Reversible and Irreversible Changes

Fundamentals of Chemical Reactions

In a chemical reaction, reactants undergo chemical transformation to form products. A reaction proceeds as reactants interact with one another, continuing until either all of the reactants are converted into products or at least one limiting reactant is completely consumed.

Irreversible Reactions

An irreversible reaction is a chemical reaction that proceeds to completion and moves in the forward direction only. Under the given reaction conditions, the products formed cannot react among themselves to regenerate the original reactants.

When stoichiometric amounts of the reactants are used in an irreversible reaction, no reactant species remain present at the end of the reaction.

An example of an irreversible reaction takes place immediately in an aqueous solution when aqueous sodium chloride ($NaCl_{(aq)}$) reacts with aqueous silver nitrate ($AgNO_{3(aq)}$) to produce solid silver chloride precipitate ($AgCl_{(s)}$) and aqueous sodium nitrate ($NaNO_{3(aq)}$):

$NaCl_{(aq)} + AgNO_{3(aq)} \rightarrow AgCl_{(s)} + NaNO_{3(aq)}$

In this equation, the single forward arrow ($\rightarrow$) denotes that the reaction moves exclusively in the forward direction until completion.

Reversible Reactions

In the majority of chemical reactions, the reaction does not go to completion. Instead, the products of the reaction react among themselves under the exact same conditions to re-form the original reactants. A reaction that moves in both forward and backward (reverse) directions under the same set of conditions is defined as a reversible reaction.

In a reversible chemical system, two opposing processes occur simultaneously:

• Forward direction: Reactants react together to form products.
• Backward direction: Products react together to give back the original reactants.

Reversible reactions are denoted in chemical equations using the reversible symbol $\rightleftharpoons$, which consists of two half-arrowheads pointing in opposite directions (the upper half-arrowhead pointing to the right for the forward process, and the lower half-arrowhead pointing to the left for the reverse process).

An example of a reversible reaction is the synthesis of gaseous ammonia ($NH_{3(g)}$) from gaseous nitrogen ($N_{2(g)}$) and gaseous hydrogen ($H_{2(g)}$):

$N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}$

This reaction takes place in the presence of an iron ($Fe$) catalyst at a temperature of $400\,^\circ\text{C}$ and a pressure of $200\,\text{atm}$. Under these specific conditions, nitrogen and hydrogen combine to yield ammonia in the forward process, while ammonia simultaneously decomposes back into nitrogen and hydrogen in the reverse process.