As Leis da Termodinâmica. O que é termodinâmica?

Transcription

1 As Leis da Termodinâmica O que é termodinâmica?

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4 Energia? Trabalho? W = F. r W = F r cos(θ )

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6 W = P ext Δ V

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8 W = Pext, j ΔV j Vf W = P dv V0 () Vf W = n R T ln V0 Gás ideal, sistema fechado, temperatura constante

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10 Calor Calor x Energia Térmica: Calor: energia térmica em trânsito ΔΔT Energia térmica: movimento das partículas

11 Por que Cmolar são tão diferentes?

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13 Trocas de calor

14 ΔU = Q + W

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16 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.

17 State Functions Usually 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.

18 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.

19 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.

20 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.

21 Hess s Law Hess s 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.

22 Hess s 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.

23 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.

24 Calculation of H C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l) Imagine this as occurring in three steps: 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.

25 Calculation of H C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l) Imagine this as occurring in three steps: 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.

26 Calculation of H C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l) Imagine this as occurring in three steps: 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.

27 Calculation of H C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l) The sum of these equations is: 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) Thermochemistry 2009, Prentice-Hall, Inc.

28 Calculation of H We can use Hess s law in this way: H = n Hf products m Hf reactants where n and m are the stoichiometric coefficients. Thermochemistry 2009, Prentice-Hall, Inc.

29 Calculation of H C3H8 (g) + 5 O2 (g) 3 CO2 (g) + 4 H2O (l) H = [3( kj) + 4( kj)] [1( kj) + 5(0 kj)] = [( kj) + ( kj)] [( kj) + (0 kj)] = ( kj) ( kj) = kj Thermochemistry 2009, Prentice-Hall, Inc.

30 U = f 1 k T 2 CV,m = ΔU 1 = f k ΔT 2

31 Entalpia H = U + P V CV,m qv ΔU = = ΔT ΔT CP, m qp ΔH = = ΔT ΔT

32 Δ H = Δ U + Δ (P V ) Δ H = Δ U + Δ (n R T ) Δ H = Δ U + R T Δ (n gás ) Δ H = qp Significado físico

33 CV,m calor C = Δ T CP,m = CP,m Δ U = Δ T Δ H = Δ T Δ H Δ U + n R ΔT = = CV,m + R Δ T Δ T

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40 Energy in Foods Most of the fuel in the food we eat comes from carbohydrates and fats. Thermochemistry 2009, Prentice-Hall, Inc.

41 Energy in Fuels The vast majority of the energy consumed in this country comes from fossil fuels. Thermochemistry 2009, Prentice-Hall, Inc.

42 A Segunda Lei da Termodinâmica

43 O que significa espontâneo? H2 (g) + O2 (g) H2O (l)

44 Entropia Entropia e Desordem? Afirmação: A entropia de todo sistema isolado aumenta quando este sofre um processo espontâneo

45 Entropia x Desordem Então, o que é entropia? dq rev ΔS = T dq rev ds = T Δ S = n CV,m ( ) T final ln T inicial ( V final Δ S = n R ln V inicial ) 2 dq rev ΔS = T 1 Gás ideal, processo reversível, sistema fechado, volume constante Gás ideal, processo reversível, sistema fechado, temperatura constante

46 Δ S = n CV,m ( ) T final ln T inicial

47 ( V final Δ S = n R ln V inicial ) E se a pressão, não o volume, fossem fornecidos? ( ) Pinicial Δ S = n R ln P final

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49 Calculate the change in entropy of mol O2(g) when its pressure is increased from atm to atm at constant temperature. ( ) Pinicial Δ S = n R ln P final

50 ΔS de Mudança de Estado Físico Δ transição H Δ transição S = T transição

51 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 K, and its enthalpy of vaporization is 29.1 kjmol-1.

52 Qual é o fundamento microscópico para a entropia? S = k B ln (Ω) N! Ω = N 1! N 2! N 3!... ( ) Nj E j α exp N0 kb T

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56 Procurando um critério de espontaneidade

57 q surr = q sist T surr = constante!!!!! Δ S surr Δ S surr Δ S surr q surr q sist q sist = = = T surr T sist T q surr q sist, P Δ H sist = = = T surr T sist T Δ H sist = T

58 E, para o Hg?

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60 Um processo é espontâneo se ele é acompanhado por um aumento da entropia do universo (sistema + vizinhanças) Δ S total >0 Δ G sistema <0

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