Video Transcript
A pale-yellow precipitate is produced in the reaction between aqueous silver nitrate
and potassium bromide solutions. What is the net ionic equation for this reaction?
In this question, our job is to write a net ionic equation for the reaction
described. However, to begin, it would be best to write a balanced molecular equation first. A molecular equation is a chemical equation in which all reactants and products have
chemical formulas of undissociated compounds. In the question, we are told there is a reaction between aqueous silver nitrate and
potassium bromide solutions. Therefore, these solutions are the reactants of the chemical reaction, and we need to
write their chemical formulas.
In a chemical compound, silver ions tend to have a one plus charge, so we can write
Ag+. The chemical formula of the polyatomic nitrate ion is NO31−. We can see that the sum of the ionic charges of these two ions is zero. In other words, to form a neutral compound, only one silver ion and one nitrate ion
are needed. Of course, we are not required to show subscripts of one in a chemical formula, but
we should be sure to include the state symbol aq to show that we have an aqueous
solution of silver nitrate.
In a compound, potassium ions tend to have a one plus charge and bromide ions tend to
have a one minus charge. Therefore, the chemical formula of potassium bromide is KBr. And we must use the state symbol aq to show that it is dissolved in an aqueous
solution. According to the question, a pale-yellow precipitate is formed in the reaction. A reaction in which an insoluble product, or precipitate, forms from the reaction of
two soluble substances in solution is called a precipitation reaction.
In our reaction, the pale-yellow precipitate is silver bromide. Solid silver bromide forms when silver ions combine with bromide ions. We can see that the sum of the ionic charges of these two ions is zero. Therefore, the chemical formula of silver bromide is AgBr, and we must include the
state symbol s to show that it is a solid precipitate.
If we look back at the reactants, we notice that the ions that do not form a
precipitate are the nitrate and potassium ions. These ions remain dissolved in the solution. So we write the chemical formula of aqueous potassium nitrate as KNO3 with the state
symbol aq. Now, we should make sure that our molecular equation is balanced. There is one silver ion, one nitrogen atom, and three oxygen atoms on both sides of
the chemical equation. We also notice that there is one potassium ion and one bromide ion on both sides of
the equation. Therefore, this molecular equation is already balanced.
Now, before we can write a net ionic equation, we will need to construct an ionic
equation. An ionic equation separates ionic compounds and acids that are dissolved in aqueous
solutions into separate ions. In our molecular equation, there are three ionic compounds that are dissolved in
aqueous solutions: silver nitrate, potassium bromide, and potassium nitrate.
Let’s start with the reactants. Each ion must be written with its ionic charge followed by the state symbol aq. Now, let’s look at the products. Notice that we did not break up the silver bromide precipitate into ions. However, we did separate potassium nitrate into ions because this ionic compound is
dissolved in an aqueous solution. At last, we’re ready to write a net ionic equation. A net ionic equation is a simplified ionic equation that shows only the species
involved in the chemical reaction.
We know that silver ions and bromide ions react to form the silver bromide
precipitate. However, after the reaction takes place, the potassium ions and nitrate ions are
still present in the aqueous solution. These ions did not participate in the chemical reaction and are called spectator
ions. To write a net ionic equation for the reaction, we need to remove these spectator
ions from the ionic equation. Therefore, the net ionic equation for the reaction between aqueous silver nitrate and
potassium bromide solutions is Ag+ aqueous plus Br− aqueous react to form AgBr
solid.
