Chemical reactions can be classified into five classes based on the type of reactant or the number of reactants and products. The first two classes you will learn about are essentially the opposite of each other. Study the information on the tabs to learn about each one.
In a synthesis reaction two or more substances react to form a single product. Sometimes synthesis reactions are called combination reactions.
Watch the video to see some examples of synthesis reactions and what the equations for these types of reactions look like.
You may want to use the study guide to follow along. If so, click below to download the study guide.
In this lesson, we are going to look at some of the fundamental types of chemical reactions, and what those reactions look like when we quantify them in chemical equations. The first type of chemical reaction that we are going to examine are synthesis reactions.
The term synthesis means bringing parts together to make a new whole, so in these types of chemical reactions, we have two or more substances combining to form a single new substance.
When you see the chemical equation for a synthesis reaction, the reactants will always be two or more elements or compounds. On the right side of the arrow, there will always only be one compound as the product.
Now, before we start looking at these types of reactions, we need to first understand how certain elements exist in their natural state. Some elements, called diatomic elements, cannot exist as just a single atom. In order to have these elements in their pure, elemental form, they have to come in pairs. Two atoms: diatomic.
We can remember what these elements are by remembering the word “HONClBrIF.” This word is ridiculous, but it helps us remember all of these diatomic elements: H is hydrogen, O is oxygen, N is nitrogen, Cl is chlorine, Br is bromine, I is iodine, and F is fluorine.
So, if you ever see any of these elements discussed as reactants in a chemical reaction, you know that they are present as diatomics. Hydrogen is actually H2, oxygen is O2, nitrogen is N2, and so on.
With that in mind, let's look at a few examples of synthesis reactions.
The first synthesis reaction we will look at is potassium reacting with chlorine to produce potassium chloride. The balanced chemical equation for this is 2K plus Cl2 yields 2KCl. So our reactants are potassium and chlorine, and the single product produced is potassium chloride. Is this equation balanced? Well, we have two potassium atoms on the left side and two potassium atoms on the right. We also have two chlorine on the left and two on the right. So yes, this equation is balanced.
Alright, let's look at a tougher example: sulfur dioxide reacts with water to produce sulfurous acid. As a chemical equation, we can write this as SO2 plus H2O yields H2SO3. Our reactants here are sulfur dioxide and water, and our singular product is sulfurous acid. Is this equation balanced? Well, there's one sulfur on either side of the equation, three oxygens on either side, and two hydrogens. So yes, this chemical equation is balanced.
Let's look at one last synthesis reaction. This one is a little more challenging than the others. For this reaction, we have aluminum reacting with oxygen to produce aluminum oxide.
Let's start creating the chemical equation for this reaction. Aluminum can exist as a single atom, but oxygen cannot, because it's part of HONClBrIF. So we write Al plus O2. Next, to figure out how much aluminum and oxygen there is in aluminum oxide, we need to criss-cross the charges. The charge on aluminum is 3+, and the charge on oxygen is 2-. Recall that we get rid of those signs, and write each charge as the subscript for the other element. So we have that Al plus O2 yields Al2O3.
Our two reactants are aluminum and oxygen, and our single product is aluminum oxide. Now, is this equation balanced? Well, right off the bat, we can see that there is only one aluminum on the left side and two on the right. Oxygen is unbalanced too, so no, this equation is not balanced yet. We'll have to take care of that.
Let's begin by balancing oxygen. The least common multiple of 2 and 3 is 6, so we need to add coefficients that will make it so there are 6 oxygen atoms on either side of the equation. We do that by writing a coefficient of 3 on the left side, and a coefficient of 2 on the right. Now there are 6 oxygen atoms on either side of the equation.
Now that oxygen is balanced, let's balance aluminum too. Right now there are 2 times 2, or 4 aluminum atoms on the right side, so we can balance this equation by putting a coefficient of 4 next to aluminum on the left side.
Now we have 4 aluminum atoms, and 6 oxygen atoms on both sides, so this equation is balanced.
