generalchem_ls_19.pdf

8/10/2019 GeneralChem_LS_19.pdf

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BITSPilani, Pilani Campus

CHEM F111 General ChemistryLecture 19

BITS

PilaniPilani Campus

Dr. Ajay K. Sah


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Thermodynamics

The First Law

Thermochemistry

The Second Law

The Third Law


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Extensive and Intensive Properties

Extensive Property: A property that depends on the

amount of the substance in the sample. e.g., mass,

volume

Intensive property:A property that is independent ofthe amount of the substance in the sample; e.g.,

pressure, temperature, molar volume.

Extensive properties are additive but intensive

properties are not.


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Extensive and Intensive Properties

Some intensive properties are ratios of two

extensive properties, e.g., density of a substance.

It is ratio of two extensive properties mass and

volume.

Specific property:

Given any extensive property, one may divide it by

the mass to obtain an intensive property called a

specific property.

For example, Specific Volume = Volume per Unit

Mass.


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System

System: That portion of

the universe set apart for

detailed study and

analysis.

Surroundings: Rest of

the universe, with which

the system can interact.

Thermally isolated system is called Adiabatic

system eg: water in vacuum flask.


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Classification of Systems

Open System One that can

exchange matter as well as

energywith its surroundings.

Closed System One which

can exchange energy but not

matter with the surroundings.

Isolated System One

which is completely

unaffected by the

surroundings.


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Homogeneous/Heterogeneous Systems

System consists of one or more parts, each ofwhich is spatially uniform in its properties, each

called a phase.

A single phase system is homogeneous, while a

system of two or more phases is heterogeneous



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1stlaw of thermodynamics

Conservation of energy in mechanical and thermal

processesWork done on a system changes its internal

energy U, a state function which represents the

sum total of all the kinetic and potential energies ofthe atoms/molecules/ionsin the system.

U = w + q

Internal energy is an extensive property.

Molar internal energy is an intensive property.

In an adiabatic process,U = w



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Work

From mechanics, Work done w = fdx

Work done against gravity in raising an object of

mass m by a distance h = mgh

Most important in thermodynamics of fluids,

work of expansion/compression,

w = -pextdV

Slow, controlled process with no friction and theexternal pressure being almost equal to the system

pressure, w = -pdV



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Work: Example


1 mole of a perfect gas at 2 bar, and 300 K is in a

piston cylinder arrangement, in thermal contact

with the surroundings at 300 K. The gas is taken to

a final pressure of 1 bar, (a) by suddenly reducing

the external pressure to 1 bar, or (b) by reducingthe external pressure in infinitesimal steps, at each

stage allowing the gas to reach equilibrium. Find q,

w,

U and

in each case. Note: The molarinternal energy of a perfect gas depends only on

the temperature.


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Work: Solution of Example


(a) w = -

PexdV = - Pex(V2V1) = -PexR(T2/P2T1/P1)= - RT(10.5) = -0.5RT = -150R.

For a perfect gas, dU = CVdT, and since here T1= T2,

U = 0.

Therefore q = -w = 150R

Again, dH = CpdT, and with T1= T2here, so = 0.


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Work: Solution of Example


(b)At any stage during the process p = nRT/V

w = -PexdV = -pdV = -RT ln(V2/V1)

= RT ln(P2/P1) = -0.693RT = -208R

Like earlier

U =

H = 0 andhence, q = -w = 208R

U and are the same as in (a) since the initial

and final states are the same, but w and q are not.

w

is greater here than in (a).

Reversible work is maximum work and greater

than irreversible work


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Heat And Enthalpy

The energy supplied as heat to a system or

released by a system at constant pressure is equalto another thermodynamic property called

ENTHALPYwhich is defined as H = U + pV

It is a state function and an extensive property

like U and V.

Molar enthalpy Hmis an intensive property like Um,p and Vm

Hm= Um+ pVm= Um+ RT for a perfect gas.



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Heat Capacity

Heat capacity at const. volume CV

= (

U/

T)V

Heat capacity at const. pressureCP= (H/T)P

Hm= Um+ RT

or Hm- Um = RT

or Hm- Um = R T

or

Hm/ T - Um/ T= R

or CP,mCV,m= R



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Thermochemistry

Thermochemistry is the study of the heat evolved

or absorbed in chemical reactions

And hence it deals with calculations of quantities

like heat capacity, heat of combustion, heat of

formation, etc.

Hess Law- Combination of Reaction Enthalpies

Born-Haber Cycle- Determination of lattice

enthalpy of an ionic crystal

Kirchhoffs Equation- Variation of H with

temperature



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Hess Law: Example


The standard enthalpy of a reaction is the sum of

the standard enthalpies of the reactions into whichthe overall reaction may be divided. Example:

C3

H6

(g) + (9/2) O2

(g) 3CO2

(g) + 3H2

O(l) r

= ?

