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A net ionic equation is a form of chemical equation in which substances are represented by their ionic formulas and only the substances that participate directly in the chemical reaction are included. This type of equation is usually used to show the reactions of chemical compounds that exist in an aqueous, or liquid, solution called an electrolyte. Electrolytes are substances consisting of freely floating ions, positively or negatively charged atoms that enable the solution to conduct electricity. Some of the ions in an electrolyte can react with each other to form a product, while other ions remain in the solution and do not participate in the reaction. A net ionic equation cuts out the non-participating ions and shows only the ions that change into a different product during the course of the reaction.
In a general chemical equation, substances are represented by their full chemical formulas — for example, the reaction of aqueous (aq) solutions of sodium hydroxide (NaOH) and hydrochloric acid (HCl) would be shown as NaOH(aq) + HCl(aq), followed by an arrow and the formulas of the products, indicating what the reaction would yield. In reality, however, both sodium hydroxide and hydrochloric acid are strong electrolytes, which means that they dissociate completely into their constituent ions when they are in solution. Sodium hydroxide does not exist as a NaOH molecule in solution, it exists as a positively charged sodium ion (Na+) and a negatively charged hydroxide ion (OH-), both floating freely in the mixture. An ionic equation would therefore show strong electrolytes as individual ions with positive or negative charges. Non-electrolyte substances that did not dissociate into ions would stay written in molecular form.
The net ionic equation goes one step further. Instead of simply representing all the separate ions and their products in the equation, it omits the ions that do not change during the reaction. These non-participant ions are called spectator ions. By simplifying the reaction in this way, a net ionic equation shows only the actual reaction that is taking place.
For example, in the reaction of sodium hydroxide and hydrochloric acid mentioned above, a pure liquid (l) product, water (H2O), is formed. A water molecule consists of two hydrogen atoms bonded to one oxygen atom. In the water product, the single oxygen atom and one of the hydrogen atoms both come from the hydroxide (OH-) ion of sodium hydroxide, while the second hydrogen atom is the hydrogen ion (H+) of hydrochloric acid. Since the OH- and H+ ions change form and bond to create liquid water, they are included in the net ionic equation.
The sodium ion (Na+) from sodium hydroxide (NaOH) and the chlorine ion (Cl-) from hydrochloric acid (HCl), however, do not interact to create the final product. At the end of the reaction, they are still free-floating ions in the solution. Sodium and chlorine are therefore the spectator ions of this reaction, and are not included in the net ionic equation. The net ionic equation in the reaction described above would read OH-(aq) + H+(aq) --> H2O(l).