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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.
For a system undergoing a reversible reaction described by the general chemical equation + + + + 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.
[a] While processes in isolated systems are never reversible, [3] cyclical processes can be reversible or irreversible. [4] Reversible processes are hypothetical or idealized but central to the second law of thermodynamics. [3] Melting or freezing of ice in water is an example of a realistic process that is nearly reversible.
The reaction of an acid in water solvent is often described as a dissociation + + where HA is a proton acid such as acetic acid, CH 3 COOH. The double arrow means that this is an equilibrium process, with dissociation and recombination occurring at the same time.
where k f is the rate constant for the forward reaction and k b is the rate constant for the backward reaction and the square brackets, […], denote concentration. If only A is present at the beginning, time t = 0 , with a concentration [A] 0 , the sum of the two concentrations, [A] t and [B] t , at time t , will be equal to [A] 0 .
In 1884, Jacobus van 't Hoff proposed the Van 't Hoff equation describing the temperature dependence of the equilibrium constant for a reversible reaction: = where ΔU is the change in internal energy, K is the equilibrium constant of the reaction, R is the universal gas constant, and T is thermodynamic temperature.
While chemical kinetics is concerned with the rate of a chemical reaction, thermodynamics determines the extent to which reactions occur. In a reversible reaction, chemical equilibrium is reached when the rates of the forward and reverse reactions are equal (the principle of dynamic equilibrium) and the concentrations of the reactants and ...