Tuesday, November 8, 2016

Balancing Equations and Reactions

"Chemical reactions must be balanced." What does that mean?

The number of atoms that begin the reaction must end up somewhere. If there are ten atoms at the beginning, when the reaction is done, there will still be ten atoms. However, how the atoms are combined into compounds changes!

For this discussion, the way chemical notation is represented will be slightly altered. The coefficient will, as always, precede the molecule and identify how many molecules or sets of molecules (moles) are needed for the reaction. However, rather than using the subscripted number FOLLOWING the atom, hereafter, the number of atoms will be indicated by the use of a normal number.

Thus, water will be represented as H2O. Aluminum chloride will be AlCl3. Three waters will be 3•H2O. Three aluminum chloride will be 3•AlCl3. The • is added to help set off the coefficient. In other places notation can be found that look like 3H2O or 3ALCL3 or 5C6H12O6.

Clearly, the use of subscripts is preferred, but for this discussion, they will be set aside.

The process of balancing an equation begins by writing out all of the reactants and the products, putting them on opposite sides of either an equal sign or an arrow. Water and aluminum chloride will be used as examples.

Suppose it is desirable to combine oxygen and hydrogen to make water. The formula for those two elements are O2 and H2, meaning that each molecule of oxygen has 2 oxygen atoms and each molecule of hydrogen has two hydrogen atoms. The formula for water is H2O, meaning there are two hydrogen atoms and one oxygen atom in each molecule.

So the reaction looks like this, to begin with:

H2 + O2 = H2O

However, this equation is NOT in balance. On the left side (reactant side), there are two hydrogen and two oxygen, but on the right side (product side), there is only ONE oxygen. To correct this, the following can be done:

H2 + O2 = 2•H2O

Adding the coefficient of 2 in front of the water molecule results in there being two oxygens present, so the number of oxygens balance. HOWEVER, adding the coefficient means that there are, now, 4 hydrogen present. Another adjustment needs to take place. 

The equation can be brought into balance if 4 hydrogens can be on the left side of the equation. This can happen by adding yet another coefficient:

2•H2 + O2 = 2•H20

At last, everything balances—on the left are 4 hydrogen and 2 oxygen, and on the right are the same numbers of the same atoms.

Look at the unbalanced formula for making aluminum chloride:

Al + HCL = AlCl3 + H2

To balance this, several coefficients are needed. That HCL has one of each hydrogen and chlorine on the left, but there are 3 and 2 of them on the right, results in some "tricky" math. 

Trying a coefficient of 2 on the left results in:

Al + 2•HCL = ?•AlCl3 + H2

That brought the hydrogen into balance, but the chlorine still did not work out. Where 3 chlorines are needed, only two were available. The balanced equation requires this:

2•AL + 6•HCL = 2•AlCl3 + 3•H2

Thus, there are on BOTH sides of the equal sign: 2 Al, 6 H, and 6 Cl.

Summary Thoughts:

To balance a chemical equation:
  • Begin with the symbols for each element or compound involved.
  • Place the reactants on the left of the equal sign and the products on the right.
  • Change the coefficients as needed until—VITAL CONCEPT—there are the same number of each atom on both sides of the equal sign.

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