Chemistry » Chemical Reactions and Stoichiometry » Writing and Balancing Chemical Equations

Equations for Ionic Reactions

Equations for Ionic Reactions

Given the abundance of water on earth, it stands to reason that a great many chemical reactions take place in aqueous media. When ions are involved in these reactions, the chemical equations may be written with various levels of detail appropriate to their intended use.

To illustrate this, consider a reaction between ionic compounds taking place in an aqueous solution. When aqueous solutions of CaCl2 and AgNO3 are mixed, a reaction takes place producing aqueous Ca(NO3)2 and solid AgCl:

\(\mathrm{CaCl_2}(aq) + \mathrm{2AgNO_3}(aq) \longrightarrow \mathrm{Ca(NO_3)_2}(aq) + \mathrm{2AgCl}(s)\)

This balanced equation, derived in the usual fashion, is called a molecular equation because it doesn’t explicitly represent the ionic species that are present in solution. When ionic compounds dissolve in water, they may dissociate into their constituent ions, which are subsequently dispersed homogeneously throughout the resulting solution (a thorough discussion of this important process is provided in a later tutorial that covers solutions). Ionic compounds dissolved in water are, therefore, more realistically represented as dissociated ions, in this case:

\(\mathrm{CaCl_2}(aq) \longrightarrow \mathrm{Ca}^{2+}(aq) + \mathrm{2Cl}^{−}(aq)\)

\(\mathrm{2AgNO_3}(aq) \longrightarrow \mathrm{2Ag}^{+}(aq) + \mathrm{2NO_3}^{−}(aq)\)

\(\mathrm{Ca(NO_3)_2}(aq) \longrightarrow \mathrm{Ca}^{2+}(aq) + \mathrm{2NO_3}^{−}(aq)\)

Unlike these three ionic compounds, AgCl does not dissolve in water to a significant extent, as signified by its physical state notation, s.

Explicitly representing all dissolved ions results in a complete ionic equation. In this particular case, the formulas for the dissolved ionic compounds are replaced by formulas for their dissociated ions:

\(\mathrm{Ca}^{2+}(aq) + \mathrm{2Cl}^{−}(aq) + \mathrm{2Ag}^{+}(aq) + \mathrm{2NO_3}^{−}(aq) \longrightarrow \mathrm{Ca}^{2+}(aq) + \mathrm{2NO_3}^{−}(aq) + \mathrm{2AgCl}(s)\)

Examining this equation shows that two chemical species are present in identical form on both sides of the arrow, \(\mathrm{Ca}^{2+}(aq)\) and \(\mathrm{NO_3}^{−}(aq)\). These spectator ions—ions whose presence is required to maintain charge neutrality—are neither chemically nor physically changed by the process, and so they may be eliminated from the equation to yield a more succinct representation called a net ionic equation:

\(\require{enclose}\enclose{horizontalstrike}{\mathrm{Ca}^{2+}(aq)} + \mathrm{2Cl}^{−}(aq) + \mathrm{2Ag}^{+}(aq) + \enclose{horizontalstrike}{\mathrm{2NO_3}^{−}(aq)} \longrightarrow \enclose{horizontalstrike}{\mathrm{Ca}^{2+}(aq)} + \enclose{horizontalstrike}{\mathrm{2NO_3}^{−}(aq)} + \mathrm{2AgCl}(s)\)

\(\mathrm{2Cl}^{−}(aq) + \mathrm{2Ag}^{+}(aq) + \longrightarrow \mathrm{2AgCl}(s)\)

Following the convention of using the smallest possible integers as coefficients, this equation is then written:

\(\mathrm{Cl}^{−}(aq) + \mathrm{Ag}^{+}(aq) + \longrightarrow \mathrm{AgCl}(s)\)

This net ionic equation indicates that solid silver chloride may be produced from dissolved chloride and silver(I) ions, regardless of the source of these ions. These molecular and complete ionic equations provide additional information, namely, the ionic compounds used as sources of Cl and Ag+.

Molecular and Ionic Equations

When carbon dioxide is dissolved in an aqueous solution of sodium hydroxide, the mixture reacts to yield aqueous sodium carbonate and liquid water. Write balanced molecular, complete ionic, and net ionic equations for this process.

Solution

Begin by identifying formulas for the reactants and products and arranging them properly in chemical equation form:

\(\mathrm{CO_2}(aq) + \mathrm{NaOH}(aq) \longrightarrow \mathrm{Na_2CO_3}(aq) + \mathrm{H_2O}(l) \text{ (unbalanced)}\)

Balance is achieved easily in this case by changing the coefficient for NaOH to 2, resulting in the molecular equation for this reaction:

\(\mathrm{CO_2}(aq) + \mathrm{2NaOH}(aq) \longrightarrow \mathrm{Na_2CO_3}(aq) + \mathrm{H_2O}(l)\)

The two dissolved ionic compounds, NaOH and Na2CO3, can be represented as dissociated ions to yield the complete ionic equation:

\(\mathrm{CO_2}(aq) + \mathrm{2Na}^{+}(aq) + \mathrm{2OH}^{−}(aq) \longrightarrow \mathrm{2Na}^{+}_{aq} + \mathrm{CO_3}^{2−}(aq) + \mathrm{H_2O}(l)\)

Finally, identify the spectator ion(s), in this case Na+(aq), and remove it from each side of the equation to generate the net ionic equation:

\(\require{enclose}\mathrm{CO_2}(aq) + \enclose{horizontalstrike}{\mathrm{2Na}^{+}(aq)} + \mathrm{2OH}^{−}(aq) \longrightarrow \enclose{horizontalstrike}{\mathrm{2Na}^{+}_{aq}} + \mathrm{CO_3}^{2−}(aq) + \mathrm{H_2O}(l)\)

\(\mathrm{CO_2}(aq) + \mathrm{2OH}^{−}(aq) \longrightarrow \mathrm{CO_3}^{2−}(aq) + \mathrm{H_2O}(l)\)

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