Avanço da reação

Em físico-química, o avanço da reação ou extensão da reação é uma grandeza que mede a extensão à qual uma reação prossegue, conceito introduzido pelo cientista belga Théophile de Donder.[1]

É normalmente representado pela letra grega ξ e tem como unidade o mol.[2]

Definição

Considere a reação:

Suponha uma quantidade infinitesimal dξ de reagente A que se transforma em B. A mudança na quantidade de A pode ser representada pela equação dnA = – dξ, e a mudança na quantidade de B por dnB = dξ.[3]

O avanço da reação é, então, definido como:[4][5]

onde indica a quantidade do i-ésimo reagente, e  é o número estequiométrico do i-ésimo reagente (note-se que, aqui, é definido como sendo negativo para reagentes, e como positivo, para produtos). Em outras palavras, é a quantidade de substância que está sendo alterada em um reação em equilíbrio químico.

Considerando transformações finitas em vez de infinitesimais, pode-se escrever a equação para o avanço de uma reação como:

O avanço é definido como zero no começo da reação. Assim, a mudança no ξ é o avanço por si só:

Relações

A relação entre a alteração na energia de reação de Gibbs e energia livre de Gibbs pode ser definida como o coeficiente angular da energia de Gibbs plotada contra o avanço da reação em pressão e temperatura constantes:[6]

Analogamente, a relação entre a alteração na entalpia de reação e entalpia pode ser definida como:[7]

Uso

O avanço da reação é uma grandeza útil em cálculos com reações em equilíbrio. Considerando a reação:

onde as quantidades iniciais são:  e a quantidade de A em equilíbrio é 0,5 mol. Pode-se calcular o avanço da reação pela sua definição:

é negativo pois o número estequiométrico de um reagente é negativo, por definição. Com o grau de avanço calculado, é possível rearranjar a equação e calcular os valores de equilíbrio de B e C.

Referências

  1. ↑ Kondepudi, Dilip; Prigogine, Ilya (5 de novembro de 2014). Modern Thermodynamics: From Heat Engines to Dissipative Structures (em inglês). [S.l.]: John Wiley & Sons. p. 68. ISBN 9781118698709 
  2. ↑ Baeza-Baeza, Juan José; García-Alvarez-Coque, María Celia (23 de janeiro de 2014). «Extent of Reaction Balances: A Convenient Tool to Study Chemical Equilibria». World Journal of Chemical Education (em inglês). 2 (4). doi:10.12691/wjce-2-4-2 
  3. ↑ Atkins, Peter; de Paula, Julio (2006). Physical chemistry 8 ed. [S.l.: s.n.] p. 201. ISBN 0-7167-8759-8 
  4. ↑ Lisý, Ján Mikuláš; Valko, Ladislav (1979). Príklady a úlohy z fyzikálnej chémie. [S.l.: s.n.] p. 593 
  5. ↑ Ulický, Ladislav (1983). Chemický náučný slovník. [S.l.: s.n.] p. 313 
  6. ↑ «Chemical Equilibrium» (PDF). NC State University. Consultado em 21 de junho de 2017 
  7. ↑ Lisý, Ján Mikuláš; Valko, Ladislav. Príklady a úlohy z fyzikálnej chémie. [S.l.: s.n.] 

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