Competency #7
Answer and Explanation
MgCl2 (aq) + 2AgNO3 (aq) 
Mg(NO3)2 (aq) + 2AgCl (s)
There are 4 steps that can be followed to determine the products in an exchange (also called ‘double displacement') reaction, such as this one.
Step 1: Identify the types of ions present in solution
The physical state ‘(aq)' means that each of the reactants are dissolved in water. When an ionic compound dissolves in water, it breaks apart into its ions. In this example MgCl2 will break apart into Mg2+ and Cl1- ions (it is known that elements in the group IIA of the periodic table, such as Be, Ca, and Mg will form 2+ ions, while elements in group VIIA, such as F, Cl, and Br, will form 1- ions). AgNO3 will break apart into Ag? ions (we can't tell the charge by looking at the periodic table) and NO31- ions. NO31- is a group of atoms called a polyatomic ion. This ion, and other common observed polyatomic ions are found in this table, your textbook, and nearly any general chemistry text. Since we have 1 Ag? ion and 1 NO31- ion, we know the charge on the Ag ion must be +1.
Types of ions present: Mg2+ Cl1-Ag1+ NO31-
Step 2: Rearrange the ions to form neutral products
Mg2+ was bonded to Cl1- in the reactant. To form a new product, we must see if it can bond to either of the other ions present. Mg cannot bond to Ag1+ because both are positive and like charges repel (also it is impossible to form a neutral compound with these two ions).
The only remaining possibility is for Mg2+ to bond with NO31- . However if one Mg2+ ion bonds to one NO31- ion, the overall charge on this compound would be +1. Since we want to form neutral compounds (overall charge of zero), we will have to use one Mg 2+ ion and two NO31- ions (overall charge = +2 –1 –1 = 0): Mg(NO3) 2.
The other product uses the other two ions: Ag1+ and Cl1-. Using one of each of these, we get a neutral compound (overall charge = +1 –1 = 0) AgCl.
Note that in step 2, we are determining what the subscripts must be to give us neutral compounds. Once we determine the correct values for the subscripts, we cannot change them.
Step 3: Determine the physical state for each product (NOT REQUIRED)
For ionic compounds, look it up on the table of solubilities. In this case, Mg(NO3) 2is listed as soluble, which means it can be designated with an ‘(aq).' AgCl is listed as insoluble, which means that it will be a solid when formed, and can be designated with an ‘(s)' in the chemical equation.
MgCl2 (aq) + 2AgNO3 (aq) Mg(NO3)2 (aq) + AgCl (s)
Step 4: Balance the chemical equation
Now, we must make sure there is the same number of each type of atom in the reactants as there is in the products. Rules for balancing the equation:
- We are allowed to change the coefficients
- If no coefficient is shown, it has an implied value of ‘1'
- Only one coefficient per molecule/compound. The coefficient tells us how many of that molecule/compound are needed or are formed in the chemical reaction. For example ‘NH 3' means there is one NH3 molecule (1 N atom and 3 H atoms), while ‘2 NH3' means there are 2 NH3 molecules (a total of 2 N atoms and 6 H atoms).
- We are not allowed to change the subscripts. In Step 2, we determined what they must be and we cannot change them at this point.
Making a table will help us to keep track of the number of each atom on each side:
MgCl2 (aq) + 2AgNO3 (aq) Mg(NO3)2 (aq) + AgCl (s)
| Reactants | Products | Reactants |
|---|---|---|
| 1 | Mg | 1 |
| 2 | Cl | 1 |
| 1 | Ag | 1 |
| 1 | N | 2 |
| 3 | O | 6 |
We can see that the above chemical equation is not balanced.
By changing the coefficients in front of AgNO3 and AgCl from ‘1' to ‘2,' we can get a balanced equation:
MgCl2 (aq) + 2AgNO3 (aq) Mg(NO3)2 (aq) + 2AgCl (s)
| Reactants | Products | Reactants |
|---|---|---|
| 1 | Mg | 1 |
| 2 | Cl | 2 |
| 2 | Ag | 2 |
| 2 | N | 2 |
| 6 | O | 6 |
Often, it will take some trial and error to get a balanced chemical equation.