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Question Video: Balancing a Net Ionic Equation Chemistry • 10th Grade

Consider the following equation: Fe³⁺ (aq) + 𝑋OH⁻ (aq) ⟶ 𝑌 (s). What are the correct substitutions for 𝑋 and 𝑌 to complete and balance the partial ionic equation?

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Video Transcript

Consider the following equation: Fe3+ aqueous plus 𝑋OH− aqueous react to form 𝑌, a solid. What are the correct substitutions for 𝑋 and 𝑌 to complete and balance the partial ionic equation? (A) 𝑋 equals one, 𝑌 equals FeOH. (B) 𝑋 equals three, 𝑌 equals Fe2O3. (C) 𝑋 equals two, 𝑌 equals Fe(OH)2. (D) 𝑋 equals three, 𝑌 equals Fe(OH)3. (E) 𝑋 equals one, 𝑌 equals Fe(OH)3.

Let’s begin by examining the given partial ionic equation. The “aq” 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 “s” symbol next to the 𝑌 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 𝑋 and 𝑌 in the provided equation. 𝑋 is a coefficient, which is a whole number used to balance a chemical equation. On the other hand, 𝑌 is the chemical formula of the ionic compound formed in the reaction.

Let’s 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 it’s called the iron(III) ion. OH− 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. Let’s 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 OH− ions, the net charge is zero. So the correct ratio of ions in the compound is one Fe3+ ion to three OH− 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 we’ll need to use a subscript to the right of the hydroxide ion in the chemical formula.

The hydroxide ion is a polyatomic ion. It’s 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 𝑋 in front of the OH− ion in the equation is three.

We have successfully determined the substitutions for 𝑋 and 𝑌. 𝑋 is equal to three, and 𝑌 is equal to Fe(OH)3. The correct answer is choice (D).

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