Competency #7

Answer and Explanation

ZnCl2 (aq) + Na2S(aq) Arrow. 2 NaCl (aq) + ZnS (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 ZnCl2 will break apart into Zn+?(we can't tell the charge by looking at the periodic table) and Cl1- ions (it is known that elements in group VIIA, such as F, Cl, and Br, will form 1- ions). Since we have two –1 ions, the charge on one Zn must be +2 . Na2S will break apart into 2 Na+1 ions and S-2 ions. Group IA always forms +1 ions, and oxygen and sulfur form –2 ions (remember this is the number of electrons an atom needs to gain or lose to get a filled outer shell. You can always figure this out using a periodic table).

Types of ions present: Zn2+ Cl1- Na1+ S-2

Step 2: Rearrange the ions to form neutral products

Zn2+ 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. Zn2+ cannot bond to Na1+ 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 Zn2+ to bond with S2-. Since the charges are equal, this will be a neutral compound.

The other product uses the other two ions: Na1+ and Cl1-. Using one of each of these, we get a neutral compound (overall charge = +1 –1 = 0) NaCl.

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, ZnCl2 is listed as soluble, which means it can be designated with an ‘(aq).' ZnS 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.

ZnCl2 (aq) + Na2S(aq) 2 NaCl (aq) + ZnS (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:

  1. We are allowed to change the coefficients
  2. If no coefficient is shown, it has an implied value of ‘1'
  3. 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 ‘NH3' means there is one NH3 molecule (1 N atom and 3 H atoms), while ‘2 NH 3' means there are 2 NH3 molecules (a total of 2 N atoms and 6 H atoms).
  4. 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.

Make a table will help us to keep track of the number of each atom on each side:

ZnCl2 (aq) + Na2S (aq) NaCl (aq) + ZnS (s)

Reactants Products Reactants
1 Zn 1
2 Cl 1
2 Na 1
1 S 1

We can see that the above chemical equation is not balanced.

By changing the coefficients in front of NaCl from ‘1' to ‘2,' we can get a balanced equation:

ZnCl2 (aq) + Na2S (aq) 2 NaCl (aq) + ZnS (s)

Reactants Products Reactants
1 Zn 1
2 Cl 2
2 Na 2
1 S 1

Often, it will take some trial and error to get a balanced chemical equation.

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