A chemical reaction is a process in which chemical bonds are either formed or broken. In other words, chemical reactions are processes in which chemical compounds either form (from simpler compounds or elements) or disintegrate (into simpler compounds or elements). The elements or compounds that exist before the reaction are called reactants; the ones that exist afterward are called products.
Chemical reactions are represented by chemical equations. The formulas for the reactants are shown on the left side of the equation, and the formulas for the products are shown on the right side. An arrow points from the reactants to the products, and a ‘+’ sign separates the formulas on each side. For instance, here’s a chemical equation representing the reaction of hydrogen molecules (H2) with oxygen molecules (O2) to form water molecules (H2O).
2H2 + O2 → 2H2O
Notice that some of the formulas are prefixed by a non-subscript number—in this case the number “2,” which I have highlighted in yellow. The reason for this prefix is that each side of the equation must have exactly the same number of each type of atom. Each oxygen atom bonds with two hydrogen atoms when forming water, and we began with two oxygen atoms (in the form of an O2 molecule). So we need a total of four hydrogen atoms, and hence two of the H2 molecules. That’s why the reactant H2 is prefixed with “2” on the left side of the equation. Likewise, the prefix “2” is used on the right side of the equation, because there will be two H2O molecules when the reaction is finished.
Notice that there are a total of 4 hydrogen atoms and 2 oxygen atoms in the reactants, and that same number of each type of atom in the products. An equation is said to be balanced if its reactants and products contain the same number of each type of atom. When writing a chemical equation, it is often easiest to write the reactants and products first, then prefix them as needed to balance the equation. (Examples will be given below.)
Predicting what products will form in a chemical reaction can be difficult, but it is relatively easy to predict what will happen when a metal near the left side of the periodic table (especially an alkali or alkaline metal) reacts with a non-metal near the right side. Recall that elements near the left side prefer to give away their valence electrons, while elements near the right side want to fill their outer shells. For this reason, the products of such reactions are usually ionic compounds—that is, compounds involving ionic bonds. And it is easy to determine how many electrons each metal atom will give to each non-metal atom, because we know how many valence electrons each metal atom has to give away and how many extra electrons each non-metal atom needs to fill its outer shell. We can use this information to predict exactly what ionic compound will form.
Mg + F2 → MgF2
Notice that the equation is already balanced: we don’t have to prefix any of the reactants or products with a number, because both sides of the equation already contain the same number of each type of atom—namely, one magnesium atom and two fluorine atoms.
For another example, consider what happens when magnesium metal burns (i.e., combines with oxygen from the air). What is the chemical equation describing that reaction? Oxygen in the air consists of O2 molecules. So the reactants are Mg and O2. What about the products? Well, magnesium has two valence electrons, and oxygen needs two more electrons to fill its outer shell. So, if each magnesium atom pairs up with exactly one oxygen atom, everyone is happy! Thus, the result of the reaction will be magnesium oxide (MgO). To write a chemical equation describing this reaction, we begin by writing the reactants on the left side of the arrow and the products on the right side, like this:
Mg + O2 → MgO
But the above equation is unbalanced, because there are two oxygen atoms on the left side and only one on the right. Since each Mg atom combines with just one O atom, we’ll need two Mg atoms for each O2 molecule, and the product will be two molecules of MgO. So, here’s the correct (balanced) chemical equation describing what happens when magnesium burns:
2Mg + O2 → 2MgO
Let’s try one more example. What’s the chemical equation describing what happens when potassium burns? The reactants are atoms of potassium metal (K) and molecules of oxygen (O2 ). What about the products? Potassium has just one valence electron to give away, but each oxygen atom wants two. So two K atoms will team up with each O atom to form a molecule of potassium oxide (K2O). To balance the equation, we’ll need four atoms of potassium for each oxygen molecule in the reactants, and the product will be two molecules of potassium oxide. Here’s the chemical equation:
4K + O2 → 2K2O
Reactions can be exothermic (give off heat energy) or endothermic (absorb heat energy). The energy released (in the form of heat) in exothermic chemical reactions comes from chemical potential energy. But chemical potential energy is really a form of electromagnetic potential energy, since chemical bonds involve the electromagnetic force. Likewise, the heat absorbed in endothermic chemical reactions is converted into chemical potential energy—a form of electromagnetic potential energy.
In this interactive activity, you can practice balancing chemical equations. Adjust the number of molecules of each reactant and product so that both sides of the equation have the same number of atoms of each element.