15.4 Describing Reactions in Solutions by Writing Molecular, Complete Ionic, and
15.4 Describing Reactions in Solutions by Writing Molecular, Complete Ionic, and Net Ionic Equations
Learning Objectives
By the end of this section, you will be able to:
- Write and balance chemical equations in molecular, complete ionic, and net ionic formats.
Ionic Compounds in Solution
We have learned that one important aspect about ionic compounds that differs from molecular compounds has to do with dissolving in a liquid, such as water. When molecular compounds, such as sugar, dissolve in water, the individual molecules drift apart from each other. When ionic compounds dissolve, the ions physically separate from each other. We can use a chemical equation to represent this process—for example, with NaCl:
When NaCl dissolves in water, the ions separate and go their own way in solution; the ions are now written with their respective charges, and the (aq) phase label emphasizes that they are dissolved (Figure 15.4a and Figure 15.4b). When an ionic compound dissociates in water, water molecules surround each ion and separate it from the rest of the solid. Each ion goes its own way in solution.
All ionic compounds that dissolve behave this way. (This behaviour was first suggested by the Swedish chemist Svante August Arrhenius [1859–1927] as part of his PhD dissertation in 1884. Interestingly, his PhD examination team had a hard time believing that ionic compounds would behave like this, so they gave Arrhenius a barely passing grade. Later, this work was cited when Arrhenius was awarded the Nobel Prize in Chemistry.)
Keep in mind that when the ions separate, all of the ions separate. Thus, when CaCl2 dissolves, the one Ca2+ ion and the two Cl− ions separate from each other:
CaCl2(s) → Ca2+(aq) + Cl–(aq) + Cl–(aq)
which is simplified to
CaCl2(s) → Ca2+(aq) + 2Cl–(aq)
That is, the two chloride ions go off on their own. They do not remain as Cl2 (that would be elemental chlorine; these are chloride ions); they do not stick together to make Cl2− or Cl22−. They become dissociated ions in their own right. It is important to note that polyatomic ions retain their overall identity when they are dissolved. It is important to understand and write the chemical equation that represents the dissociation of ions, as it is a crucial concept when writing complete ionic chemical equations, and net ionic equations.
Example 15.4a
Write the chemical equation that represents the dissociation of each ionic compound.
- KBr
- Na2SO4
- (NH4)3PO4
Solution
- KBr(s) → K+(aq) + Br−(aq)
- Not only do the two sodium ions go their own way, but the polyatomic sulfate ion stays together as the sulfate ion. The dissolving equation is Na2SO4(s) → 2Na+(aq) + SO42−(aq)
- Not only do the three ammonium ions stay together, but the one phosphate ion stays together as well since they are both polyatomic ions. The dissolving equation is (NH4)3PO4(s) → 3NH4+(aq) + PO43−(aq)
Exercise 15.4a
Equations for Ionic Reactions
Given the abundance of water on earth, it stands to reason that a great ma