Gastric acid is commonly represented in general chemistry as aqueous hydrochloric acid, HCl(aq). In water, HCl behaves as a strong acid and is treated as fully ionized, with acidity carried by hydronium, H3O+(aq), while chloride, Cl−(aq), is typically a spectator ion.
\[ \mathrm{HCl(aq)} + \mathrm{H_2O(l)} \rightarrow \mathrm{H_3O^+(aq)} + \mathrm{Cl^-(aq)} \]
Many neutralization equations are simplest in net ionic form by tracking \(\mathrm{H_3O^+}\) (or equivalently \(\mathrm{H^+}\)) reacting with the basic species supplied by an antacid.
Common antacid chemistry provides the set of reactions used to write the neutralization equations that take place in the stomach: hydroxides neutralize acidity by forming water, while carbonates and bicarbonates neutralize acidity by forming water and carbon dioxide.
Core neutralization reactions (net ionic)
Net ionic equations remove spectator ions and highlight the acid–base event. The following forms cover the most common antacid ingredients.
\[ \mathrm{H_3O^+(aq)} + \mathrm{OH^-(aq)} \rightarrow 2\,\mathrm{H_2O(l)} \]
\[ \mathrm{H_3O^+(aq)} + \mathrm{HCO_3^-(aq)} \rightarrow \mathrm{CO_2(g)} + 2\,\mathrm{H_2O(l)} \]
\[ 2\,\mathrm{H_3O^+(aq)} + \mathrm{CO_3^{2-}(aq)} \rightarrow \mathrm{CO_2(g)} + 3\,\mathrm{H_2O(l)} \]
Molecular equations for common antacid bases
Molecular equations are often requested alongside net ionic equations because they show the specific salt produced (typically a chloride in gastric conditions). Solid-state antacids dissolve or react at the solid–solution interface; the equations below represent the overall stoichiometry in aqueous solution.
| Antacid base (typical) | Overall molecular equation with HCl(aq) | Corresponding net ionic equation | Notable products |
|---|---|---|---|
| Sodium bicarbonate, NaHCO3(s) | \(\mathrm{NaHCO_3(s)} + \mathrm{HCl(aq)} \rightarrow \mathrm{NaCl(aq)} + \mathrm{CO_2(g)} + \mathrm{H_2O(l)}\) | \(\mathrm{H_3O^+(aq)} + \mathrm{HCO_3^-(aq)} \rightarrow \mathrm{CO_2(g)} + 2\,\mathrm{H_2O(l)}\) | CO2(g), H2O(l), NaCl(aq) |
| Calcium carbonate, CaCO3(s) | \(\mathrm{CaCO_3(s)} + 2\,\mathrm{HCl(aq)} \rightarrow \mathrm{CaCl_2(aq)} + \mathrm{CO_2(g)} + \mathrm{H_2O(l)}\) | \(2\,\mathrm{H_3O^+(aq)} + \mathrm{CO_3^{2-}(aq)} \rightarrow \mathrm{CO_2(g)} + 3\,\mathrm{H_2O(l)}\) | CO2(g), H2O(l), CaCl2(aq) |
| Magnesium hydroxide, Mg(OH)2(s) | \(\mathrm{Mg(OH)_2(s)} + 2\,\mathrm{HCl(aq)} \rightarrow \mathrm{MgCl_2(aq)} + 2\,\mathrm{H_2O(l)}\) | \(\mathrm{H_3O^+(aq)} + \mathrm{OH^-(aq)} \rightarrow 2\,\mathrm{H_2O(l)}\) (applied twice per Mg(OH)2) | H2O(l), MgCl2(aq) |
| Aluminum hydroxide, Al(OH)3(s) | \(\mathrm{Al(OH)_3(s)} + 3\,\mathrm{HCl(aq)} \rightarrow \mathrm{AlCl_3(aq)} + 3\,\mathrm{H_2O(l)}\) | \(\mathrm{H_3O^+(aq)} + \mathrm{OH^-(aq)} \rightarrow 2\,\mathrm{H_2O(l)}\) (applied three times per Al(OH)3) | H2O(l), AlCl3(aq) |
Spectator ions and net ionic simplification
Chloride, \(\mathrm{Cl^-}\), appears on both sides when HCl(aq) is written explicitly and therefore cancels in the net ionic equation. The chemically essential change is the consumption of \(\mathrm{H_3O^+}\) by a base.
Carbonate and bicarbonate neutralization produces dissolved carbonic acid intermediates that rapidly decompose to CO2(g) and water. The overall net ionic equations above already reflect this combined acid–base plus decomposition outcome.
Visualization of stomach neutralization chemistry
Common pitfalls
Carbonate stoichiometry involves 2 equivalents of acidity per \(\mathrm{CO_3^{2-}}\) because two protonation events occur overall. Hydroxide stoichiometry involves one equivalent of acidity per \(\mathrm{OH^-}\), with solids such as Mg(OH)2 supplying two hydroxide equivalents per formula unit.