Video: Identifying Which Element Is out of Balance in a Chemical Equation

Which element is out of balance in the chemical equation CH₃CH₂OH + O₂ ⟶ 2CO₂ + 3H₂O?

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Video Transcript

Which element is out of balance in the chemical equation CH₃CH₂OH plus O₂ react to form two CO₂ plus three H₂O.

Checking whether a chemical equation is balanced requires three steps. Step one involves identifying the elements involved. The second step involves counting the atoms of each element on each side of the equation, on the reactants side and the products side. The third step involves looking at each element and seeing where the number of atoms on the reactants side and the products side are the same or not.

Let’s start by identifying the elements. Every element has a chemical symbol. And all the symbols are either a capital letter, or a capital letter followed by a lowercase letter. For instance, nitrogen has the chemical symbol N, while helium has the chemical symbol He. What we need to do is look at the chemical equation and break it down into the individual letter groups.

Let’s start with the first letter C. We isolate C because C is followed by another capital letter H. Therefore, they refer to different elements. If you look at your periodic table, you can see that the element symbol C refers to the element carbon. The next letter in the formula refers to hydrogen. We can ignore any numbers. The next C also refers to carbon. So, we can ignore it. And the next unique letter grouping is O. O refers to oxygen. And if we continue to look at the rest of the equation, we can see that we’ve accounted for all the elements. With this, we’ve completed step one, identifying the elements as carbon, hydrogen, and oxygen.

Now we can move on to step two. For this part, we can draw out this grid, with the equation at the top and the elements that we’ve identified to one side. We split the equation down the middle and work out the number of atoms of each element on the left-hand side and then on the right.

On the left, we have two C symbols, meaning we have two carbon atoms. So, we can do this to record that. On the right-hand side, we have only one symbol C, but we have two CO₂s. That means two molecules of carbon dioxide. So, we have two carbon atoms overall.

For hydrogen, we have three symbols for hydrogen on the left-hand side, but these subscript numbers indicate the number of atoms of that element. So, we have six hydrogen atoms in total. That’s three plus two plus one. On the right-hand side, we have three water molecules, which means we have three equivalents of H₂. This means that we have six hydrogen atoms on the right-hand side. That’s three multiplied by two.

With oxygen, our first reactant has one oxygen atom, and our second has two. In our first product, we have four oxygen atoms because we have two CO₂ molecules. And in our second product, we have three oxygen atoms because we have three water molecules.

Now we can move on to step three. For carbon, we have two atoms on the left-hand side and two on the right-hand side. This means that carbon is balanced in this equation. We have as much carbon in the reactants as we have in the products. For hydrogen, we have six on the left-hand side and six on the right-hand side. So, hydrogen is also balanced. Finally, for oxygen, we have three on the left-hand side and seven on the right-hand side.

Since we don’t have the same number of oxygen atoms in the reactants as we have in the products, oxygen is out of balance. Therefore, the element out of balance in our chemical equation is oxygen. If you’re interested, the balanced form of this equation would have two extra oxygen molecules on the reactants side, CH₃CH₂OH plus three O₂ react to form two CO₂ plus three H₂O.

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