Most importantly, though, remember that this is a synthesis reaction because we took multiple elements or compounds and combined - or synthesized them together - into a single compound.
Question
How can you tell, just by looking at the number of reactants and products, that a certain chemical reaction is a synthesis reaction?
Synthesis reactions will have at least two reactants, but only one product.
You've just seen how substances combine during synthesis reactions. The counterpart to these types of reactions are decomposition reactions.
In a decomposition reaction a single compound breaks down into two or more simpler substances.
Watch the video to see some examples of decomposition reactions and what the equations for these types of reactions look like.
You may want to use the study guide to follow along. If so, click below to download the study guide.
You just looked at synthesis reactions, sometimes called combination reactions. Decomposition reactions are like the exact opposite of synthesis reactions. While a synthesis reaction begins with multiple substances and ends with a single compound, decomposition reactions do the reverse of this: one compound breaks down into two or more simpler substances. These substances might be elements, or they might themselves be compounds.
So, just like with synthesis reactions, you can identify a decomposition reaction by examining the reactants and products. In these types of reactions, there will be only one compound as a reactant. For the products, there will be two or more elements or compounds.
An example of a decomposition reaction is breaking water down into its constituent parts. We start with one compound, H2O, and at the end of the reaction, we have two substances, hydrogen, H2, and oxygen, O2. Notice that we had to use a couple coefficients to balance this equation. That will often be the case with chemical reactions.
To better understand these types of reactions, let's look at a couple of examples.
This example asks you to write the balanced chemical equation for the decomposition of lead four oxide. First, let's figure out what the chemical formula for lead four oxide is. We know that it will be composed of lead and oxygen. The charge on lead four is right in the name: four plus. The charge for oxygen is two minus. We drop the signs from those charges, and then we criss cross the charges, like this. Next, we have to reduce the subscripts by their greatest common factor of two, which leaves us with Pb1 O2. We can ignore that one, and we get a formula of PbO2.
This compound will decompose into its parts of lead and oxygen. Since oxygen is one of the honclbrif elements, we need to make sure that it is present as the diatomic O2, which it is. So in this reaction, our reactant is lead four oxide, and our products are lead and oxygen. Is this equation balanced? Well, we have one lead on both sides, and 2 oxygens, so this is balanced.
Let's look at one more example. This one asks you to find the equation for the decomposition of hydrogen iodide. Well, this compound will be made up of hydrogen and iodine. Hydrogen has a charge of 1+, and iodine has a charge of 1-. We can drop those signs, and criss cross the charges. Since they are both ones, we drop the subscripts to find that the chemical formula for hydrogen iodide is just HI. Hi! This will decompose into hydrogen and iodine.
Hydrogen and iodine are both honclbrif elements, so they both need to be present as diatomics, H2 and I2. Notice, however, that doing this unbalances the equation, so we need to add a coefficient of 2 next to the reactant.
So in this reaction, our reactant was hydrogen iodide, our products were hydrogen and iodine, H2 and I2, and we've verified that this equation is balanced.
So these are decomposition reactions. They are characterized by the presence of a single compound as a reactant, which breaks apart into multiple simpler substances during the chemical reaction.
Question
How can you identify a decomposition reaction based only on the numbers of reactants and products?
A decomposition reaction will always have exactly one reactant that breaks apart into multiple, simpler substances.
Question
Ammonia is a compound that is produced at chemical plants like the one shown here. The synthesis of ammonia from hydrogen and nitrogen is represented by this equation:
\(3\text{H}_{2\text{(g)}} + \text{N}_{2\text{(g)}} \rightarrow 2\text{NH}_{3\text{(g)}}\)
How is the decomposition of ammonia related to this reaction?
In the synthesis reaction, hydrogen and nitrogen combine to produce ammonia. In the decomposition reaction, ammonia is broken down into hydrogen and nitrogen according to this equation:
\(3\text{NH}_{3\text{(g)}} \rightarrow \text{H}_{2\text{(g)}} + 2\text{N}_{2\text{(g)}}\)
