Video Transcript
Small quantities of hydrogen peroxide are used in hair dyes. Hydrogen peroxide will naturally break down into water and oxygen. Which of the following equations shows a balanced chemical equation for this decomposition? (A) Two H2O2 reacts to produce H2O plus O2. (B) H2O2 reacts to produce two H2O plus O2. (C) Two H2O2 reacts to produce two H2O plus O2. (D) Two H2O2 reacts to produce H2O plus two O2. (E) H2O2 reacts to produce H2O plus O2.
In a balanced chemical equation, there are the same number of each type of atom on both sides of the reaction arrow. In each of the reaction equations, hydrogen and oxygen appear on both sides of the reaction arrow. So to determine which of the equations is balanced, we need to count the total number of hydrogen and oxygen atoms on both sides of the arrow.
Let’s start by looking at the equation in answer choice (A). This equation includes a coefficient. A coefficient is a number written in front of a species in a chemical equation. We can think of coefficients as multipliers. One molecule of H2O2 contains two atoms of hydrogen and two atoms of oxygen. So two molecules of H2O2 contain four atoms of hydrogen and four atoms of oxygen. In a molecule of water, there are two atoms of hydrogen and one atom of oxygen. There are two atoms of oxygen in a molecule of oxygen gas, so the total number of oxygen atoms on the right-hand side of the reaction arrow is three. The number of hydrogen and oxygen atoms on both sides of the reaction arrow is not the same, so the equation in answer choice (A) is not balanced.
In the equation in answer choice (B), there are two hydrogen atoms and two oxygen atoms on the left-hand side of the reaction arrow. Two molecules of water contain four atoms of hydrogen and two atoms of oxygen. Adding the oxygen atoms from the oxygen molecule gives a total of four oxygen atoms on the right-hand side of the equation. We can see that the number of hydrogen and oxygen atoms is not equal. So the equation in answer choice (B) is not balanced.
The products of equation (C) are the same as equation (B). So there are four atoms of hydrogen and four atoms of oxygen on the right-hand side of the arrow. There are also four atoms of hydrogen and four atoms of oxygen on the left-hand side of the arrow. The number of hydrogen and oxygen atoms is the same on both sides of the reaction arrow. So the equation in answer choice (C) is balanced and is likely the answer to the question. But just to be sure, let’s take a look at answer choices (D) and (E).
The reactants in answer choice (D) are the same as in (C). So there are four hydrogen atoms and four oxygen atoms on the left-hand side of the arrow. There are two hydrogen atoms and five oxygen atoms on the right-hand side of the arrow. So, the equation in answer choice (D) is not balanced. In the equation in answer choice (E), the number of hydrogen atoms is the same on both sides of the reaction arrow, but the number of oxygen atoms is different. Therefore, this equation is not balanced.
In conclusion, the equation that shows a balanced chemical equation for the decomposition of hydrogen peroxide is answer choice (C): two H2O2 reacts to produce two H2O plus O2.