Criteria of equilibrium and spontaneity

INTRODUCTION

Observable properties such as colour, temperature, pressure, concentration, and so on do not change during the course of a process when an equilibrium is reached.

The word equilibrium literally means ‘balance,’ and it refers to the fact that a chemical reaction represents a state of equilibrium between the reactants and products that are involved in the reaction. Certain physical processes, such as the melting point of ice at 0°C, are observed to be in an equilibrium state, where both ice and water are present at the same time.

Types of Equilibrium

Physical equilibrium is used to describe the state of affairs in physical processes such as the melting of solids, the dissolution of salt in water, and so on, whereas chemical equilibrium is used to describe the state of affairs in chemical reactions.

Physical changes  in a  equilibrium

This equilibrium is associated with the physical process that is taking place in the world. These are the ones:

1. i) Equilibrium between solids and liquids

For example, H2O(s) H2O(l); the rate of melting of ice equals the rate of freezing of ice; etc.

(ii) The equilibrium between liquid and gas

For example, H2O(l) H2O (g)

(iii) The equilibrium between solids and gases

For example, I2(s) = I2 (vapour)

Chemical changes in a equilibrium

Chemical changes occur in a state of equilibrium.

The state of a reversible reaction in which both forward and backward reactions occur at the same rate is referred to as the chemical equilibrium state.

The equilibrium state of a reversible reaction is the stage of the reaction during which the concentrations of the reactants and products do not change with time.

According to the definition, an equilibrium state is defined as one in which the system’s measurable properties (such as pressure or density) don’t undergo any further noticeable changes with time when subjected to a given set of conditions.

Characteristics of Equilibrium states

1. i) Equilibrium states can only be achieved if a reversible reaction is carried out in a closed space.
2. ii) In chemical equilibrium at a given temperature, the consistency of certain properties such as pressure, concentration, density, or colour is characterised.

(iii) At equilibrium, the concentrations of each reactant and product are fixed, and this is true regardless of whether the reaction is started with the reactants or with the products as the starting point. 

 2HI =H2 + I2 

 H2 + I2 = 2HI

(iv) Equilibrium is achieved in a shorter period of time through the use of a positive catalyst.

(v) It is a dynamic force in the natural world. It appears that the reaction has come to a halt, however, because the concentrations of the reactants and products do not appear to have changed.

Ionic equilibrium

Equilibrium between ions Chemical reactions can also take place in a solution, in which the majority of the time ions are involved. The substance that reacts with an ion in solution is referred to as an electrolyte. Ionic Equilibrium is defined as the state of equilibrium that exists between the unionised molecules of a particular substance and the ion that has formed in a solution.

Acids, bases, and salts are the most common types of ionic compounds. As a result, when they are dissolved in water or any other solvent, the ionic equilibrium is maintained in them. The equilibrium constant is related to the strength of these electrolytes, i.e. the number of ions that they supply in solution, because all of them are electrolytes.

Electrolytes that are strong and weak 

The strength of an electrolyte is measured in terms of the degree of ionisation (or ionisation).

Strengthening electrolytes are those electrolytes that have been ionised almost completely, whereas weakening electrolytes are those electrolytes that have only been ionised to a lesser extent.

When it comes to strong electrolytes, alpha is equal to 1.

For weak electrolytes, the value of alpha is less than one .

There is no equilibrium in strong electrolytes because they ionise completely when dissolved in a solvent, so there is no equilibrium in strong electrolytes. However, only a small amount of ionisation occurs in weak electrolytes. The equilibrium between unionised electrolytes and ions formed in solution can be achieved as a result of this process.

COOH + H2O CH3COOH + H2O+ CH3COOH + H2O+

NH4OH+ H2O NH4+ + OH– NH4+ + OH–

Spontaneity

Let us make an attempt to comprehend the meaning of spontaneity. A spontaneous process is an irreversible process that can only be reversed by the intervention of some external factors. The entropy of a system is defined as the degree to which it is subject to chance.

Predicting the spontaneous nature of a reaction 

Generally speaking, the total entropy change is the most important parameter in determining the spontaneity of any given process. Because most chemical reactions fall into one of two categories: closed system and open system, we can say that there is a change in enthalpy along with the change in entropy in most chemical reactions. Because the change in enthalpy also affects the randomness of the process by affecting the molecular motions, the change in entropy alone cannot explain the spontaneity of such a process. As a result, the Gibbs energy change is employed to explain the spontaneity of a process. Gibbs’ energy is both a state function and a property with a large range. The following is the general expression for Gibbs energy change at constant temperature:

The Gibbs Equation is defined as

∆ Gsys = ∆Hsys – T∆Ssys.

Where,

In this case, ∆Gsys is the Gibbs energy change of the system.

∆Hsys is the system’s enthalpy change, written as

∆Ssys = the change in entropy of the system

T is the system’s temperature in degrees Celsius.

The Gibbs equation is the name given to this situation.

Whenever there is a spontaneous process, the total entropy change, ∆Stotal, is consistently greater than zero.

ΔStotal=ΔSsys + ΔSsurr

 Where,

Total entropy change for the process is equal to one.

∆Ssys = the change in entropy of the system

∆Ssurr is the entropy change of the surrounding environment.

In the case of thermal equilibrium between the system and the surrounding environment, the difference in temperature between the system and the surrounding environment is zero, i.e. DT=0. As a result, the enthalpy lost by the system is recovered by the environment.

Aside from that, for a spontaneous process, the total change in entropy is zero, which is expressed as ∆Stotal> 0.

Therefore;

TΔSsys – ΔHsys>0

ΔHsys– TΔSsys<0

It is possible to state, using the Gibbs equation, that

ΔGsys< 0

As a result, it can be concluded that any process is spontaneous if the change in Gibbs energy of the system is less than zero, or that the process is not spontaneous if the change in Gibbs energy of the system is greater than zero.

As a result, the spontaneity of a reaction can be easily predicted with the help of the relationship described above.

• When it comes to exothermic reactions, the enthalpy of the system is negative, resulting in all exothermic reactions being spontaneously initiated.
• For endothermic reactions, the Gibbs free energy is only negative when the temperature is extremely high or the entropy change is extremely high, and neither of these conditions is met.

CONCLUSION

The term equilibrium refers to the concept that a chemical reaction represents a condition of equilibrium between the reactants and products involved in the reaction. Physical equilibrium describes the situation in physical processes such as solid melting, salt dissolving in water, and so on, whereas chemical equilibrium describes the set of circumstances in chemical reactions. Equilibrium states can only be achieved in a closed space by performing a reversible reaction.  Equilibrium between ions Chemical reactions can also take place in a solution, in which the majority of the time ions are involved.