### Abstract

Original language | English |
---|---|

Pages (from-to) | 587-588 |

Number of pages | 2 |

Journal | Journal of Chemical Education |

Volume | 75 |

Issue number | 5 |

DOIs | |

Publication status | Published - May 1998 |

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### Keywords

- thermodynamics
- reaction equilibrium composition
- non ideal mixtures

### Cite this

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**Is the reaction equilibrium composition in non-ideal mixtures uniquely determined by the initial composition?** / Sefcik, J .

Research output: Contribution to journal › Article

TY - JOUR

T1 - Is the reaction equilibrium composition in non-ideal mixtures uniquely determined by the initial composition?

AU - Sefcik, J

PY - 1998/5

Y1 - 1998/5

N2 - Reaction equilibrium can be mathematically described by the equilibrium equation and the reaction equilibrium composition can be calculated by solving this equation. It can be proved by non-elementary thermodynamic arguments that for a generic system with given initial composition, temperature and pressure there is a unique stable equilibrium state corresponding to the global minimum of the Gibbs free energy function. However, when the concept of equilibrium is introduced in undergraduate chemistry and chemical engineering courses, such arguments are generally not accessible. When there is a single reaction equilibrium among mixture components and the components form an ideal mixture, it has been demonstrated by a simple, elegant mathematical argument that there is a unique composition satisfying the equilibrium equation. It has been also suggested that this particular argument extends to non-ideal mixtures by simply incorporating activity coefficients. We show that the argument extension to non-ideal systems is not generally valid. Increasing non-ideality can result in non-monotonicity of the function crucial for the simple uniqueness argument, and only later it leads to non-uniqueness and hence phase separation. The main feature responsible for this is a composition dependence of activity coefficients in non-ideal mixtures.

AB - Reaction equilibrium can be mathematically described by the equilibrium equation and the reaction equilibrium composition can be calculated by solving this equation. It can be proved by non-elementary thermodynamic arguments that for a generic system with given initial composition, temperature and pressure there is a unique stable equilibrium state corresponding to the global minimum of the Gibbs free energy function. However, when the concept of equilibrium is introduced in undergraduate chemistry and chemical engineering courses, such arguments are generally not accessible. When there is a single reaction equilibrium among mixture components and the components form an ideal mixture, it has been demonstrated by a simple, elegant mathematical argument that there is a unique composition satisfying the equilibrium equation. It has been also suggested that this particular argument extends to non-ideal mixtures by simply incorporating activity coefficients. We show that the argument extension to non-ideal systems is not generally valid. Increasing non-ideality can result in non-monotonicity of the function crucial for the simple uniqueness argument, and only later it leads to non-uniqueness and hence phase separation. The main feature responsible for this is a composition dependence of activity coefficients in non-ideal mixtures.

KW - thermodynamics

KW - reaction equilibrium composition

KW - non ideal mixtures

U2 - 10.1021/ed075p587

DO - 10.1021/ed075p587

M3 - Article

VL - 75

SP - 587

EP - 588

JO - Journal of Chemical Education

JF - Journal of Chemical Education

SN - 0021-9584

IS - 5

ER -