Definition of Dynamic Chemical Equilibrium

The definition of the dynamic equilibrium of the chemical reaction is the point where no further action apart. The reaction occurs by the change of Gibbs free energy and entropy to reach this dynamic equilibrium point, thus reversible endothermic and exothermic reactions are the example of such heat changes.

Therefore the chemical reaction proceeds to some extent than stop by converting some portion of reactant to product.

Example of Dynamic Equilibrium

If hydrogen and iodine vapor kept at a constant temperature in a closed vessel. The reaction proceeds to some extent and then stop, by converting some portion of hydrogen and iodine to hydrogen iodide.

Example of dynamic equilibrium in reversible chemical reaction

Because after this dynamic point, the reactant and product attained equilibrium.

H2 + I2 ⇆ 2HI

If you take some hydrogen iodide in the closed vessel at the same temperature. A faction of hydrogen iodide converted into hydrogen and iodine and the rest of the hydrogen iodide remains unchanged.

2HI ⇆ H2 + I2

But in both the experiment, the amount of hydrogen, iodine, and hydrogen iodide remains unchanged. Therefore this chemical reaction uses for the definition of dynamic equilibrium.

So the equilibrium of the chemical reaction maintains the following criteria

  1. Approachability from both ends
  2. Permanency of the equilibrium
  3. The incompleteness of the reaction
  4. Dynamic equilibrium

Example of Reversible equilibrium

When water 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 the example of the reversible nature of the dynamic equilibrium.

Energy Change in the Chemical Reaction

How a chemical reaction occurs? This leading question can not be answered in a simple sentence. Because the chemical reaction going through energy changes of components. The two primary factors are responsible for the feasibility of a chemical reaction.

  1. The potential energy of the reacting system must be lowered by the reaction.
  2. Reactants posses potentially to react because they must find a suitable path to react at a participle rate under specific conditions.

The fast concern with thermodynamic energy and studies under chemical equilibrium but the second one constitutes the study of kinetics.

Heat Change in the Chemical Reaction

At the equilibrium, all reversible chemical reaction, the total thermodynamic heat change is zero.

ΔStotal = ΔSsystem + ΔSsurr

But for all irreversible processes or natural chemical reactions, the value of total entropy increases and greater than zero.

Thus for an irreversible chemical reaction, the total heat change goes to increases and when it reached an equilibrium point, the entropy attains maximum value.

Because at equilibrium, no further reaction takes place or no further possible entropy change occurs. Thus at equilibrium

ΔStotal = 0

Exothermic and Endothermic Reactions

All the natural reaction follow a general trend because they take place in a direction which results in an ultimate decrease in the chemical energy of the universe. Thus the exothermic and endothermic reactions differ by definition as

  1. Exothermic chemical reactions are accompanied by the emission of energy.
  2. But endothermic chemical reactions are accompanied by the absorption of energies.

Many exothermic reactions are reversible thus, the reverse process must be endothermic in nature.

For example, the synthesis of ammonia is an endothermic reversible chemical reaction but the reverse process going through exothermic.

Forward reaction

2NH3 → N2 + 3H2 (ΔH = +92.22 KJ mol-1)

Backword reaction

N2 + 3H2 → 2NH3 (ΔH = -92.22 KJ mol-1)

Gibbs Free Energy and Equilibrium

For the spontaneous process, two factors control the ultimate energy change of the chemical reactions.

  1. Enthalpy change (ΔH).
  2. Entropy change (ΔS).

But these two changes collectively determined by another fundamental property of the system which known Gibbs free energy (ΔG). Van’t Hoff equation proposed the relation between free energy and the equilibrium constant of a reaction.

He used the Gibbs – Helmholtz equation to derive the free energy at the equilibrium of the chemical reaction.

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

Solution
The heat of a reaction or enthalpy is a state function because ΔH does not change if the initial state and final state are the same.

Thus a catalyst cannot change the initial and final state of reactions, hence ΔH remains the same whether a catalyst used or not.

Law of Mass Action and Chemical Equilibrium

Norwegian Physicists, Guldberg, and Waage in 1867 developed the quantitative relation between the amount of the chemical product and reactant at the equilibrium point and this relation is known as mass action law.

When temperature kept constant, the rate of a chemical reaction at equilibrium, proportional to the active masses of the reacting system.

  1. If the solution dilutes, molar concentration or moles/liter used.
  2. Thus when the reactants and product in the gaseous state, palatial pressure used.
  3. For pure solid and pure liquid, active mass assumed to be unity.