C3H6(g) + H2(g) C3H8(g) r1= 124 kJ

C3H8(g)+5O2(g) 3CO2(g)+4H2O(l) r2= 2220 kJ

H2O(l) H2(g) + O2(g) r3= 286 kJ

r= r1

+ r2 + r3

= 2058 kJ


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Born-Haber Cycle


Determination of lattice enthalpy of an ionic crystal


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Kirchhoffs Equation


Kirchhoffs Equation- Variation of

H

withtemperature

HT2

= HT1

+ Cp

( T2

- T1

)

Cp= is the difference between the weighted

sums of the standard molar heat capacities of

products and the reagents ie.

Cp= n Cp(products)- nCp(reactants)


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Natural / Spontaneous Processes


Spontaneous change: Change that has tendency to

occur without work having to be done to bring itabout.

A spontaneous change has a natural tendency to

occur.

Non-spontaneous change: Change that can be

brought about only by doing work.

A non-spontaneous change has no natural tendency

to occur.


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Spontaneous

GAS VAC GAS

Not Spontaneous

Rigid, Adiabatic Container, Or

Isothermal free expansion of a perfect gas

Forward process spontaneous; Reverse process

non-spontaneous although consistent with First Law


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Hot

block

Cold

block

Spontaneous

Non-Spontaneous

Flow of energy as heat

Blocks are

adiabatically isolated

from the rest ofuniverse.

In either direction, theprocess is consistent

with the First Law.

bl d bl


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Irreversible and Reversible Processes


A process I

Fis irreversible if the system cannotbe returned to the initial state Iwithout leaving a

change in the surroundings. An irreversible

process is also said to be spontaneous or natural.

A process I F is reversible if the system can be

returned to the initial state I without leaving any

change in the surroundings.

ibl d ibl


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Irreversible and Reversible Processes


Reversible Irreversible

Infinitesimal driving force Finite driving force

System traverses sequence of

equilibrium states.

System traverses non-

equilibrium states.

At any stage, an infinitesimalchange in some property can

cause the process to reverse, and

the same sequence of states

followed in reverse.

No dissipative effects such as

friction.

i d /


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Disorder / Entropy


Matter tends to become disordered.

Energy tends to become disordered.

The extent of disorderness of a system is

measured by entropy (s).

The entropy S, a state function with dimensions

J/K, is an extensive property, defined bydS = dqrev/T

E


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Entropy


For the isothermal process in example 1(b), the

entropy change S = nR ln(V2/V1) = Rln2. Since S is

a state function, the change must be the same forthe free expansion 1(a) also, and Rln2 for the

change in state in the opposite direction

d f h


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Entropy: 2ndLaw of Thermodynamics


Second Law: The entropy of an isolated system (theuniverse) tends to increase.

More generally

The entropy of a closed system in an adiabatic

enclosure can never decrease.It increases in an

irreversible process, and remains constant in a

reversible process.

dS 0 (adiabatic enclosure)

dS d /T ( h ?)


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dS = dqrev/T (why?)


The greater the amount of heat transferred to

the system, greater the thermal motion stimulatedin itand hence the greater the dispersal of energy.

This suggests that,

dS

dq

More entropy is generated when a given quantity

of energy is transferred as heat to a cold system

than to a hot system. The simplest way of taking

this dependence on temperature into account is to

write, dS = dq/T

h ibl h f d ?


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Why reversible heat transfer, dqrev ?


Suppose, we want to determine the entropy change

when a system changes between two specified states.Consider that, entropy change = dSsys

Actual change occurs reversibly or irreversibly, not

known.

Say, we have found a path that joins the same two

states and which is reversible.

Entropy change = dSsys, as entropy is a state function.

But energy absorbed as heat, dqrevfor the reversible

path, might be different.

Wh ibl h f d ?


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Why reversible heat transfer, dqrev ?


Let the system to restore to its initial state

reversibly following the same path.

Its entropy changes bydSsys (since entropy is a state

function) and heat released by system will bedqrev.

Therefore entropy of surroundings changes bydqrev/T.

The total entropy change of the adibatically isolated

system during this restoration will be zero, since it iscarried out reversibly.

Therefore, - dSsys+ dqrev/T = 0, or, dSsys= dqrev/T

E Ch


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For a measurable change, the entropy change is the

sum of the infinitesimal changes:

Sf

i

= dqrev/T

Therefore in case of reversible isothermal

expansion of a gas, the entropy change of the

system,dqrev = qrev/Tf

i

S = (1/T)

Entropy Change

F d l E i


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Fundamental Equation

Combined First and Second laws for a closed

system of fixed composition, only pdV work:

dU = q + w, qrev= TdS, wrev= -pdV

Hence, dU = TdSpdV

Since, the equation relates changes in functions of

the state, it may be used to calculate these changes

for irreversible processes also.

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