O segundo e terceiro principios da Termodinámica

Cambio espontáneo : Aquél que tende a ocurrir sen necesidade de ser impulsado por unha influencia externa

“ Der Energie der Welt ist konstant; die Entropy der Welt strebt einem Maximum zu” A entropía e o segundo principio dS> 0 proceso espontáneo nun sistema illado dS=0 proceso reversible nun sistema illado . S é unha función de estado  S = S 2 – S 1 Rudolf Julius Emmanuel Clausius (1822-1888)

Interpretación molecular da entropía S = k B 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 Ludwig Boltzmann (1844-1906)

3º 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)

Entropía de reacción Variación coa temperatura

J. Phys. Chem. B, 102 (40), 7871 -7876, 1998 Web Release Date: September 12, 1998 Copyright © 1998 American Chemical Society Heat Capacity of MgSiN 2 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 MgSiN 2 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 MgSiN 2 were about a factor 2 larger than those of AlN. The entropy S T , the enthalpy ( H T - H 0 ), and the energy function ( G T - H 0 ) in the range of 0-800 K were calculated using standard thermodynamic formulas. By extrapolating the Cp data to high temperatures at which G T is known, H 0 was estimated to equal -534 kJ mol -1 . Introduction MgSiN2 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.