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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 Termodinâmica

O que é termodinâmica?

Energia?

Trabalho? W = F⃗ . r⃗

W = |F⃗| |⃗r| cos (θ )

W = − Pext ΔV

W = ∑j

Pext , j ΔV

W = ∫V 0

V f

P dV

W = n R T ln(V f

V 0)

Gás ideal, sistema fechado, temperatura constante

Calor

Calor x Energia Térmica:

Calor: energia térmica em trânsito →ΔΔT

Energia térmica: movimento das partículas

Por que Cmolar

são tão diferentes?

?

Trocas de calor

ΔU = Q + W

Thermochemistry

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

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

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

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

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

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

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

Standard Enthalpies of Formation

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

Thermochemistry

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

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

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

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

Calculation of H

We can use Hess’s law in this way:

H = nHf°products – mHf° reactants

where n and m are the stoichiometric coefficients.

Thermochemistry

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 = f12

k T CV ,m =ΔUΔT

= f12

k

Entalpia

H = U + P V

CV ,m = ΔUΔT

=qV

Δ T

CP,m = Δ HΔT

=qP

ΔT

Δ H = ΔU + Δ (P V )

Δ H = ΔU + Δ(n R T )

Δ H = ΔU + R T Δ(ngás)

CV ,m =Δ UΔ T

CP,m =Δ HΔ T

C =calorΔ T

CP ,m =Δ HΔ T

=Δ U + n R ΔT

Δ T= CV ,m + R

Thermochemistry

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

Thermochemistry

Energy in Fuels

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

A Segunda Lei da Termodinâmica

O que significa espontâneo ?

H2 (g)

+ O2 (g)

H2O

(l)

Entropia

Entropia e Desordem?

Afirmação:

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

Entropia x Desordem

Então, o que é entropia?

dS =dqrev

TΔ S = ∫ dqrev

TΔ S = ∫

1

2 dqrev

T

Δ S = n CV ,m ln( T final

T inicial) Gás ideal, processo reversível,

Δ S = n R ln( V final

V inicial)

Gás ideal, processo reversível, sistema fechado, temperatura constante

Δ S = n CV ,m ln( T final

T inicial)

Δ S = n R ln( V final

V inicial)

Δ S = n R ln( Pinicial

P final)

E se a pressão, não 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.

Δ S = n R ln( Pinicial

P final)

ΔS de Mudança de Estado Físico

Δ transição S =Δ transição H

T transição

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 (Ω)

Ω =N!

N1! N2! N3! ...

N j

N 0

α exp( −E j

kB T )

Qual é o fundamento microscópico para a entropia?

qsurr = − qsist

T surr = constante ! ! ! ! !

Δ Ssurr =qsurr

T surr

=− qsist

T sist

= −qsist

T

Δ Ssurr =qsurr

T surr

=− qsist , P

T sist

= −Δ H sist

T

Δ Ssurr = −Δ H sist

T

E, para o Hg ?

Um processo é espontâneo se ele é

acompanhado por um aumento da entropia do

universo (sistema + vizinhanças)

Δ S total>0⇒ΔG sistema<0