Question

Identify the Lewis acid and Lewis base in each of the following reactions: (a) ${\mathrm{HNO}}_2(aq)+{\mathrm{OH}}^{-}(aq)\rightleftharpoons {\mathrm{NO}}_2^{-}(aq)+{\mathrm{H}}_2\mathrm{O}(t)$ (b) ${\mathrm{FeBr}}_3(s)+{\mathrm{Br}}^{-}(aq)\rightleftharpoons {\mathrm{FeBr}}_4^{-}(aq)$ (c) ${\mathrm{Zn}}^{2+}(aq)+4{\mathrm{NH}}_3(aq)\rightleftharpoons \mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4^{2+}(aq)$ (d) ${\mathrm{SO}}_2(g)+{\mathrm{H}}_2\mathrm{O}(l)\rightleftharpoons {\mathrm{H}}_2{\mathrm{SO}}_3(aq)$

Short answer

(a) Lewis base: ${\mathrm{OH}}^{-}$; Lewis acid: ${\mathrm{HNO}}_2$ (b) Lewis base: ${\mathrm{Br}}^{-}$; Lewis acid: ${\mathrm{FeBr}}_3$ (c) Lewis base: ${\mathrm{NH}}_3$; Lewis acid: ${\mathrm{Zn}}^{2+}$ (d) Lewis base: ${\mathrm{H}}_2\mathrm{O}$; Lewis acid: ${\mathrm{SO}}_2$

Step-by-step solution

Reaction (a) - Identifying the Lewis Acid and Base

For the reaction: $${\mathrm{HNO}}_2(aq)+{\mathrm{OH}}^{-}(aq)\rightleftharpoons {\mathrm{NO}}_2^{-}(aq)+{\mathrm{H}}_2\mathrm{O}(t)$$ Here, the hydroxide ion (${\mathrm{OH}}^{-}$) donates an electron pair to the ${\mathrm{HNO}}_2$ molecule to form water (${\mathrm{H}}_2\mathrm{O}$) and ${\mathrm{NO}}_2^{-}$. Thus, the Lewis base is the hydroxide ion (${\mathrm{OH}}^{-}$), and the Lewis acid is the ${\mathrm{HNO}}_2$ molecule.

Reaction (b) - Identifying the Lewis Acid and Base

For the reaction: $${\mathrm{FeBr}}_3(s)+{\mathrm{Br}}^{-}(aq)\rightleftharpoons {\mathrm{FeBr}}_4^{-}(aq)$$ Here, the Bromide ion (${\mathrm{Br}}^{-}$) donates an electron pair to the ${\mathrm{FeBr}}_3$ molecule to form ${\mathrm{FeBr}}_4^{-}$. Thus, the Lewis base is the Bromide ion (${\mathrm{Br}}^{-}$), and the Lewis acid is the ${\mathrm{FeBr}}_3$ molecule.

Reaction (c) - Identifying the Lewis Acid and Base

For the reaction: $${\mathrm{Zn}}^{2+}(aq)+4{\mathrm{NH}}_3(aq)\rightleftharpoons \mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4^{2+}(aq)$$ Here, the ammonia molecules (${\mathrm{NH}}_3$) donate electron pairs to the ${\mathrm{Zn}}^{2+}$ ion to form the $\mathrm{Zn}{\left({\mathrm{NH}}_3\right)}_4^{2+}$ complex. Thus, the Lewis base is the ammonia molecule (${\mathrm{NH}}_3$), and the Lewis acid is the ${\mathrm{Zn}}^{2+}$ ion.

Reaction (d) - Identifying the Lewis Acid and Base

For the reaction: $${\mathrm{SO}}_2(g)+{\mathrm{H}}_2\mathrm{O}(l)\rightleftharpoons {\mathrm{H}}_2{\mathrm{SO}}_3(aq)$$ Here, the water molecule (${\mathrm{H}}_2\mathrm{O}$) donates an electron pair to the ${\mathrm{SO}}_2$ molecule to form the ${\mathrm{H}}_2{\mathrm{SO}}_3$ ion. Thus, the Lewis base is the water molecule (${\mathrm{H}}_2\mathrm{O}$), and the Lewis acid is the ${\mathrm{SO}}_2$ molecule.

Key concepts

Lewis Acid-Base Reactions

Understanding Lewis acid-base reactions is crucial for anyone studying chemistry. These reactions involve the transfer of electron pairs between two substances. A Lewis acid accepts an electron pair, while a Lewis base donates an electron pair.

Let's consider the reaction between hydroxide ions and nitrous acid as an example: $${\mathrm{HNO}}_2(aq)+{\mathrm{OH}}^{-}(aq)\rightleftharpoons {\mathrm{NO}}_2^{-}(aq)+{\mathrm{H}}_2\mathrm{O}(l)$$ Here, the ${\mathrm{OH}}^{-}$ ion donates an electron pair to ${\mathrm{HNO}}_2$, making ${\mathrm{OH}}^{-}$ the Lewis base and ${\mathrm{HNO}}_2$ the Lewis acid. Identifying which species is the acid or base in such reactions is key to predicting the products and understanding the reaction's direction.

Electron Pair Donation

The concept of electron pair donation plays a central role in the Lewis theory of acids and bases. In a Lewis acid-base reaction, the base donates a pair of electrons, which the acid accepts to form a new chemical bond.

For instance, in the reaction with iron(III) bromide and bromide ions: $${\mathrm{FeBr}}_3(s)+{\mathrm{Br}}^{-}(aq)\rightleftharpoons {\mathrm{FeBr}}_4^{-}(aq)$$ the bromide ion ${\mathrm{Br}}^{-}$ acts as a Lewis base by donating an electron pair to the ${\mathrm{FeBr}}_3$, the Lewis acid. This interaction demonstrates how electron pair donation leads to the formation of a more complex ion. Understanding these interactions can elucidate the properties and behaviors of the resulting compounds in chemical reactions.

Chemical Equilibrium

In the context of Lewis acid-base chemistry, chemical equilibrium is the state in which the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of reactants and products remain constant over time.

Let's analyze the reaction involving zinc ions and ammonia: $${\mathrm{Zn}}^{2+}(aq)+4{\mathrm{NH}}_3(aq)\rightleftharpoons \mathrm{Zn}({\mathrm{NH}}_3{)}_4^{2+}(aq)$$ The equilibrium is established as the zinc ion, the Lewis acid, binds with the Lewis base, ammonia, and this reversible reaction reaches a point where there are no further changes in concentration of the reactants and products. Understanding how equilibrium is established, and the conditions that affect it, such as temperature or concentration changes, is essential for predicting how a reaction will proceed and is a fundamental principle in chemical reaction dynamics.