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Chemical Equilibrium

Chemical Equilibrium Definition in Chemistry

Chemical equilibrium is a dynamic condition in chemistry for the reversible chemical reaction in which no net change in the amounts of reactants and products occurs or no further action apart from equilibria. Reversible equilibrium in the solution formed by changing Gibbs free energy, enthalpy, and entropy to produce the product at a definite temperature. At the equilibrium point, the backward and forward reactions go on at equal rates, velocities without any change in the amounts of substances. According to the law of equilibrium, the reaction proceeds to some extent than stopping by formation some portion of reactant to the product in a chemical solution.

Why does the chemical reaction occur? In a reaction, the reactant or product atoms present in the molecule change their electronic configuration by the formation of chemical bonding. But these leading questions for learning chemistry cannot be defined in a simple sentence. Primarily two facts determine the feasibility of chemical reaction under a given set of conditions. The first fact is the courses of thermodynamics energy conservation law and discusses under the chemical equilibrium formula. The second fact study under chemical kinetics change of concentration per unit time also describes by equilibrium law.

Chemical equilibrium or dynamic equilibria reaction and reactant product solution

Chemical Products in Equilibrium Solutions

Hydrogen and iodine vapour kept at a constant temperature in a closed vessel. The reaction proceeds to some extent and then stop, by converting some portion of the hydrogen atom and iodine atom to hydrogen iodide. Because after this dynamic point, the reactant and product attained equilibrium. If you take some hydrogen iodide in the closed vessel at the same temperature, a faction of hydrogen iodide is converted into hydrogen molecule and iodine molecule. Hence the rest of the hydrogen iodide remains unchanged.

But in both the chemistry test, the amount of hydrogen, iodine, and hydrogen iodide remains unchanged. Therefore, the equilibria definition of the chemical reaction maintains certain criteria, approachability established from both points of reactant and product solution, permanency of the chemical equilibrium, the incompleteness of the reversible reaction.

Equilibrium Solution of Reversible Reaction

When water is added to bismuth chloride, a milky solution appears due to the formation of bismuth oxychloride.

BiCl3 + H2O ⇆ BiOCl + 2HCl

But when hydrochloric acid solution added to this milky solution, the milkiness of the solution disappears. This is an example of the dynamic reversible nature of the equilibrium or equilibria.

Heat Change in Equilibrium Chemical Reaction

At the equilibria solution, all reversible chemical reaction, the total specific heat change is zero. But for all irreversible processes or natural chemical reactions, the value of total entropy increases and greater than zero. Reduction of stream in presence chemical element iron or diffusion of syrup by water is the examples of irreversible equilibrium where the total heat change goes to increases. Therefore, it reached an equilibrium point where the entropy attains maximum value because at equilibrium, no further chemical reaction occur or no further possible entropy change occurs (ΔStotal = 0).

Exothermic and Endothermic Reactions

All the natural reactions follow a general trend because they take place in a direction that results in an ultimate decrease in the chemical energy of our environment. Therefore, the exothermic and endothermic reactions in chemistry differ by definition, exothermic chemical equilibrium reactions are accompanied by the emission of energy but endothermic chemical reactions are accompanied by the absorption of energy.

Many exothermic reactions are reversible but the reverse process must be endothermic in nature. For example chemical equilibrium reaction, esterification of acetic acid in an alcohol solution, synthesis of ammonia, and esterification of hydrocarbon amylene (C5H10). If the forward process is an endothermic reversible equilibrium reaction then the reverse process happens through exothermic. For example, forward reaction: 2NH3 → N2 + 3H2 (ΔH = +92.22 KJ mol-1) but backward reaction: N2 + 3H2 → 2NH3 (ΔH = -92.22 KJ mol-1).

Energy Change in Chemical Reaction

For the spontaneous process, two factors control the ultimate energy change of the reactions, enthalpy change (ΔH), entropy change (ΔS). But these two changes are collectively determined by another fundamental property of the system which is known as Gibbs free energy (ΔG). Van’t Hoff equation define the relation between free energy and the equilibrium constant of a chemical reaction. Van’t Hoff used Gibbs – Helmholtz equation to represents the free energy at the equilibrium point in chemistry or chemical science.

Problem: Why equilibrium energy fixed, whether a chemical catalyst used or not?

Solutions: Enthalpy is a state function because ΔH does not change if the initial state and final state are the same. Therefore, according to the Le Chatelier principle, the chemical catalyst cannot change the initial and final state of equilibrium reactions, hence ΔH remains the same whether a catalyst used or not.

Photochemical Equilibrium Reaction

The chemical equilibrium of the thermal reversible systems greatly influences in the forward or backward process by absorption of light or photon energy. Therefore, the absorption of electromagnetic spectrum radiation or light by the reactant increases the speed of forwarding reaction but does not influence the reverse reaction. Due to increases in products, the rate of the reverse thermal process increases equally. Ultimately balanced the forward or backward chemical reaction to reached equilibrium.

For example, the dimerization of anthracene in the benzene solution carried out photochemical with the spectrum of UV-light. The anthracene in solution is irradiated with ultraviolet light and exited by absorbing quantum energy. The exited molecule forms dimer by the collision of the normal molecule. But the exited molecules may also re-emit the absorbed energy through fluorescent and be quenched.

Law of Mass Action and Chemical Equilibrium

Norwegian Physicists, Guldberg, and Waage in 1867 developed the dynamic relationship between the amount of the chemical product or reactant at the equilibrium constant, and this formula is known as mass action law. When the temperature is kept constant, the rate of a chemical reaction at equilibrium, proportional to the active masses of the reacting system.

For equilibrium systems of chemical reactions in a gaseous or dilute solution, the active mass meaning the partial pressure and molar concentration but for pure crystalline solid active masses define as unity in chemistry. The different types of physical and chemical definition or measurement in chemistry uses for the determination of the concentration of reactant and product at dynamic equilibria, such as equilibrium vapour density, refractive index, optical rotation, electrical polarity.