Whenever we hear the word equilibrium, a reversible reaction with two direction arrow comes to mind. This is because of the common concept of existence of equilibrium in a reversible reaction. But actually;

Anything under its most stable state that is lowest energy state is in equilibrium

For example, a seesaw attains the state of equilibrium when a balance of weight is achieved on both sides.

How can Equilibrium be defined?

The most elaborative definition which covers all possible equilibrium categories (from lever to chemical reaction) of any system under its umbrella is;

Equilibrium is a state when state functions of the system do not change

with respect to their surrounding

According to first law of thermodynamics, the natural tendency of any system is to achieve its lowest energy state. The equilibrium will be achieved when any system achieves its own lowest energy state within its own boundaries.
If the system is not in its most stable state then the process of change will continue till the achievement of the lowest energy/equilibrium state.

Read more about states, state functions, and processes of change at; States and State Functions

Types of Equilibrium

In chemistry, there are three types of equilibrium;

1. Mechanical equilibrium
2. Thermal equilibrium
3. Chemical equilibrium

1. Mechanical equilibrium

A system is said to be in mechanical equilibrium when there is no exchange of matter between the system and its surrounding. Closed and isolated systems are in mechanical equilibrium.

2. Thermal equilibrium

It is the state of system when there is no heat exchange between the system and its surrounding. An isolated system exists in thermal equilibrium which can be static or dynamic itself, but with respect to surrounding, heat flow towards and from the system should be zero.

3. Chemical equilibrium

A system is said to have achieved chemical equilibrium when the net change in its composition becomes static/constant with respect to its surrounding.

For example, a reversible chemical reaction after a specific teq (equilibrium time) or an irreversible reaction after tcomp (completion time).

Reversible and irreversible reactions equilibrium graphs

Reversible Reactions

A reversible reaction after equilibrium time has no net change in composition. After equilibrium, the rates of forward and reverse steps become equal. In unit time, the number of molecules of reactants (A) being converted to products (B) becomes equal to the number of B molecules being converted to A.

A ⇌ B

Therefore, it is a dynamic equilibrium- the lowest energy state of a reversible reaction.

Irreversible Reactions

For an irreversible reaction, (on completion) 100% reactants may not have been converted to products but the reaction is completed. For example, a reaction completes at 95% conversion to products after which it is not kinetically feasible.

A → B

Therefore, at completion time, the reaction mixture will be 95% B and 5% A. Over time, there is no more change in composition and the reaction mixture will be in chemical equilibrium i.e. the lowest energy state for an irreversible reaction which is always achieved after tcomp for any spontaneous reaction.

Systems with more than one equilibrium

If a reversible reaction is taking place in an isolated container such that the forward reaction is exothermic and the reverse reaction is endothermic. The heat liberated in the forward step will be used up in the reverse step to meet its endothermicity, as the isolated system prevents any heat loss to the surrounding.

Definitely, the amount of heat liberated in the forward exothermic reaction will be same as the amount absorbed in the associated reverse step. As a whole, the system will be in mechanical, thermal as well as a chemical equilibrium.

Equilibrium prevailing in different Thermodynamic systems

There are three types of thermodynamic systems.

1. Open system

A system where the exchange of heat and matter is possible between the system and surrounding.

2. Closed system

A system where the exchange of heat is possible but the exchange of matter is prohibited between the system and surrounding.

3. Isolated system

An isolated system is the one, where no heat or matter can enter or leave the system.

The types of equilibrium associated with the systems are given as;

Equilibrium systems

In an open system, only chemical equilibrium is possible where no compositional changes are involved during the exchange of heat and matter between the system and its surrounding. For example, a cup of tea placed in open air to cool.

In a closed system, both chemical and mechanical equilibriums or possible during the exchange of heat between the system and surrounding.

In an isolated system, there is no exchange of matter and heat with the surrounding. All three types of equilibrium i.e. chemical, thermal, and mechanical equilibrium exist within the system.

For a detailed account of Types of systems and examples, please visit Open, Closed and Isolated Systems with Examples

Concepts Berg

When is chemical equilibrium called dynamic equilibrium?

This is true for only reversible reactions as both forward and reverse reactions take place at the same pace, therefore, dynamic equilibrium.

How does pressure affect chemical equilibrium?

Pressure affects chemical equilibrium according to Le Chatelier’s principle. A detailed account of which is given here; Le Chatlier’s Principle

Is chemical equilibrium a macroscopic or a microscopic property?

Chemical equilibrium is not a property at all, it is a condition of state balancing.

Can I study ionic equilibrium independent of chemical equilibrium?

Yes, if the studied equilibrium does not change the chemical nature of participants, then it is regarded as mechanical equilibrium.  For example, an equilibrium in the dissolution process of sparingly soluble salt where there is no chemical change.