Competency #7
Answer and Explanation
Which of the following chemical equations are balanced?
3 MgO (s) + 2 Fe (s) 
Fe2O3 (s) + 3 Mg (s)
2 Al (s) + 3 O2 (g) Al2O3 (s)
The correct answer is A: The top equation is balanced but the lower equation is not.
You can make a table for each equation to determine if it is balanced:
3 MgO (s) + 2 Fe (s) Fe2O3 (s) + 3 Mg (s)
| Reactants | Products | Reactants |
|---|---|---|
| 3 | Mg | 3 |
| 3 | O | 3 |
| 2 | Fe | 2 |
From this table, we can see that the above chemical equation is balanced.
2 Al (s) + 3 O2 (g) Al2O3 (s)
| Reactants | Products | Reactants |
|---|---|---|
| 2 | Al | 2 |
| 6 | O | 3 |
We see that this equation is not balanced. :
Alternate Explanation:
We can also draw a more symbolic representation of what is indicated by each chemical equation.
3 MgO (s) + 2 Fe (s) Fe2O3 (s) + 3 Mg (s)
We can draw this as:
MgO FeFeFeOOO Mg
MgO Fe Mg
MgO Mg
In the above reaction, you can see that there are equal numbers of each atom on both sides.
2 Al (s) + 3 O2 (g) Al2O3 (s)
We can draw this as:
Al OO
Al OO AlAlOOO
OO
In the above reaction, you can see that there are NOT equal numbers of each atom on both sides.