Question Video: Identifying an Incorrectly Balanced Chemical Equation Chemistry

Which of the following is not a balanced chemical equation? [A] 3 HClO ⟶ HClO₃ + 2 HCl [B] 2 HClO ⟶ Cl₂O + H₂O [C] 3 HClO₂ ⟶ 2 HClO₃ + HCl [D] 3 HClO ⟶ 2 HClO₂ + HCl [E] 3 HClO₃ ⟶ HClO₄ + H₂O + 2 ClO₂

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Video Transcript

Which of the following is not a balanced chemical equation? (A) Three HClO reacts to form HClO3 plus two HCl. (B) Two HClO reacts to form Cl2O plus H2O. (C) Three HClO2 reacts to form two HClO3 plus HCl. (D) Three HClO reacts to form two HClO2 plus HCl. (E) Three HClO3 reacts to form HClO4 plus H2O plus two ClO2.

A balanced chemical equation is a reaction equation with the same number of atoms of each element on the reactants and product sides of the equation. So to determine if each of these five reaction equations is balanced, we’ll need to count the number of atoms of each element on both sides of the reaction equation. In each of these chemical reactions, there are the elements hydrogen, chlorine, and oxygen. Let’s start off by checking if the chemical equation in answer choice (A) is balanced. In the reactant molecule, there is one atom of hydrogen, one atom of chlorine, and one atom of oxygen. But the coefficient of three in front of this chemical species means that there are three of this molecule, giving us a total of three atoms each of hydrogen, chlorine, and oxygen on the reactant side.

In the first product, there is one atom of hydrogen, one atom of chlorine, and three atoms of oxygen indicated by the subscript. In the second product, there is one hydrogen atom and one chlorine atom. There are two molecules of this product, giving us a total of three hydrogen atoms, three chlorine atoms, and three oxygen atoms on the product side. We have three atoms of each element on both the reactant and product sides of the equation. So this chemical equation is balanced. Since we’re looking for an equation that is not balanced, answer choice (A) is not the correct answer. So let’s move on to the equation in answer choice (B).

In the reactant, there’s one hydrogen atom, one chlorine atom, and one oxygen atom. There are two molecules of this reactant, giving us a total of two atoms of each element on the reactant side. Moving on to the first product, there are two atoms of chlorine and there is one oxygen atom. In the second product, there are two hydrogen atoms and one more oxygen atom, giving us a total of two atoms of each element on the product side. Since there are two atoms of each element on both the reactant and product side of the equation, this equation is also balanced. So answer choice (B) is not the correct answer.

Now let’s take a look at the equation in answer choice (C). In the reactant molecule, there is one hydrogen atom, one chlorine atom, and two oxygen atoms. There are three of this reactant molecule. So there are three hydrogen atoms, three chlorine atoms, and six oxygen atoms on the reactant side. In the first product, there is one hydrogen atom, one chlorine atom, and three oxygen atoms, and there are two molecules of this product. In the second product, there is one more hydrogen atom and one more chlorine atom. So there is a total of three hydrogen atoms, three chlorine atoms, and six oxygen atoms on the product side. Once again, the number of atoms of each element are the same on the reactant and product side of the equation. So answer choice (C) is a balanced chemical equation. This is not the correct answer.

Now, let’s look at answer choice (D). In the reactant molecule, there is one hydrogen atom, one chlorine atom, and one oxygen atom. There are three molecules of this reactant, so there are three atoms each of hydrogen, chlorine, and oxygen on the reactant side. In the first product, there is one atom of hydrogen, one atom of chlorine, and two atoms of oxygen, and there are two molecules of this product. In the second product, there is one more hydrogen atom and one more chlorine atom, giving us a total of three hydrogen atoms, three chlorine atoms, and four oxygen atoms on the product side.

As we can see, we have three hydrogen and chlorine atoms on both sides of the equation, but we have three oxygen atoms on the reactant side and four oxygen atoms on the product side. Since the number of oxygen atoms is not the same on both sides of the equation, this chemical equation is not balanced. So answer choice (D) is the correct answer. But let’s look at answer choice (E) to make sure that we have the hang of this. In the reactant molecule, there is one hydrogen atom, one chlorine atom, and three oxygen atoms. Since there are three molecules of this reactant, there are three atoms of hydrogen, three atoms of chlorine, and nine atoms of oxygen on the reactant side.

In the first product molecule, there’s one hydrogen atom, one chlorine atom, and four oxygen atoms. In the second product molecule, there are two hydrogen atoms and there is one oxygen atom. So far, there are three hydrogen atoms, one chlorine atom, and five oxygen atoms on the product side. Moving on to the final product, there is one chlorine atom and two oxygen atoms, and there is two of this product molecule. This gives us a total of three hydrogen atoms, three chlorine atoms, and nine oxygen atoms on the product side. The number of hydrogen, chlorine, and oxygen atoms are the same on both the reactant and product side of the equation. So this equation was correctly balanced as well. It’s no surprise that this is not the correct answer.

Of the equations we were given, the only one that was not a balanced chemical equation was answer choice (D). Three HClO reacts to form two HClO2 plus HCl.

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