Video Transcript
Consider the following equation:
Fe3+ aqueous plus πOHβ aqueous react to form π, a solid. What are the correct substitutions
for π and π to complete and balance the partial ionic equation? (A) π equals one, π equals
FeOH. (B) π equals three, π equals
Fe2O3. (C) π equals two, π equals
Fe(OH)2. (D) π equals three, π equals
Fe(OH)3. (E) π equals one, π equals
Fe(OH)3.
Letβs begin by examining the given
partial ionic equation. The βaqβ symbol next to each ion
stands for an aqueous solution, which means the ions are dissolved in a solution
with water. On the product side of the
equation, the βsβ symbol next to the π indicates that a solid forms when the ions
react. Chemists call this solid a
precipitate.
The type of compound formed when
two ions react is called an ionic compound. In this question, we are being
asked to find the correct substitutions for π and π in the provided equation. π is a coefficient, which is a
whole number used to balance a chemical equation. On the other hand, π is the
chemical formula of the ionic compound formed in the reaction.
Letβs take a closer look at the
provided partial ionic equation. A partial ionic equation is also
known as a net ionic equation. This type of equation shows ions
involved in a chemical change. What ions are reacting in the
provided equation? Fe is the chemical symbol for
iron. This iron ion has a three plus
charge, so itβs called the iron(III) ion. OHβ is the chemical formula for the
hydroxide ion. This ion has a charge of one
minus.
When the ions react, they combine
in a specific ratio to form a neutral compound. Letβs determine how many of each of
these ions will combine to form the neutral compound. We can try a few ratios. When one Fe3+ ion combines with one
hydroxide ion, there is a net charge of positive two. The compound must be neutral, so it
needs to have a zero charge. This is not the correct ratio. If one Fe3+ ion combines with two
hydroxide ions, there is a net charge of positive one. This ratio does not give us a net
charge of zero. So this is also not a correct
ratio. If one Fe3+ ion combines with three
OHβ ions, the net charge is zero. So the correct ratio of ions in the
compound is one Fe3+ ion to three OHβ ions.
Now we need to write a correct
chemical formula for the ionic compound. When writing chemical formulas for
ionic compounds, the metal cation is always written first and the nonmetal anion is
written second. The number of ions in the compound
is written as a subscript to the right of each ion. Because the compound contains only
one iron cation, we do not need to use a subscript to the right of the iron ion. However, the compound contains
three hydroxide anions. So weβll need to use a subscript to
the right of the hydroxide ion in the chemical formula.
The hydroxide ion is a polyatomic
ion. Itβs composed of two atoms: one
oxygen atom and one hydrogen atom. We need to use parentheses when we
have more than one polyatomic ion in a chemical formula. The chemical formula of the ionic
compound formed is Fe(OH)3. Knowing this allows us to eliminate
answer choices (A), (B), and (C) because these choices do not contain the correct
chemical formula.
Now we need to focus on balancing
the net ionic equation. On the left side of the equation,
there is currently one iron ion. And on the right side of the
equation, there is also one iron ion. There is one oxygen atom on the
left side of the equation, and there are three oxygen atoms on the right side. The three outside of the
parentheses is distributed to both of the atoms inside the parentheses. On the left side of the equation,
there is currently one hydrogen atom. And there are three hydrogen atoms
on the right side of the equation.
The iron atoms are currently
balanced in the equation. However, the oxygen and hydrogen
atoms are not. We need to multiply both the oxygen
and the hydrogen atoms on the left side by a coefficient of three. This means that the π in front of
the OHβ ion in the equation is three.
We have successfully determined the
substitutions for π and π. π is equal to three, and π is
equal to Fe(OH)3. The correct answer is choice
(D).