Organic Chemistry

Chemistry at your Fingertips! Balancing Equations

Equation balancing is essential in chemistry.  To fully understand how a chemical equation even works, we need to define a few terms.

Balancing Equations

The first term is the reactantsReactants are the substances on the left side of an equation, before the arrow.
Products are the terms on the right side of the equation, after the arrow. Chemistry Balancing Equations
Logically these two terms make sense.  The products are what you have before the reaction, and the reactants are what you have after the reaction has occurred.
In order to have a balanced equation, both the reactants and products must have the same number of atoms.
When this has been achieved, the equation is said to have been balanced.  The below equation is balanced since the reactants equal the products.  We have 4 H’s on the left side and 4 H’s on the right side, 2 O’s on the left side and 2 O’s on the right side.


Requirements to balance an equation

Good question!  In order to balance an equation, you can only modify the coefficients (the numbers in front of the atom).  Chemistry Balancing Equations
For example, I can only modify the 2 in front of the H(on the reactants side), I cannot modify the subscript on the H and make that an H1.  This would make the world implode, and we don’t want that to happen.
Therefore, we tweak with the coefficients on each of the terms.  Another Error that is frequently made is trying to add coefficients where they don’t belong.


On the product side of the above equation, I cannot stick in a coefficient on the O.  What I’m trying to say is, you cannot make the above equation balanced by doing this:

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Therefore, you can only modify the coefficients as a term.  Let’s see if you get the hang of it with a simple balancing problem.

Problem 1

Problem: Which are reactants and which are products in the following reaction?2H2O(l) 2H2O(g)

Plan: By looking at the definitions above, the reactants are on the left side of the equation and the products are on the right side of the equation. Chemistry Balancing Equations
The H2O liquid is a reactant and the H2O gas is a product.
Discussion:  If you got this wrong, read the definitions above again.

Problem 2

Problem:  Balance the following equation      N2(g)+ H2(g) NH3(g)

Plan: We see that we have 2 N’s on the reactants side and 1 N on the product side, therefore we must multiply the product side by 2.
Once we multiply the product side by 2, we will have 6 H’s.  This means we need to multiply the reactant H2 term by 3.
Discussion: We can see by looking at the equation that everything is balanced.

Now that we have the general idea behind balancing, let’s try a harder problem.

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Problem 3

Problem:  Balance the following equation        C6H12O6(s) + O2(g) CO2(g) + H2O(l)

Plan: The first step is to put a coefficient of 1 in front of the longest term.  This helps you see what needs to be done to make the complicated term equal to the non-complicated terms.  C6H12O6 is the longest term, therefore a 1 is placed in front of it.
Since we have 6 C’s on the left side and 1 C on the right, we will multiply the CO2 by 6.  Since we have 12 H’s on the left and 2 on the right side, we’ll multiply the H2O by 6 also.  Now we need to check what we’ve got.
We’ve got the right number of H’s and the right number of C’s, but the O’s don’t match up.  We have 8 O’s on the left but 24 O’s on the right.  Therefore a 6 needs to be placed in front of the O2.
C6H12O6(s) + 6O2(g) 6CO2(g) + 6H2O(l)
Comment: Now you can see how balancing equations can get tricky!  The most important step shown here is to place a 1 in front of the most complicated term.  It really helps speed up the process.

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