In the case of chemical reactions, complete conversion of substrates to products is observed only for a small part of them. The vast majority, including those occurring in our body, are reversible. For such processes, a state of chemical equilibrium is established. This is an extremely important concept in chemistry, which is associated with, for example, Le Chatelier’s principle (or the Equilibrium Law). Knowledge of the basic laws and principles regarding chemical equilibrium allows, for example, to predict many reactions or conduct technological processes more efficiently.
Reversible and irreversible reactions
Taking the course of chemical reactions into consideration, a general division can be made – into reversible and irreversible reactions. However, it should be remembered that in reality there are no 100% irreversible reactions, and the qualification to a particular group is based on the prevailing final effect.
Irreversible reactions
In this case, the substrates in the system react and result in specific products. However, there is no reverse transformation – products into substrates. In the formula, a single arrow is marked between the reactants, and the arrow is pointing towards the products.
Irreversible reactions are primarily all those in which precipitates form. In practice, they are insoluble in water and cannot be reconstituted. Reversibility of the process is also difficult to achieve in the case of reactions taking place in open systems, i.e. in a beaker or a spherical flask, from which one of the resulting products, e.g. carbon dioxide, can escape freely, but by changing the conditions of the reaction, i.e. by transferring it to a closed system, it can be made reversible. The group of irreversible reactions also includes all processes where the interaction between product molecules occurs to a much lesser extent than between substrates. Thus, such a process takes place in both directions, but due to the fact that the reconstitution of the substrates is negligible, in practice they are classified as irreversible reactions.
Reversible reactions
These are reactions in which products are formed from substrates and, at the same time, the reverse process occurs with comparable intensity – i. e. the reconstitution of substrates from products. In the reaction formula, two arrows are placed between the reactants, with the arrows pointing in opposite directions, to indicate the reversibility of the reactions taking place.
Reversible reactions are mainly those that are carried out in closed systems. Molecules of substrates and products are unable to leave the reaction environment and they collide with each other, creating products and reconstituting substrates. An example of such a reaction is the production of hydrogen iodide from iodine and hydrogen.
The state of chemical equilibrium
Whether a particular reaction can occur, or if it is reversible or irreversible, depends largely on the particular process conditions and the type of the process. If a reversible reaction takes place, i.e. substrates change into products as a result of a chemical reaction and, at the same time, as a result of collisions of product molecules, substrates undergo recreation, then at some point in time when these reactions occur, a state of chemical equilibrium is established. In other words, the concentrations of substrates and products in the system are at a constant level – their amount does not change despite the continuous reaction. In the chemical equilibrium state, the reaction rates in both directions are the same. The chemical equilibrium, under certain conditions, means the most stable state for a particular system. The energy requirement is then very low.
The chemical equilibrium constant, K, is a coefficient that describes the equilibrium of two reversible reactions. It is defined as the quotient of the concentrations of products and substrates raised to the powers corresponding to the stoichiometric coefficients that were measured in the chemical equilibrium state. The formula for the constant was derived in 1864 as the so-called the law of mass action. It should be remembered that this is a characteristic value for a particular reaction.
A number of factors affect the state of chemical equilibrium. These are some of them:
- Temperature,
- volume of the reaction system,
- pressure,
- concentration of reactants.
What is important, the position of the equilibrium state is not affected by the addition or change of the reaction catalyst. Its task is only to shorten the time during which equilibrium is reached.
Le Chatelier’s principle (the Equilibrium Law)
By changing certain reaction conditions, we can influence its equilibrium. The system will always strive to maintain equilibrium, because it is energetically favourable for it (lowest energy expenditure). As a consequence, there will be changes counteracting the factors throwing the system out of balance. This phenomenon was explained in 1881 and is known today as Le Chatelier-Braun’s principle or as the Equilibrium Law. It allows to understand the response of the system to changes in conditions of a reaction, which is in a state of chemical equilibrium.
The addition of one of the reacting substances to the system causes a disturbance to the equilibrium. The concentrations of individual reactants change. Thus, to counteract this, the system tends to produce more product (when a substrate has been added) or to reconstitute the substrate (when a certain amount of product has been added).
Changing the reaction temperature has a huge impact, especially for endothermic and exothermic processes. In the case of the former, it is necessary to supply energy to the system in the form of heat, so increasing the temperature of the entire process will shift the equilibrium to the right (lowering to the left), towards the formation of more product. The opposite will be the case for exothermic reactions, where one of the products is heat. The temperature has no effect on the equilibrium of the reaction, where no thermal effect is observed.
In the case of reactions taking place in the gas phase, pressure is a very important aspect. Increasing the value of this parameter, i.e. reducing the reaction volume of the system, will cause the reaction equilibrium to shift. The direction of changes depends on the stoichiometric coefficients in the reaction equation. This parameter will not affect the chemical equilibrium of the reaction in which the sum of the stoichiometric coefficients of the gaseous substrates and products is the same.
It is worth remembering that when the system is thrown out of the chemical equilibrium, the Equilibrium Law will apply and, as a result, a new state of equilibrium will be reached.