Dr. Fleming's Help Pages and Tutorials

What is a Balanced Chemical Reaction?

A balanced Chemical Reaction is a representation of what chemical chnages occur in a reaction. It will reflect the remifications of both Dalton's Atomic Theory of matter as well as the Law of Conservation of Mass.

In order to do this, a properly balanced reaction will:

have the same number of each type of atom on either side of the reaction
Use the smallest whole number coefficients that will achieve the above goal
Indicate the mole ratios of reactants and products as related through a chemical reaction

A balanced equation makes it possible to quantitatively determine how much product one can expect from a given ammount of reacts utilized in a chemical reaction.

Now how much would you pay?

An example of a very simple balanced chemical reaction is given below:

Ba(OH)₂ + H₂SO₄ → BaSO₄ + 2 H₂O

Note that in this reaction, the numbers of atoms of each type are the same on either side of the reaction.

Element	# reactants	# products
Ba	1	1
O	6	6
H	4	4
S	1	1

The reaction can be interprested that one mole of barium hydroxide (Ba(OH)₂) will react with one mole of sulfuric acid (H₂SO₄) to form one mole of barium sulfate (BaSO₄) and two moles of water (H₂O).

Some hints for Balancing Chemical Reactions

Balance major atoms first

A major atom is generally something other than H or O

Balance ions as single units whenever possible
Always save pure elements for last
Practice, practice, practice!

An Example

Balance the reaction

___ NaCl + ___ Pb(NO₃)₂ → ___ NaNO₃ + ___ PbCl₂

In this reaction, we can treat nitrate (NO₃^-) as a single unit as it does not change in the reaction. Because there are two nitrates on the reactant side and only one on the product side, we can introduce a stoichiometric coefficient of 2 in front of sodium nitrate (NaNO₃).

___ NaCl + ___ Pb(NO₃)₂ → 2 NaNO₃ + ___ PbCl₂

Now there is an imbalance of sodium ions. This can be fixed by putting a 2 in front of sodium chloride (NaCl).

2 NaCl + ___ Pb(NO₃)₂ → 2 NaNO₃ + ___ PbCl₂

This balances the chloride (Cl^-) and the lead (Pb²⁺) so the reaction is balanced!

2 NaCl + Pb(NO₃)₂ → 2 NaNO₃ + PbCl₂

Note: When no numerical coefficient is present, the coefficient is 1.

Balancing a difficult reaction

Consider the reaction

___ PF₅ + ___ S → ___ PF₃ + ___ SF₄

What makes this challanging is that F shows up in both products. Balancing it is going to involve both reactants and one of the reactants!

A simple approach

Since each PF₅ molecule is giving up two F atoms, two PF₅ molecules will provide enough F to satisfy the formation of one SF₄ molecule. So let's put a 2 in front of the phosporus species.

2 PF₅ + ___ S → 2 PF₃ + ___ SF₄

The creates 10 F atoms on both sides. It also leaves the S balanced, with one atome on each side. So actually, the reaction is balanced at this point.

2 PF₅ + S → 2 PF₃ + SF₄

A Super-Cool Algebraic Solution

Remember all fo that algebra you leanred once upon a time? Here is one place you can put it to work to your advantage. If we note the the coefficients in front of the phosphorous species must be the same (in order to balance P) and that the coefficients in front of the S containing species must be the same, We can write the reaction as

x PF₅ + y S → x PF₃ + y SF₄

Now, we can use the F atoms to develop a relationship between x and y in our balanced equation. The total number of F atoms provided on the reactant side is 5x. And the toal on the products side is 3x + 4y. And, these totals must be equal! Therefore, we can write

5x = 3x + 4y

Soving for x yields

2x = 2

The smallest whole-numbers that satisfy this relationship are x = 1 and y = 2. So using these as the stoichiometric coefficients yields