A reversible reaction is a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously.
:
\mathit aA + \mathit bB <=> \mathit cC + \mathit dD
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
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
.
Weak
acids
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...
and
bases undergo reversible reactions. For example,
carbonic acid:
: H
2CO
3 (l) + H
2O
(l) ⇌ HCO
3−(aq) + H
3O
+(aq).
The
concentration
In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', '' number concentration'', ...
s of reactants and products in an equilibrium mixture are determined by the
analytical concentration
Molar concentration (also called molarity, amount concentration or substance concentration) is a measure of the concentration of a chemical species, in particular of a solute in a solution, in terms of amount of substance per unit volume of solut ...
s of the reagents (A and B or C and D) and the
equilibrium constant, ''K''. The magnitude of the equilibrium constant depends on the
Gibbs free energy
In thermodynamics, the Gibbs free energy (or Gibbs energy; symbol G) is a thermodynamic potential that can be used to calculate the maximum amount of work that may be performed by a thermodynamically closed system at constant temperature and ...
change for the reaction. So, when the free energy change is large (more than about 30 kJ mol
−1), the equilibrium constant is large (log K > 3) and the concentrations of the reactants at equilibrium are very small. Such a reaction is sometimes considered to be an irreversible reaction, although small amounts of the reactants are still expected to be present in the reacting system. A truly irreversible chemical reaction is usually achieved when one of the products exits the reacting system, for example, as does carbon dioxide (volatile) in the reaction
: CaCO
3 + 2HCl → CaCl
2 + H
2O + CO
2↑
History
The concept of a reversible reaction was introduced by
Berthollet in 1803, after he had observed the formation of
sodium carbonate crystals at the edge of a
salt lake (one of the
natron
Natron is a naturally occurring mixture of sodium carbonate decahydrate ( Na2CO3·10H2O, a kind of soda ash) and around 17% sodium bicarbonate (also called baking soda, NaHCO3) along with small quantities of sodium chloride and sodium sulfate. ...
lakes in Egypt, in
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
):
:2NaCl + CaCO
3 → Na
2CO
3 + CaCl
2
He recognized this as the reverse of the familiar reaction
: Na
2CO
3 + CaCl
2→ 2NaCl + CaCO
3
Until then,
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
s were thought to always proceed in one direction. Berthollet reasoned that the excess of
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
in the lake helped push the "reverse" reaction towards the formation of sodium carbonate.
[Claude-Louis Berthollet,"Essai de statique chimique", Paris, 1803]
(Google books)
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*Anita Waage (born 1971), Norwegian footballer
*Elsa Waage (born 1959), Icelandic opera singer
*Geir Waage (born 1967), Norwegian politician
*Hilde Waage (born 1959), Norwegian ...
and Guldberg formulated their law of mass action
In chemistry, the law of mass action is the proposition that the rate of the chemical reaction is directly proportional to the product of the activities or concentrations of the reactants. It explains and predicts behaviors of solutions in dy ...
which quantified Berthollet's observation. Between 1884 and 1888, Le Chatelier and Braun formulated Le Chatelier's principle, which extended the same idea to a more general statement on the effects of factors other than concentration on the position of the equilibrium.
Reaction kinetics
For the reversible reaction A⇌B, the forward step A→B has a rate constant and the backwards step B→A has a rate constant . The concentration of A obeys the following differential equation:
If we consider that the concentration of product B at anytime is equal to the concentration of reactants at time zero minus the concentration of reactants at time , we can set up the following equation:
Combining and , we can write
:.
Separation of variables is possible and using an initial value , we obtain:
:
and after some algebra we arrive at the final kinetic expression:
:.
The concentration of A and B at infinite time has a behavior as follows:
:
:
:
:
Thus, the formula can be linearized in order to determine :
:
To find the individual constants and , the following formula is required:
:
See also
* Dynamic equilibrium
In chemistry, a dynamic equilibrium exists once a reversible reaction occurs. Substances transition between the reactants and products at equal rates, meaning there is no net change. Reactants and products are formed at such a rate that the co ...
* Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which both the Reagent, reactants and Product (chemistry), products are present in concentrations which have no further tendency to change with time, so that there is no observable chan ...
* Irreversibility
* Microscopic reversibility The principle of microscopic reversibility in physics and chemistry is twofold:
* First, it states that the microscopic detailed dynamics of particles and fields is time-reversible because the microscopic equations of motion are symmetric with resp ...
* Static equilibrium
References
{{reflist
Equilibrium chemistry
Physical chemistry