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As Leis da Termodinâmica O que é termodinâmica?
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### Transcript of O que é termodinâmica? - quimica.ufpr.br · Calor Calor x Energia Térmica: Calor: energia...

As Leis da Termodinmica

O que termodinmica?

Energia?

Trabalho? W = F . r

W = |F| |r| cos ( )

W = Pext V

W = jPext , j V

W = V 0

V f

P dV

W = n R T ln(V fV 0)Gs ideal, sistema fechado,

temperatura constante

Calor

Calor x Energia Trmica:

Calor: energia trmica em trnsito T

Energia trmica: movimento das partculas

Por que Cmolar so to diferentes?

?

Trocas de calor

U = Q + W

Thermochemistry

2009, Prentice-Hall, Inc.

First Law of Thermodynamics Energy is neither created nor destroyed. In other words, the total energy of the universe is

a constant; if the system loses energy, it must be gained by the surroundings, and vice versa.

Thermochemistry

2009, Prentice-Hall, Inc.

State FunctionsUsually we have no way of knowing the internal energy of a system; finding that value is simply too complex a problem.

Thermochemistry

2009, Prentice-Hall, Inc.

State Functions However, we do know that the internal energy

of a system is independent of the path by which the system achieved that state. In the system below, the water could have reached

room temperature from either direction.

Thermochemistry

2009, Prentice-Hall, Inc.

State Functions Therefore, internal energy is a state function. It depends only on the present state of the

system, not on the path by which the system arrived at that state.

And so, E depends only on Einitial and Efinal.

Thermochemistry

2009, Prentice-Hall, Inc.

State Functions

However, q and w are not state functions.

Whether the battery is shorted out or is discharged by running the fan, its E is the same. But q and w are different

in the two cases.

Thermochemistry

2009, Prentice-Hall, Inc.

Hesss Law

Hesss law states that [i]f a reaction is carried out in a series of steps, H for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.

Thermochemistry

2009, Prentice-Hall, Inc.

Hesss Law

Because H is a state function, the total enthalpy change depends only on the initial state of the reactants and the final state of the products.

Thermochemistry

2009, Prentice-Hall, Inc.

Standard Enthalpies of Formation

Standard enthalpies of formation, Hf, are measured under standard conditions (25 C and 1.00 atm pressure).

Thermochemistry

2009, Prentice-Hall, Inc.

Calculation of H

Imagine this as occurringin three steps:

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C (graphite) + 4 H2 (g)3 C (graphite) + 3 O2 (g) 3 CO2 (g)4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

2009, Prentice-Hall, Inc.

Calculation of H

Imagine this as occurringin three steps:

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C (graphite) + 4 H2 (g)3 C (graphite) + 3 O2 (g) 3 CO2 (g)4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

2009, Prentice-Hall, Inc.

Calculation of H

Imagine this as occurringin three steps:

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C (graphite) + 4 H2 (g)3 C (graphite) + 3 O2 (g) 3 CO2 (g)4 H2 (g) + 2 O2 (g) 4 H2O (l)

Thermochemistry

2009, Prentice-Hall, Inc.

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

C3H8 (g) 3 C (graphite) + 4 H2 (g)3 C (graphite) + 3 O2 (g) 3 CO2 (g)4 H2 (g) + 2 O2 (g) 4 H2O (l)

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

The sum of these equations is:

Thermochemistry

2009, Prentice-Hall, Inc.

Calculation of H

We can use Hesss law in this way:

H = nHfproducts mHf reactants

where n and m are the stoichiometric coefficients.

Thermochemistry

2009, Prentice-Hall, Inc.

H = [3(-393.5 kJ) + 4(-285.8 kJ)] [1(-103.85 kJ) + 5(0 kJ)]= [(-1180.5 kJ) + (-1143.2 kJ)] [(-103.85 kJ) + (0 kJ)]= (-2323.7 kJ) (-103.85 kJ) = -2219.9 kJ

C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l)

Calculation of H

U = f 12 k T CV ,m =UT = f

12 k

Entalpia

H = U + P V

CV ,m =UT

=qV T

CP,m = HT

=qPT

H = U + (P V )

H = U + (n R T )

H = U + R T (ngs)

CV ,m = U T

CP,m = H T

C = calor T

CP ,m = H T =

U + n R T T = CV ,m + R

Thermochemistry

2009, Prentice-Hall, Inc.

Energy in FoodsMost of the fuel in the food we eat comes from carbohydrates and fats.

Thermochemistry

2009, Prentice-Hall, Inc.

Energy in Fuels

The vast majority of the energy consumed in this country comes from fossil fuels.

A Segunda Lei da Termodinmica

O que significa espontneo ?

H2 (g) + O2 (g) H2O (l)

Entropia

Entropia e Desordem?

Afirmao:

A entropia de todo sistema isolado aumenta quando este sofre um processo espontneo

Entropia x Desordem

Ento, o que entropia?

dS =dqrevT

S = dqrevT S = 12 dqrev

T

S = n CV ,m ln( T finalT inicial )Gs ideal, processo reversvel, sistema fechado, volume constante

S = n R ln( V finalV inicial )Gs ideal, processo reversvel, sistema fechado, temperatura constante

S = n CV ,m ln( T finalT inicial )

S = n R ln( V finalV inicial )

S = n R ln( PinicialP final )

E se a presso, no o volume, fossem fornecidos?

Calculate the change in entropy of 0.321 mol O2(g) when its pressure is increased from 0.300 atm to 12.00 atm at constant temperature.

S = n R ln( PinicialP final )

S de Mudana de Estado Fsico

transio S = transio HT transio

Calculate the entropy of vaporization of acetone at 296 K with an external pressure of 1 bar. The molar heat capacity of liquid acetone is 127 JK-1mol-1, its boiling point is 329.4 K, and its enthalpy of vaporization is 29.1 kJmol-1.

S = k B ln ()

=N!

N1! N2! N3! ...

N jN 0

exp( E jkB T )

Qual o fundamento microscpico para a entropia?

qsurr = qsist

T surr = constante ! ! ! ! !

Ssurr =qsurrT surr

= qsistT sist

= qsistT

Ssurr =qsurrT surr

= qsist , P

T sist=

H sistT

Ssurr = H sistT

E, para o Hg ?

Um processo espontneo se ele

acompanhado por um aumento da entropia do

universo (sistema + vizinhanas)

S total>0G sistema

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