JEE Main & Advanced Chemistry Equilibrium / साम्यावस्था Reversible And Irreversible Reactions

A chemical reaction is said to have taken place when the concentration of reactants decreases, and the concentration of the products increases with time. The chemical reactions are classified on the basis of the extent to which they proceed, into the following two classes;

(1) Reversible reactions : Reaction in which entire amount of the reactants is not converted into products is termed as reversible reaction.

(i) Characteristics of reversible reactions

(a) These reactions can be started from either side,              

(b) These reactions are never complete,

(c) These reactions have a tendency to attain a state of equilibrium, in which Free energy change is zero (DG = 0),

(d) This sign \[(\rightleftharpoons )\] represents the reversibility of the reaction,

(ii) Examples of reversible reactions

(a) Neutralisation between an acid and a base either of which or both are weak e.g.,

\[CH{}_{3}COOH+Na\,OH\]\[\rightleftharpoons \]\[C{{H}_{3}}COONa+{{H}_{2}}O\]

(b) Salt hydrolysis, e.g.,

\[Fe\,C{{l}_{3}}+3{{H}_{2}}O\]\[ \rightleftharpoons \] \[Fe\,{{\left( OH \right)}_{3}}+3HCl\,\]\[\]

(c) Thermal decomposition, e.g.,

\[PC{{l}_{5}}_{(g)\,}\]\[\rightleftharpoons \]\[PC{{l}_{3}}_{(g)}\,+C{{l}_{2}}_{(g)}\]\[-Q\]

(d) Esterification, e.g.,

 \[C{{H}_{3}}COOH+{{C}_{2}}{{H}_{5}}OH\] \[\rightleftharpoons \]\[C{{H}_{3}}COO{{C}_{2}}{{H}_{5}}+{{H}_{2}}O\]

(e) Evaporation of water in a closed vessel, e.g.,

\[{{H}_{2}}{{O}_{(l)}}\] \[\rightleftharpoons \]\[{{H}_{2}}{{O}_{(g)}}\]\[-Q\]

(2) Irreversible reactions : Reaction in which entire amount of the reactants is converted into products is termed as irreversible reaction.

(i) Characteristics of irreversible reactions

(a) These reactions proceed only in one direction (forward direction),

(b) These reactions can proceed to completion,

(c) In an irreversible reaction, DG < 0,

(d) The arrow \[(\to )\] is placed between reactants and products,

(ii) Examples of irreversible reactions

(a) Neutralisation between strong acid and strong base e.g.,

\[NaOH+HCl\to NaCl+{{H}_{2}}O\]\[+13.7\ kcal\]

(b) Double decomposition reactions or precipitation reactions e.g.,

\[BaC{{l}_{2}}_{(aq)}+{{H}_{2}}S{{O}_{4}}_{(aq)}\to BaS{{O}_{4}}_{(s)}\downarrow +2HC{{l}_{(aq)}}\]

(c) Thermal decomposition, e.g.,

\[2KCl{{O}_{3}}_{(s)}\xrightarrow{Mn{{O}_{2}},\Delta }2KC{{l}_{(s)}}+3{{O}_{2}}\uparrow \]          

(d) Redox reactions, e.g.,

\[SnC{{l}_{2}}_{(aq)}+2FeC{{l}_{3}}_{(aq)}\to SnC{{l}_{4}}_{(aq)}+2FeC{{l}_{2}}_{(aq)}\]