In Lesson 2, "What Happens to Mass in a Chemical Reaction?" you learned about the law of conservation of mass. Recall that the law of conservation of mass states that matter (mass) cannot be created or destroyed. Therefore the total mass of the reactants in a chemical reaction must be equal to the total mass of the products. Chemical reactions represent different quantities or amounts of reactants and products using coefficients. Adding the coefficients insures equal masses on both sides of the equation; this process is known as balancing an equation.
The chemical reaction shown below is unbalanced, that is, it does not follow the law of conservation of mass.
| Mg |
+ |
HCl |
 |
MgCl2 |
+ |
H2 |
We can determine the reaction is unbalanced by performing an atom inventory. An atom inventory is counting different types of atoms on both sides of the equation to see if they are equal. An atom inventory for the equation is shown below.
| Mg |
+ |
HCl |
|
MgCl2 |
+ |
H2 |
| 1 |
Mg |
1 |
| 1 |
H |
2 |
| 1 |
Cl |
2 |
In order to balance the equation, we added a coefficient of 2 in front of the hydrochloric acid (HCl). The updated atom inventory is shown below.
| Mg |
+ |
2 HCl |
|
MgCl2 |
+ |
H2 |
| 1 |
Mg |
1 |
2 1 |
H |
2 |
2 1 |
Cl |
2 |
Notice now all elements have equal numbers of atoms on both the reactants and products side of the equation. This equation is now considered to be balanced.
