KEY2CHEM

Writing Chemical Equations

Chemical equations are a useful way to convey a change in a written way. The chemical equation describes the conversion of reactants into products and can be written as a molecular (includes all components in their neutral form), ionic (separating aqueous ionic compounds into individual ions, as they actually exist in solution), or net ionic (only including ionic species that change during the process) equation, depending on the information one wishes to emphasize.

Example 1.

Aqueous solutions of potassium chloride ($$\require{mhchem}\ce{KCl}$$) and lead (II) nitrate ($$\require{mhchem}\ce{Pb(NO3)2}$$) are combined to form solid lead (II) chloride ($$\require{mhchem}\ce{PbCl2}$$) and aqueous potassium nitrate ($$\require{mhchem}\ce{KNO3}$$). Write the balanced molecular equation for this process.

A. $$\require{mhchem}\ce{2 KCl(aq) + Pb(NO3)2(aq)->PbCl2(s) + 2 KNO3(aq)}$$

B. $$\require{mhchem}\ce{Pb^{2+} (aq) + 2Cl^{-}(aq)->PbCl2(s) }$$

C. $$\require{mhchem}\ce{KCl(aq) + Pb(NO3)2(aq)->PbCl2(s) + KNO3(aq)}$$

Solution

A. $$\require{mhchem}\ce{2 KCl(aq) + Pb(NO3)2(aq)->PbCl2(s) + 2 KNO3(aq)}$$

The molecular equation shows all the compounds together, even if they are aqueous. Balancing the equation requires a $$2$$ in front of $$\require{mhchem}\ce{KCl}$$ and $$\require{mhchem}\ce{KNO3}$$.

Example 2.

Write the ionic equation corresponding to the molecular equation given below.

$$\require{mhchem}\ce{NaCl(aq) + AgNO3(aq)->AgCl(s) + NaNO3(aq)}$$

A. $$\require{mhchem}\ce{NaCl(aq) + AgNO3(aq)->AgCl(s) + NaNO3(aq)}$$

B. $$\require{mhchem}\ce{Na+(aq) + Cl^{-}(aq) + Ag+(aq) + NO3^{-}(aq)->AgCl(s) + Na+(aq) + NO3^{-}(aq)}$$

C. $$\require{mhchem}\ce{Na(s) + Cl2(g) + Ag(s) + NO3^{-}(aq)->AgCl(s) + Na(s) + NO3^{-}(aq)}$$

Solution

B. $$\require{mhchem}\ce{Na+(aq) + Cl^{-}(aq) + Ag+(aq) + NO3^{-}(aq)->AgCl(s) + Na+(aq) + NO3^{-}(aq)}$$

The ionic equation separates aqueous ionic compounds into individual ions, since this is actually how they exist in solution. Solids, liquids, and gases stay together, as they do not dissociate in aqueous solution.

Example 3.

Write the net ionic equation corresponding to the following balanced molecular equation.

$$\require{mhchem}\ce{HCl(aq) + NaOH(aq)->H2O(l) + NaCl(aq)}$$

A. $$\require{mhchem}\ce{H+(aq) + Cl^{-}(aq) + Na+(aq) + OH^{-}(aq) -> H2O(l) + Na+(aq) + Cl^{-}(aq) }$$

B. $$\require{mhchem}\ce{Na+(aq) + Cl^{-}(aq) -> NaCl(aq)}$$

C. $$\require{mhchem}\ce{H+(aq) + OH^{-}(aq) -> H2O(l)}$$

C. $$\require{mhchem}\ce{H+(aq) + OH^{-}(aq) -> H2O(l)}$$

The net ionic equation, like the ionic equation, separates aqueous species into their component ions. The spectator ions (ions that are identical on both the reactant and product sides of the equation) are then canceled out to leave the net ionic equation. The spectator ions in this equation are $$\require{mhchem}\ce{Na+}$$ and $$\require{mhchem}\ce{Cl-}$$.