Chemical equations show the substances that exist before and after a chemical reaction occurs. To accurately represent a chemical reaction, a chemical equation must show that matter is conserved. In other words, the chemical equation must show that the number of each type of atom on the reactant side is equal to the number of each type of atom on the product side.
Reminder
Number and type of atoms in the reactants = Number and type of atoms in the products
To show that mass is conserved during a chemical reaction, a chemical equation must be balanced. In a balanced chemical equation, the number of atoms of all elements are equal on both sides of the arrow.
Let's Watch
Watch this video to learn how chemical equations are balanced to accommodate for this requirement. As you watch the video, pay attention to how the coefficients are adjusted in the process of balancing equations. Recall that a coefficient is the number written in front of a reactant or product that shows the relative proportions of each reactant or product in the reaction. Note that the coefficients are usually not written if the value is one.
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All chemical reactions obey the law of conservation of mass, which says that the total mass of the reactants must be exactly equal to the total mass of the products. Along with this, the quantities of each element involved in a chemical reaction must be the same before and after the reaction occurs. Balancing chemical equations allows us to write chemical equations in a way that shows this conservation of quantity.
Let’s quickly review the law of conservation of mass. To understand this, let’s look at a chemical reaction between solid zinc and an aqueous solution of copper sulfate. On the left side of the equation, we have 1 zinc, just like we do on the right side of the equation. Similarly, each side of the equation has 1 copper, 1 sulfur, and 4 oxygens. You can also view the sulfate ions as distinct units, meaning we have 1 sulfate on each side of the equation. Since the quantities for all elements are equal on both sides of the equation, this is a balanced chemical equation.
But when a chemical equation is unbalanced, there are some steps we need to take to balance it. First, we use whole number coefficients in front of each chemical formula. We only ever change these coefficients, never the subscripts. To make the process a little simpler, you can often treat polyatomic ions as single units. Let’s look at a few examples of balancing chemical equations.
For the first example, let’s look at the following reaction: hydrogen gas reacts with oxygen gas to produce water. This is the unbalanced equation for this reaction. Well, on the left side of the equation, we have 2 hydrogens and 2 oxygens. On the right side of the equation, we have 2 hydrogens, but only 1 oxygen. To fix this, we are going to add a coefficient of 2 next to the H2O. As a result of this, there are now 4 hydrogens and 2 oxygens on the right side. We will need to increase the amount of hydrogen on the left side. If we put a coefficient of 2 there, we now have 4 hydrogens and 2 oxygens on each side of the equation, so the equation is balanced. And this is our final, balanced equation: 2H2 plus O2 yields 2H2O.
Let’s look at a slightly tougher example now. Zinc hydroxide reacts with phosphoric acid to produce zinc phosphate and water. If we write that as an unbalanced equation, we get Zn(OH)2 plus H3PO4 yields Zn3(PO4)2 plus H2O. With reactions like this, it can be helpful to rewrite H2O as HOH, so that we can see that water is just hydrogen attached to a hydroxide ion. On the left side, we have 1 zinc, 2 hydroxides, 3 hydrogens, and 1 phosphate. On the right side, we have 3 zincs, 1 hydroxide, 1 hydrogen, and 2 phosphates.
Let’s begin by balancing the zincs. To do that, we put a coefficient of 3 on next to the zinc-containing substance on the left side. The makes it so there are 3 zincs in the reactants, but now we have 6 hydroxides. To balance the hydroxides, we go back to the right side, and put a 6 in front of the HOH. Now we have balanced the hydroxides, but we need to balance the hydrogens. The right side has twice as many hydrogens, so if we add a coefficient of 2 on the left side, that gives us 6 hydrogens and 2 phosphates. And now we have a balanced chemical equation.
This process can be challenging, and it often takes time, but the result is a valid chemical equation that accurately represents what occurs during a given reaction.
Question
When balancing a chemical equation, which should be adjusted: coefficients or subscripts? Explain.
The coefficients are adjusted to make the number of atoms of each element equal on both sides of the equation. The subscripts are never adjusted because doing so changes the identity of the substance.
