GED Science Practice Test: How To Balance Chemical Reactions

We have discussed factors that affect the rate of a chemical reaction, as well as some characteristics of reactions (enthalpy and entropy).  Now we look at chemical reactions from an atomic level.  Chemical reactions are often represented through a chemical equation.  The following is an example of a chemical equation:

sci210

Remember that the reactants are the substances that start a chemical reaction.  They are found on the left side of the arrow.  The arrow in the diagram above means “yields,” which can be thought of as “react to make.”  In other words, if you read the chemical equation from left to right, it reads:  the reactants react to make the products.  Remember that the products are what results from a chemical reaction.

Also notice that this reaction has two reactants:  SO2 (which is sulfur dioxide) and O2 (which is an oxygen gas molecule). Different reactants are separate by a plus sign (+). Remember from our lesson on chemical bonding that little numbers, the subscripts, tell you how many atoms of an element are found in a compound.  For example, sulfur dioxide (SO2) is made up one sulfur atom and two oxygen atoms.  This reaction has one product, which is sulfur trioxide (SO3).  This equation also shows you that all of the substances in the reaction are gases, because you see the (g) beside each of the reactants and products.  Lastly, you see large numbers beside some of the substances, like the “2” in 2SO3.  The large numbers are called coefficients.  Coefficients will be explained in more detail in the section on balancing chemical equations.

Remember from earlier in this lesson that reactions can be classified based on their energy profiles.  If energy is released in a chemical reaction, we say that the reaction is exothermic.  If energy is absorbed in a chemical reaction, we say that the reaction is endothermic. There are, however, different ways to categorize chemical reactions, based on what happens at the molecular level. If you do some research, you will find that chemists differ in how many types of reactions there are.  We will begin by talking about three categories of types of chemical reactions:

sci211

In reaction one, a chemical bond is formed between two reactants to make a product.  In reaction two, a complex product is broken down into two different products.  In reaction three, two types of reactants break bonds, rearrange to find new “partners” and then form new bonds.  If you understand these categories, you can understand the four basic types of reactions:

sci212

While some chemists identify further types of reactions, such as combustion and neutralization reactions, these are the four basic types.  These four basic types also help us to understand what happens in a chemical reaction:  chemical bonds between atoms are broken and/or reformed.  Let us return to our sample chemical equation:

sci213

This is an example of a synthesis reaction, since two reactants combine to make a single product.  In this reaction, extra chemical bonds are made.

So, chemical reactions can have different energy profiles, be of different types, and have different reaction rates.  A feature common to all chemical reactions is that the Law of Conservation of Mass is always followed.  The Law of Conservation of Mass states that matter can be changed from one form into another, but the total amount of mass remains constant. Let us first start by examining a very simple chemical reaction in order to understand the Law of Conservation of Mass:

sci214

This chemical reaction is represented by a diagram and an equation.  Notice that the reaction is a decomposition reaction, since a reactant is breaking down into smaller products.  According to the Law of Conservation of Mass, the mass of the reactants and products must be equal.  Matter cannot disappear, nor can it appear out of nowhere.  So if you look at the picture on the left, the reactant includes one copper (Cu) atom and two chlorine (Cl) atoms.  On the right side of the arrow, you find the products. The products also show one copper atom and two chlorine atoms. Thus, matter is conserved.

This notion that mass remains constant even when chemical change occurs is important, because it helps to explain why some equations need to be balanced to reflect that the mass of reactants and products is equal.  Observe another equation:

sci215

If we again count up the number of atoms of each element on the reactant side by looking at the pictures, we see that there are 6 hydrogen atoms (white circles), and there are 2 nitrogen atoms (blue circles).  On the product side, there are also six hydrogen atoms and two nitrogen atoms.  This equation obeys the Law of Conservation of Mass.  However, this equation is different from the one above.  Notice that in this equation, if you look at the equation, rather than the pictures, you see coefficients in front of some of the chemical formula.  For example, in 3H2, the number “3” is a coefficient which means that you should have 3 of the H2 molecules in order for the equation to be balanced.  In addition, you see the coefficient “2” in front of the product, NH3. This means that you will end up with two NH3 molecules.  The pictures above each reactant and product help you to visualize what the coefficient means.

In the examples above, you were given an already balanced equation to show how each equation obeyed the Law of Conservation of Mass.  However, sometimes you are asked to balance an equation. Chemical equations are balanced by changing the coefficients on the products and reactants of a chemical formula so that there are the same number of atoms of each element on each side of the arrow. If there is no coefficient before a chemical formula, it is understood that the coefficient in this case is 1; that there is one of these molecules. In the example above, there is one molecule of N2 and three molecules of H2 on the left (reactant) side of the reaction.

The example below walks you through the specific steps of balancing a chemical reaction:

Step 1: Write out the unbalanced reaction

Write down reactants, arrow, and products.

Example:

sci216

Step 2:  Perform an initial count of atoms in reactants and atoms in products

It may help you to draw pictures for this step, but you don’t have to.  If you do draw pictures, don’t worry about getting the correct structure, just make sure you count each compound correctly.  For example, CH4 in the equation above represents one carbon and four hydrogens:

sci217

Step 3:  Determine if the elements on each side of the arrow are equal:

If each element has the same number of atoms on each side of the arrow, the equation is balanced, and you are finished.  However, that is not the case with this equation.  While the carbon is balanced (one carbon atom on each side of the arrow), the hydrogens and oxygens are not balanced.  There are four hydrogens on the reactant side, but only two on the product side.  There are two oxygens on the reactant side, but three on the product side:

sci218

Step 4: Balance the equation and Recount

Next, you must add one coefficient at a time to try and balance the equation.  Let us start by trying to balance the hydrogen atoms.  Right now, there are four hydrogen atoms in the reactants, but only two hydrogen atoms in the products.  So we will start by placing the coefficient “2” in front of the H2O on the product side.  The reason for this is that the coefficient “2” multiplied by the subscript “2” will give you four hydrogens on the product side:

sci219

Each time you add a coeffient, it is important to count up the atoms again.  Notice that though we were adding a coefficient to balance the number of hydrogens, we also changed the number of hydrogens.  So after you add a coefficient, you recount the atoms.  If everything is equal, you are finished and the equation is balanced.  If everything is not equal, you continue doing step 4 until the equation is balanced.

For this equation, the number of oxygens is still not balanced, so we you will need to add a coefficient to the reactant side of the equation, since there are fewer oxygens (2) on the reactant side than on the product side (4):

sci220

Now that the number of each element, carbon (C), hydrogen (H), and oxygen (O) are equal on the reactant and product side, you are finished and the equation is balanced.  The final balanced equation can be written as follows:

sci221

It might also help to visualize the elements on the reactant and product side physically balancing each other:

sci222

Notice that at no point while we were balancing the chemical equation did we change the subscripts.  We only added coefficients.  To change the subscripts is to change the actual products and reactants that are formed in an equation.

You have seen 1 out of 15 free pages this month.
Get unlimited access, over 1000 practice questions for just $29.99. Enroll Now