In a precipitation reaction the balanced chemical equation has ions as the reactants and a neutral ionic solid as the product. Although one reactant carries positive charge and the other carries negative charge, total charge must be conserved in the balanced equation.
To write net ionic equations, remove all spectator ions from the total ionic equation. Spectator ions are species that do not change as the reaction proceeds from reactants to products. In an ionic equation, the formulas for all soluble strong electrolytes is written in dissociated form. In a net ionic equation, the ions that are actually involved in the chemical reaction are shown.
DefinitionBack to Top
Net ionic equations are commonly written for chemical reactions in aqueous solution because they describe the actual chemical species involved in a reaction. To write net ionic equations one should know which compounds exist as ions in solution.
In the complete ionic equation representing the reaction of calcium hydroxide and sodium carbonate, some ion (OH- and Na+) appear on both sides of the equation. This means that nothing happens to these ions as the reaction occurs. They are called spectatro ions. A spectator ion is an ion in an ionic equation that does not take part in the reaction. Cancel ions from both sides to express the essential reaction that occurs.
2OH-(aq) + 2Na+(aq) + CO32-(aq) $\rightarrow$ CaCO3(s) + 2Na+(aq) + 2OH-(aq)
The resulting equation is
Ca2+(aq) + CO32-(aq) $\rightarrow$ CaCO3(s)
This is the net ionic equation, an ionic equation from which spectator ions have been canceled.
How to Write a Net Ionic Equation?Back to Top
Net ionic equations are commonly written for chemical reactions in aqueous solution because they describe the actual chemical species involved in a reaction. To write net ionic equations one must know which compounds exist as ions in solution.
- Strong acids, soluble strong bases and soluble salts exist as ions in solution. Examples include the acids HCl and HNO3 a base such as NaOH and salts such as NaCl and CuCl2.
- All other species should be represented by their complete formulas. Weak acids such as acetic acid exist in solutions primarily as molecules. Insoluble salts such as CaCO3 or insoluble bases such as Mg(OH)2 should be written in ionic form even though they are ionic compounds.
Follow the steps to write the net ionic equation.
- Write a complete balanced equation. Indicate the state of each substance.
- Rewrite the equation writing all strong acids, strong soluble bases and soluble salts as ions.
- Some ions may remain unchanged in the reaction. The ions that appear in the equation as both reactants and products cancel each other.
ExamplesBack to Top
Net ionic equations are useful in writing acid-base reactions. Consider the acid base reaction that occurs when a solution of potassium hydroxide is mixed with a solution of hydrochloric acid. The overall equation is
HCl(aq) + KOH(aq) $\rightarrow$ KCl(aq) + H2O(l)
However, all of the compounds except water exist in solution as ions (HCl ionizes in water to produce H+ and Cl-). So the complete ionic equation is
Cl-(aq) + K+(aq) + OH-(aq) $\rightarrow$ K+(aq) + Cl-(aq) + H2O(l)
The K+ and Cl- ions are present in the same forms on both sides of the equation. Because they undergo no change, they can be omitted from the equation. The net ionic equation is
H+(aq) + OH-(aq) $\rightarrow$ H2O(l)
Ionic and Net Ionic EquationsBack to Top
Ionic equations are written by assuming the strong electrolyte dissociate in aqueous solution into the corresponding ions. The ionic equation for the reaction between BaCl2 and Na2SO4 would be written as follows.
Ba2+(aq) + 2Cl-(aq) + 2Na+(aq) + SO42-(aq) $\rightarrow$ BaSO4(s) + 2Na+(aq) + 2Cl-(aq)
The three soluble ionic compounds BaCl2, Na2SO4 and NaCl are all dissociated into their ions. BaSO4 however insoluble in water and therefore listed in this equation as a solid. Certain ions appear both sides and remain unchanged during the chemical reaction, they are referred to as spectator ions. Spectator ions can be cancelled from both sides of the equation on a one to one basis. This yileds the following net ionic equation for the reaction between aqueous solutions of BaCl2 and Na2SO4.
Ba2+(aq) + SO42-(aq) $\rightarrow$ BaSO4(s)
Net ionic equations contain all the information necessary to understand the chemical reaction in atomic scale. The number of atoms or ions of each element and the net charge on both sides of the equation must be balanced.
Balancing Net Ionic EquationsBack to Top
Balancing net ionic equation will be done by counting the ions and adding coefficients to equalize the numbers of each ion on both sides of the equation. Every substance in net ionic equation will have a positive and a negative ion and the subscripts will tell how many ions is present. Once the ionic equation if formed we have to identify and cancel the spectator ions which is absolutely identical in which they cancel each other.
For example, consider the equation
FeCl3(aq) + Pb(NO3)2(aq) $\rightarrow$ Fe(NO3)3(aq) + PbCl2(s)
Break the substances into ions except PbCl2. Therefore the ionic equation for the above equation is
Cancel the spectator ions from the reactant side and the product side. Then the equation takes the following form.
3Pb2+(aq) + 6Cl-(aq) $\rightarrow$ 3PbCl2(s)
Pb2+(aq) + 2Cl-(aq) $\rightarrow$ PbCl2(s)
The net ionic equation becomes
Pb2+(aq) + 2Cl-(aq) $\rightarrow$ PbCl2(s)
RulesBack to Top
Equations for chemical equilibria are made easier to write if the dissolved ionic compounds involved in them are written as separate ions. It is often found that in equilibria involving such ionic compounds one of the ions appears both as reactant and as product. These ions are known as spectator ions. Deleting spectator ions from an ionic equation simplifies the equation and gives a net ionic equation.
While writing ionic equation we should follow certain rules.
- Soluble ionic salts are written as separate ions.
- Insoluble ionic salts are written as complete formula units.
- Very strong and strong oxo acidsexist in solution predominantly as separate ions and are written as such.
- Moderate and weak oxo acids exist in solution mostly as molecules and are written as complete formula units.
Net Ionic Equation of Mg and HClBack to Top
Consider the reaction between magnesium hydroxide and hydrochloric acid. Th overall balanced equation is
2HCl(aq) + Mg(OH)2(s) $\rightarrow$ 2H2O(l) + MgCl2(aq)
The acid and base furnish hydrogen ions and hydroxide ions respectively.
2HCl(aq) $\rightarrow$ 2H+(aq) + 2Cl-(aq)
Mg(OH)2(s) $\rightleftharpoons$ Mg2+(aq) + 2OH-(aq)
By using the above information the complete ionic equation is written as
The spectator ions cancelled each other resulting in the net ionic equation.
2H+(aq) + 2OH-(aq) $\rightarrow$ 2H2O(l)
If the reaction takes place between strong acid and strong base then neutralization reaction takes place and the net ionic equation is obtained as shown above.