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When we describe a precipitation reaction using a chemical equation, we have the choice of either writing the net ionic equation (i.e. leaving out spectator ions) or writing the respective ionic compounds:

$$ \begin{align} &\text{molecular(?)} &\quad \ce{NaCl(aq) + AgNO3(aq) &<=> AgCl(s) + NaNO3(aq)}\tag{1}\\ &\text{net ionic} &\quad \ce{Cl-(aq) + Ag+(aq) &<=> AgCl(s)}\tag{2} \end{align} $$

The net ionic equation does contain ions, plus an ionic compound, so I understand the name. However, what is the reason to call the other equation "molecular" - there are no molecules in this reaction (other than water).

Nevertheless, this is the term I find in textbook (e.g. OpenStax). Is there a better name ("compound equation" perhaps, or "implicit ions")? Is there a context where molecular equation makes more sense, and this is just a special case where it does not?

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  • $\begingroup$ I think the term molecular equation would be unfair for this precipitation reaction. Why don't you would use a "chemical equation" for the first reaction. $\endgroup$
    – M. Farooq
    Sep 28 at 20:12
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    $\begingroup$ @M.Farooq I am asking about the term used in textbooks. "Chemical equation" would refer to all three variations (showing compounds, showing all ions, omitting spectator ions). $\endgroup$ Sep 28 at 23:14
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As a brief recap, when calling such reaction a "molecular equation", we are supposed to forget that the empirical formulas for ionic substances refer to formula units, not molecules. We treat all components as if they were molecules.

The quirk with "molecular equation" lies within the willful ignorance striving for a simplification in terminology to the point of being solipsic. This happens presumably because balancing chemical reactions in solutions precedes basics of solid state chemistry and crystallography in the majority of curricula.

In my opinion, such simplification is unjust despite many textbooks still using it to this day. I usually try to either refer to these reactions by their name ("precipitation reaction", "neutralization reaction"), or use the universal term formula equation. The latter applies both to molecules and formula units equally well.

I haven't found an explicit style guide recommendation to back up this suggestion. However, several engineering and general chemistry textbooks prefer this term. So does, for example, Zumdahl's Chemistry [1, p. 137]:

Three Types of Equations Are Used to Describe Reactions in Solution

  • The formula equation gives the overall reaction stoichiometry but not necessarily the actual forms of the reactants and products in solution.

  • The complete ionic equation represents as ions all reactants and products that are strong electrolytes.

  • The net ionic equation includes only those solution components undergoing a change. Spectator ions are not included.

Reference

  1. Zumdahl, S. S.; Zumdahl, S. A.; DeCoste, D. J. Chemistry, 10th ed.; Cengage: Boston, MA, 2017. ISBN 978-1-305-95740-4.
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