Chemical equilibrium reaction

Chemical equilibrium in thermodynamics

Chemical equilibrium in thermodynamics for all reversible reactions, the total change of entropy is zero.

ΔStotal = ΔSsys + Δsurr

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

Thus for an irreversible reaction, the total entropy goes to increases and when the equilibrium reached the entropy attain maximum value.

Because at equilibrium no further reaction takes place or no further entropy change is possible.

Thus at equilibrium
ΔStotal (equilibrium) = 0

But every student and reader has a common question in his/her mind that why do chemical reactions occur?

Why do chemical reactions occur?

This leading question can not be answered in a simple sentence. Because details study reveals that the two primary factors 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 thermodynamics because it studies under chemical equilibrium but the second one constitutes the study of chemical kinetics.

Heat change in chemical reactions

All the natural chemical 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.

Exothermic reaction and endothermic reaction

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

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

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

Forward reaction

N2 + 3H2 → 2NH3
ΔH = -92.22 kJ mol-1

Backword reaction

2NH3 → N2 + 3H2
ΔH = +92.22 kJ mol-1

To study the spontaneous process of a chemical reaction two factors control the ultimate energy change of the chemical reaction.

  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 proposed the equilibrium of a chemical reaction constant at a given temperature.

He proposed the quantitative relation between chemical equilibrium and Gibbs free energy by using the Gibbs – Helmholtz equation.

Chemical equilibrium experiment

The product of the chemical reaction can not be cited if the favorable external condition not maintained.

Hence the reaction proceeds to some extent and they stop by converting some portion of reactant to product.

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

Because after this the reactant and product are attained equilibrium.

H2 + I2 ⇆ 2HI

If the reverse reaction, at the same temperature taken some hydrogen iodide in the closed vessel, a faction of hydrogen iodide converted into hydrogen and iodine. But the rest of the hydrogen iodide remains unchanged.

The amount of hydrogen, iodine, and hydrogen iodide remain unchanged in both the experiment.

So when a chemical reaction reached such a stage where no further action apert is called the equilibrium of the chemical reaction.

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 nature of the equilibrium.

Is bismuth chloride soluble in water?

If hydrolysis of bismuth chloride by added water, the milky-like solution appears due to the formation of product bismuth oxychloride.

BiCl3 + H2O ⇆ BiOCl + 2HCl

When hydrochloric acid solution added the milkiness disappears showing the reversible reaction occurs.
This shows the reversible nature of this chemical reaction.

Problem
Why heat of reaction is the same 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 of a process are the same.

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

Thus the statement is correct.

What is the Law of mass action in chemistry?

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 equation known as mass action law.

When temperature kept constant, the rate of a chemical reaction 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.
Chemical equilibrium reaction
Chemical equilibrium