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A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. This is distinct from a reversible process in thermodynamics. Weak acids and bases undergo reversible reactions. For example, carbonic acid: H 2 CO 3 (l) + H 2 O (l) ⇌ HCO 3 − (aq) + H 3 O + (aq).
The concept of chemical equilibrium was developed in 1803, after Berthollet found that some chemical reactions are reversible. [4] For any reaction mixture to exist at equilibrium, the rates of the forward and backward (reverse) reactions must be equal.
File:Biochem reaction arrow Y XXXX Z med.svg. Y is a word indicating the chemical direction of the arrow, either "forward", "reverse", or "reversible". XXXX is a list of four characters indicating whether each side has an arrow or not. Each X is replaced with "Y" or "N" to indicate the presence or absence of a side arrow.
The forward and reverse reactions are competing with each other and differ in reaction rates. These rates depend on the concentration and therefore change with the time of the reaction: the reverse rate gradually increases and becomes equal to the rate of the forward reaction, establishing the so-called chemical equilibrium.
a thermodynamic equilibrium constant, denoted by , is defined to be the value of the reaction quotient Q t when forward and reverse reactions occur at the same rate. At chemical equilibrium , the chemical composition of the mixture does not change with time, and the Gibbs free energy change Δ G {\displaystyle \Delta G} for the reaction is zero.
Chemical equilibrium is a dynamic state in which forward and backward reactions proceed at such rates that the macroscopic composition of the mixture is constant. Thus, equilibrium sign ⇌ symbolizes the fact that reactions occur in both forward ⇀ {\displaystyle \rightharpoonup } and backward ↽ {\displaystyle \leftharpoondown } directions.
At equilibrium, the chemical force driving the forward reaction must be equal to the chemical force driving the reverse reaction. Writing the initial active masses of A,B, A' and B' as p, q, p' and q' and the dissociated active mass at equilibrium as ξ {\displaystyle \xi } , this equality is represented by
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