When we need to express or compare reaction rates, it is always best to consider individual ions. For that reason, we should always convert formula equations to ionic equations. When a piece of zinc is dropped into a solution of hydrochloric acid, bubbles of hydrogen gas form and the piece of zinc gradually shrinks. An aqueous solution of zinc chloride also forms. The balanced formula equation for this reaction is Zn solid plus 2 HCl aqueous gives H2 gas plus ZnCl2 aqueous.
Notice the two aqueous solutions, 2 HCl aqueous and ZnCl aqueous. These are both soluble compounds, so an aqueous solution, they actually consist of individual ions, so we can dissociate them both into individual ions. When you are dissociating compounds, always have your ion table handy so you can check to see that all ions have their correct charges.
We'll dissociate HCl into H plus ions and Cl minus ions. Because the HCl in the formula equation has a coefficient of 2, it means both of its ions, H plus and Cl minus, will also have a coefficient of 2. Also, because HCl is aqueous, both of its ions, are aqueous. The other aqueous compound is ZnCl2 on the right side of the equation.
Because it's a soluble aqueous compound, we can also dissociate it into its separate ions. The zinc cation has a charge of 2 plus, and the chloride anion has a charge of minus 1. There is no ion with the formula Cl2. When ZnCl breaks up, it gives off two chloride ions.
So the subscript 2 in the formula equation becomes the coefficient 2 in front of the Cl-in this equation. And because ZnCl2 in the formula equation is aqueous, both of its individual ions are aqueous. Solid species in the formula equation are not in the form of ions, so they remain in the solid form unchanged in the equation. Similarly, gases do not occur as ions and remain in the molecular form, unchanged in this equation.
We'll add an arrow here to make this an equation. This is called the complete ionic equation for this reaction. To summarize, in the complete ionic equation, aqueous species in the formula equation are dissociated into individual ions, but solids and gases are always written exactly the same way as they are in the formula equation.
Notice there are two C-ions on the left side and the right side of this equation. The C-ions haven't changed at all in this reaction. People who attend sporting events but don't participate in the game are called spectators. Similarly, the C-ions are present but do not participate in the reaction. So Cl-ions are called spectator ions.
This leads us to the net ionic equation. To write the net ionic equation, we just write the complete ionic equation, and leave out any spectator ions. So in the net ionic equation, all the species in the complete ionic equation are included, and the spectator ions, 2Cl-are left out. When we are dealing with reaction rates, the net ionic equation is the most useful form of the equation, because it shows only the species that react and the products they form.
In summary, all three forms of this equation for this reaction are useful in their own way. The formula equation shows the molecular formulas for the reactants and products, as well as their states, but does not show individual ions. The complete ionic equation, which is sometimes called the total ionic equation, shows all the individual ions present in the reaction, as well as any other species.
It includes any spectator ions on both sides of the equation. And the net ionic equation shows only the species that are actually reacting. This includes individual aqueous ions, solids, gases, and any liquids if they are present.
it does not include spectator ions. Let's do another example. When aqueous bromine is added to an aqueous solution of iron iodide, aqueous iodine and aqueous iron bromide are formed.
Here is the formula equation showing this reaction. The aqueous bromine and iodine are in elemental molecular form, and they do not dissociate into ions. As long as the dissolved iodine is dilute, it will remain dissolved in water, rather than forming solid iodine.
FeI3 and FeBr3 are soluble aqueous ionic compounds, so they will occur in dissociated form in the solution. Now we can write the complete ionic equation. 2FeI3 releases 2 Fe3 plus ions.
1FeI3 releases 3 I minus ions. So 2FeI3 release a total of 6 I minus ions. 2FeBr3 released 2 Fe3 plus ions and a total of 6 Br minus ions.
Aqueous bromine is in molecular form in the formula equation, and it remains in molecular form in the complete ionic equation. Similarly, aqueous iodine is in the molecular form in the formula equation, and it remains in molecular form in the complete ionic equation. We now have the complete ionic equation.
Notice the two Fe3 plus aqueous ions are identical on each side of the equation. Therefore we can say that the aqueous Fe3 plus ion is a spectator ion in this reaction. So when we write the net ionic equation, we can leave out the two Fe3 plus ions on each side, giving us this as our final net ionic equation. for this reaction. In summary, here are the formula equation, complete ionic equation, and net ionic equation for the reaction of aqueous bromine with aqueous iron iodide solution.