2 3 principios
TRANSCRIPT
O segundo e terceiro O segundo e terceiro principios da Termodinámicaprincipios da Termodinámica
Cambio espontáneo: Aquél que tende a ocurrir sennecesidade de ser impulsado por unha influencia externa
Rudolf Julius Emmanuel Clausius(1822-1888)
“Der Energie der Welt ist konstant;die Entropy der Welt strebt einem Maximum zu”
A entropía e o segundo principio
T
qdS revδ=
dS> 0 proceso espontáneo nun sistema illadosistema illadodS=0 proceso reversible nun sistema illadosistema illado..
S é unha función de estado
∆S = S2 – S1
T
qdS
δ≥
Ludwig Boltzmann (1844-1906)
Interpretación molecular da entropía
S = kB Ln W
Microestado: Disposición das partículas nos distintos niveis de enerxía
Macroestado: Estado observable caracterizado por un conxunto de variables macroscópicas (n, P, T)
Duplex Solomillo Perete
W = nº microestados compatibles cun macroestado determinado
3º Principio da Termodinámica3º Principio da Termodinámica
A entropía de todas as substancias cristalinas
perfectas é cero cando T=0.
O cambio de entropía que ten lugar en calquera proceso físico ou químico achégase a 0 cando a temperatura achégase a 0 sempre e cando as substancias implicadas estean perfectamente ordenadas.
Walther Hermann Nernst (1864-1941)
Max Planck (1858-1947)
TTT
HT
TT
HT
TS
298
T
(g)p
ebull
oT
T
(l)p
fus
oT
0
(s)po
298ebull
ebull
fus
fus
dC
dC
dC vapfus ∫∫∫ +
∆++
∆+=
SSS
r
N
i
ii
PT
∆==
∂∂ ∑
=1,
νξ
Entropía de reacciónEntropía de reacción
T
C
T
S Pr
P
r ∆=
∂∆∂
Variación coa temperaturaVariación coa temperatura
J. Phys. Chem. B, 102 (40), 7871 -7876, 1998Web Release Date: September 12, 1998 Copyright © 1998 American Chemical Society
Heat Capacity of MgSiN2 between 8 and 800 K Richard J. Bruls, Hubertus T. Hintzen, Rudi Metselaar, and J. Cees van Miltenburg
Centre for Technical Ceramics, Laboratory of Solid State and Materials Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, and Debye Institute, Department of Interfaces and Thermodynamics, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands Received: March 18, 1998
Abstract:The specific heat at standard pressure (Cp) of MgSiN2 was determined by adiabatic calorimetry in the range of 8-400 K and differential scanning calorimetry in the range of 300-800 K. The measured Cp data for T < 24 K can be described using the Debye T 3 approximation: Cp = aT 3 with a = 1.3632 × 10-5 J mol-1 K-4. For temperatures between 350 and 650 K the Cp can be described with the Debye equation using a constant Debye temperature of 996 K. For temperatures between 24 and 350 K the Debye temperature is a function of temperature and has a minimum value of 740 K at about 55 K. The Cp data for T 300 K were compared with those of AlN. As expected, the Cp data of MgSiN2 were about a factor 2 larger than those of AlN. The entropy ST, the enthalpy (HT - H0), and the energy function (GT - H0) in the range of 0-800 K were calculated using standard thermodynamic formulas. By extrapolating the Cp data to high temperatures at which GT is known, H0 was estimated to equal -534 kJ mol-1.
IntroductionMgSiN2 is a ternary adamantine type compound with tetrahedral coordination of Mg and Si. It can be deduced from the well-known AlN by systematically replacing two Al ions with one Mg and one Si ion. The properties of MgSiN2 ceramics have recently been reported.1 Because the thermal and mechanical properties of MgSiN2 ceramics look promising, we have started an investigation of the preparation, characterization, and properties of MgSiN2. This paper focuses in more detail on one thermal property, viz., the specific heat.
T (K) C p º (J mol-1 K-1) S T º (J mol-1 K-1)Cp/T
S T º (J mol-1 K-1)
0 0 0 0 0.000010 0.014 0.0045 0.0014 0.0047
20 0.109 0.0364 0.00545 0.0363
30 0.403 0.12 0.013433 0.1343
40 1.133 0.0325 0.028325 0.3379
50 2.367 0.701 0.04734 0.7227
60 4.088 1.275 0.068133 1.3004
70 6.206 2.062 0.088657 2.0774
80 8.593 3.046 0.107413 3.0480
90 11.154 4.206 0.123933 4.1992
100 13.912 5.521 0.13912 5.5129
110 16.65 6.975 0.151364 6.9692
120 19.484 8.545 0.162367 8.5478
130 22.337 10.217 0.171823 10.2293
140 25.183 11.977 0.179879 11.9963 m x6 m x5 m x4 m x3 m x2 m x b
150 28.001 13.811 0.186673 13.8338 9.16E-15 -9.91602E-12 4.24092E-09 -8.91636E-07 8.8164E-05 -0.00201299 0.016048 #N/A
160 30.774 15.707 0.192338 15.7292
170 33.488 17.654 0.196988 17.6723 T= 300 S= 44.55557513
180 36.135 19.644 0.20075 19.6551 T ref 20
190 38.708 21.667 0.203726 21.6709 S(20)= 0.036333333
200 41.201 23.716 0.206005 23.7143
210 43.612 25.785 0.207676 25.7803
220 45.938 27.868 0.208809 27.8640
230 48.181 29.959 0.209483 29.9602
240 50.34 32.056 0.20975 32.0631
250 52.416 34.153 0.209664 34.1669
260 54.41 36.248 0.209269 36.2655
270 56.325 38.338 0.208611 38.3542
280 58.161 40.42 0.207718 40.4301
290 59.921 42.491 0.206624 42.4946
300 61.713 44.551 0.20571 44.5556
Cp/T vs T
y = 9.16E-15x6 - 9.92E-12x5 + 4.24E-09x4 - 8.92E-07x3 + 8.82E-05x2 - 2.01E-03x + 1.60E-02R2 = 1.00E+00
0
0.05
0.1
0.15
0.2
0.25
0 50 100 150 200 250 300
T/K
Cp
/Jm
ol-
1 K
-1
Entropía estándar
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300
T/K
Sº/
J m
ol-
1 K
-1