQU

JQaeb 80=

30

60

+=+=

=

=

bda

bdabda

t

ab

t

ba

QWQ

J

UU

JQbda 90=

8/8/2019 ChE 122 Lecture 1

49/53

Equilibrium

In thermodynamics, equilibrium means not onlythe absence of change but the absence of any

tendency toward change on a macroscopic scale.

Different kinds of driving forces bring aboutdifferent kinds of change. For example:

imbalance of mechanical forces tend to cause energy

transfer as a work.

temperature differences tend to cause the flow of heat.

Gradients in chemical potential tend to cause substance

to be transfer from one phase to another.

8/8/2019 ChE 122 Lecture 1

50/53

Equilibrium

A system is in equilibrium when it is in thermal,

mechanical, and chemical equilibrium Thermal equilibrium uniform temperature

throughout the system.

Mechanical equilibrium pressure at any pointin the system does not varywith time when system is isolated

Chemical equilibrium no tendency to changecomposition

Thermodynamic Equilibrium a systemcondition that precludes all possible changes ofstate all equilibrium conditions are satisfied

8/8/2019 ChE 122 Lecture 1

51/53

Phase rule For any system at equilibrium, the number of independent

variables that must be arbitrarily fixed to establish its intensive

state is given by J.W. Gibbs (1875).

The degrees of freedom of the nonreacting systems:

where is the number of phases,Nis the number of chemical species

A phase is a homogeneous region of matter. A gas or a mixtureof gases, a liquid or a liquid solution, and a crystalline solid are

examples of phases. Various phases can coexist, but they must

be in equilibrium for the phase rule to apply.

The minimum number of degrees of freedom for any system is

zero:

N = 1, = 3 (i.e. the triple point)

NF += 2

02 =+= NF

8/8/2019 ChE 122 Lecture 1

52/53

PROPERTIES AND STATE OFA SUBSTANCE

Phase A quantity of matter that is homogenous

throughout; solid, liquid, gas

When more than one phase is present, each

phase is separated by phase boundaries

How many degrees of freedom has each of the following systems:

8/8/2019 ChE 122 Lecture 1

53/53

y g g y(1) Liquid water in equilibrium with its vapor.

(2) Liquid water in equilibrium with a mixture of water vapor and nitrogen.(3) A liquid solution of alcohol in water in equilibrium with its vapor.

(1) 1 species, 2 phases

11222 =+=+= NF

(2) 2 species, 2 phases

22222 =+=+= NF

(3) 2 species, 2 phases

22222 =+=+= NF