Introduction
As a diligent student of chemistry, you will likely encounter tons of reactions that occur in aqueous solution (perhaps you are already drowning in them!). When ions are involved in a reaction, the equation for the reaction can be written with various levels of detail. Depending on which part of the reaction you are interested in, you might write a molecular, complete ionic, or net ionic equation.
Definitions of molecular, complete ionic, and net ionic equations
A molecular equation is sometimes simply called a balanced equation. In a molecular equation, any ionic compounds or acids are represented as neutral compounds using their chemical formulas. The state of each substance is indicated in parentheses after the formula. [Huh?]
Let's consider the reaction that occurs between \text{AgNO}_3AgNO
3
start text, A, g, N, O, end text, start subscript, 3, end subscript and \text{NaCl}NaClstart text, N, a, C, l, end text. When aqueous solutions of \text{AgNO}_3AgNO
3
start text, A, g, N, O, end text, start subscript, 3, end subscript and \text{NaCl}NaClstart text, N, a, C, l, end text are mixed, solid \text{AgCl}AgClstart text, A, g, C, l, end text and aqueous \text{NaNO}_3NaNO
3
start text, N, a, N, O, end text, start subscript, 3, end subscript are formed. Using this information, we can write a balanced molecular equation for the reaction:
\text{AgNO}_3(aq) + \text{NaCl}(aq) \rightarrow \text{AgCl}(s) + \text{NaNO}_3(aq)AgNO
3
(aq)+NaCl(aq)→AgCl(s)+NaNO
3
(aq)start text, A, g, N, O, end text, start subscript, 3, end subscript, left parenthesis, a, q, right parenthesis, plus, start text, N, a, C, l, end text, left parenthesis, a, q, right parenthesis, right arrow, start text, A, g, C, l, end text, left parenthesis, s, right parenthesis, plus, start text, N, a, N, O, end text, start subscript, 3, end subscript, left parenthesis, a, q, right parenthesis
[What kind of reaction is this?]
If we could zoom in on the contents of the reaction beaker, though, we wouldn't find actual molecules of \text{AgNO}_3AgNO
3
start text, A, g, N, O, end text, start subscript, 3, end subscript, \text{NaCl}NaClstart text, N, a, C, l, end text, or \text{NaNO}_3NaNO
3
start text, N, a, N, O, end text, start subscript, 3, end subscript. Since \text{AgNO}_3AgNO
3
start text, A, g, N, O, end text, start subscript, 3, end subscript, \text{NaCl}NaClstart text, N, a, C, l, end text, and \text{NaNO}_3NaNO
3
start text, N, a, N, O, end text, start subscript, 3, end subscript are soluble ionic compounds, they dissociate into their constituent ions in water. For example, \text{NaCl}NaClstart text, N, a, C, l, end text dissociates into one ion of \text{Na}^+Na
+
start text, N, a, end text, start superscript, plus, end superscript for every ion of \text{Cl}^-Cl
−
start text, C, l, end text, start superscript, minus, end superscript; these ions are stabilized by ion-dipole interactions with the surrounding water molecules. [I don't get it!]
Image of crystalline sodium chloride next to image of chloride and sodium ions dissociated in water. Each chloride ion is interacting with multiple water molecules through the positive dipole of the water, and each sodium ion is interacting with water molecules through the negative dipole of the water.
Image of crystalline sodium chloride next to image of chloride and sodium ions dissociated in water. Each chloride ion is interacting with multiple water molecules through the positive dipole of the water, and each sodium ion is interacting with water molecules through the negative dipole of the water.
Sodium chloride dissociates into sodium and chloride ions in water, and these ions become solvated by the highly polar water molecules. Image credit: "Salts: Figure 1" by OpenStax Anatomy and Physiology, CC-BY-NC-SA 4.0.
From the molecular formula, we can rewrite the soluble ionic compounds as dissociated ions to get the complete ionic equation:
\text{Ag}^+(aq) + \blueD{{\text{NO}_3}^-(aq)} + \maroonD{\text{Na}^+(aq)} + \text{Cl}^-(aq) \rightarrow \text{AgCl}(s) + \maroonD{\text{Na}^+(aq)} + \blueD{{\text{NO}_3}^- (aq)}Ag
+
(aq)+NO
3
−
(aq)+Na
+
(aq)+Cl
−
(aq)→AgCl(s)+Na
+
(aq)+NO
3
−
−
(aq)
+
Na
+
(aq)
+Cl
−
(aq)→AgCl(s)+
Na
+
(aq)
+
NO
3
−
(aq)
