Clarke Earley, Ph.D.
Department of Chemistry
Kent State University Stark Campus
North Canton, OH 44720
The following diagrams graphically illustrate two simple chemical reactions. Notice the difference between the subscripts (ex. H2) and the coefficients (shown in red) in these reactions. When balancing reactions, the coefficients can (and usually need to be) changed, but the subscripts present in the molecular formula can not be altered.
H2 + Cl2 --> 2 HCl
2 H2 + O2 --> 2 H2O
In a chemical reaction, atoms are NOT created or destroyed. What changes in a chemical reaction is the connections (bonds) between atoms. In the first reaction shown above, two hydrogen atoms (H) start out bonded to each other. During the course of the chemical reaction, this H-H bond breaks and a new H-Cl bond is formed.
A balanced chemical equation is a 'description' that accounts for all of the atoms in a chemical reaction. Every atom that appears in the reactant side (left side = starting material) MUST be accounted for in the products (right side). If five carbon atoms are present on the left side of a chemical equation, there must be exactly five carbon atoms on the right side. This is accomplished by changing the coefficients (the numbers in front of the formula). You are NOT allowed to change to formula to balance a chemical equation.
While there is not set procedure for balancing equations that works every time, the following steps will generally produce a balanced equation without requiring excess steps.
In general, you only want to change any coefficient one time. If you change a coefficient a second time, you will probably cause one of the 'earlier' elements to become 'unbalanced'.
Question: Balance the following equation for the combustion of ethanol.
CH3CH2OH + O2 ----> CO2 + H2O
In this reaction, all of the compounds have the correct formulas. The next step is to select the 'simplest' element. Either carbon (C) or hydrogen (H) could be used. For this example, we will select carbon. Following step #3, we change the coefficients in front of ethanol (CH3CH2OH) and carbon dioxide (CO2). Since there are two carbon atoms in ethanol and only one carbon atom in CO2, we add a two in front of CO2.
1 CH3CH2OH + ____ O2 ----> 2 CO2 + ____ H2O
This equation now has two carbon atoms on the reactant side and two carbon atoms on the product side, so the carbon atoms are balanced. The 'next simplest element' is hydrogen (H). Like before, we change the coefficients in front of ethanol and water (H2O) to balance this element. One ethanol molecule contains six (6) hydrogen atoms, and one water molecule contains two (2) hydrogen atoms. To balance this, we need three water molecules (3x2=6).
1 CH3CH2OH + ____ O2 ----> 2 CO2 + 3 H2O
Finally, we need to balance the oxygen atoms. On the reactants (left) side of this equation we have three (3) oxygen atoms, while on the product (right) side we have seven (7) oxygen atoms. (Two O atoms/CO2 times 2 = 4 oxygen atoms PLUS one O atom/H2O times 3 = 3 oxygen atoms. 4+3=7).
Since the reactant side has fewer oxygen atoms, we need to change one or both coefficients to increase the number of oxygen atoms. However, we do NOT want to change the coefficient in front of ethanol because we have already used this to balance carbon. (This is why I put a one in front of this molecule). Changing the coefficient in front of oxygen (O2) will give the correct answer.
1 CH3CH2OH + 3 O2 ----> 2 CO2 + 3 H2O
Finally, it is a good idea to check to be sure the equation really is still balanced.
|1 CH3CH2OH||+||3 O2||---->||2 CO2||+||3 H2O|