Question

Explain what it means for a system to be in equilibrium. How do we express this in a chemical reaction (give an example)?

Answer 1

Equilibrium condition of a system can be expressed in the terms of concentration of the reactant and concentration of the product which are involved in the chemical process of the system.

Complete step by step answer:
Equilibrium is that state of any chemical reaction where the concentration of reactant as well as of product becomes constant but it doesn’t mean that the concentration of reactant and concentration of product is the same in equilibrium condition.
-We can also understand by the fact that at equilibrium condition concentration of reactant and product does not change with time.
-In other words, equilibrium is that state of any chemical reaction in a system where the rate of forward reaction (where formation of product takes place) is equal to the rate of backward reaction (where formation of reactant takes place in reverse direction).
-Equilibrium condition of any chemical reaction under a system can be expressed by putting two direction arrows ($\rightleftharpoons$) in the middle part of the reaction i.e. in between the left hand side & right hand side of the chemical reaction.
Example of expression of equilibrium is as follow:
Hydrogen gas (${{\text{H}}_{\text{2}}}$) and iodine gas (${{\text{I}}_{\text{2}}}$) reacts together to form hydrogen iodide gas (${\text{HI}}$) and reaction is expressed as:
${{\text{H}}_{\text{2}}}\left( {\text{g}} \right){\text{ + }}{{\text{I}}_{\text{2}}} \rightleftharpoons \left( {\text{g}} \right){\text{2HI}}\left( {\text{g}} \right)$
Where the arrow represents the equilibrium condition of the reaction.

Note: By the definition of equilibrium some of you may think that concentration of reactant and product is same but it is not so, because concentration is only constant not same.