Chemical equilibrium

Japanese: 化学平衡 - かがくへいこう（英語表記）chemical equilibrium

When a chemical reaction is occurring from a source system to a product system, after a sufficient amount of time has passed, the progress of the chemical reaction will appear to stop. This state is called chemical equilibrium. However, at chemical equilibrium, the reaction does not stop completely; it only appears to have stopped because the rate of the reaction moving from the source system to the product system (forward reaction) and the rate of the reaction moving from the product system to the source system (reverse reaction) are exactly balanced. For example, in the reaction A + BC + D, if the rates of the forward and reverse reactions are proportional to the product of the concentrations of each chemical species (concentrations are represented in [ ]), then the rate of the forward reaction is [A][B] and the rate of the reverse reaction is [C][D] (the rate constants), so at equilibrium,

Therefore,

([ ] eq represents the concentration at equilibrium.) In other words, this concentration ratio always remains constant once equilibrium is reached. This relationship is called the law of mass action, and K is the equilibrium constant. K is a constant value as long as the temperature does not change.

In chemical equilibrium, the sum of the free energies of the source and product systems is equal for the chemical reactions that occur.

Here, μ _i ^l represents the chemical potential of the chemical species i on the left side of the chemical reaction equation ( l , source system), n _i ^l represents the coefficient in the reaction equation, and the same is true for the right side ( r , product system). From this, Δ G ⁰ ＝－ RTlnK
^This leads to the following relationship ( ΔG0 is the increase in standard free energy when the reaction occurs according to the reaction equation, R is the gas constant, and T is the absolute temperature).

[Toda Genjiro]

"Chemical Equilibrium, by Masao Koizumi (1957, Kyoritsu Publishing)" ▽ "The Concept of Chemical Equilibrium: How Far Do Chemical Reactions Proceed, by Kei Watanabe (1998, Shokabo Publishing)" ▽ "Chemical Equilibrium and Analytical Chemistry, by Kai Kanzaki, Masahiko Senkuma, and Takao Kurosawa, 2nd Edition (2003, Hirokawa Publishing)"

[Reference] | Chemical reactions | Law of mass action

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

化学反応が原系から生成系に向かっておこっている場合、十分に時間がたったあとでは、その化学反応の進行が見かけ上停止する。この状態を化学平衡という。しかし、化学平衡ではまったく反応が止まってしまったのではなく、原系から生成系に進む反応（正反応）の速さと、逆に生成系から原系に進む反応（逆反応）の速さとが、ちょうどつり合っているために、見かけ上、反応が止まっているようにみえるだけである。たとえば、A＋BC＋Dの反応で、正逆両反応の速度がそれぞれの化学種の濃度（[　]により濃度を表す）の積に比例するとすると、正反応の速度は[A][B]、逆反応の速度は[C][D]となるから（とは速度定数）、平衡では

となり、したがって、

（[　]eqは平衡時の濃度を表す）すなわち、平衡に達すればこの濃度比がつねに一定値になることを表している。この関係を質量作用の法則といい、Κを平衡定数という。Κは温度が変わらない限り一定値である。

　化学平衡では、このなかでおこる化学反応について、原系、生成系それぞれの自由エネルギーの和が等しくなるので

と書くことができる。ここでμ_i^lは化学反応式の左辺（l、原系）にある化学種iの化学ポテンシャル、n_i^lは反応式中の係数を表し、右辺（r、生成系）についても同様に表す。これから
　　ΔG⁰＝－RTlnK
の関係が導かれる（ΔG⁰は反応式に従って反応がおきたときの標準自由エネルギーの増加、Rは気体定数、Tは絶対温度）。