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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)
H = qP Significado fsico
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?
Procurando um critrio de espontaneidade
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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