Chapter 7

In this chapter there are five important ideas.

All ionic compounds are solids in their standard state (25 degrees Celsius and 1 atmosphere). Ionic compounds are composed of ions; elements or molecules which are positively charged are called cations and elements or molecules which are negatively charged are called anions. A simple model to be used to imagine what ions (cations or anions) look like at the atomic level, is a sphere. For simple monoatomic ions and most polyatomic ions we can think of as charged spheres of different sizes.

In the state the spheres (ions) are arranged so that any particular ion is surrounded by the oppositely charged ion. What ever pattern which arises depends on several factors which we will not be discussing at this point in our class. Below is a picture of a model of a simple ionic compound. Notice the spheres (ions) of different sizes and how each sphere (ion) is surrounded by the other spheres (ion).

What happens when an ionic compound dissolves in water? Below is a figure of a small crystal of sodium chloride surrounded by water molecules. We all know when sodium chloride is added to water the crystals of sodium chloride disappear, dissolve.

What happens when the sodium chloride dissolves? Below is a picture showing a chloride ion (larger sphere) and a sodium ion (smaller sphere) dissolved in water.

Notice hop water surrounds (hydrates) each ion. For the negatively charged chloride ion the hydrogen's of each water molecule orient themselves towards the chloride ion. For the positively charged sodium ion the oxygen's of each water molecule orient themselves towards the sodium ion.

The specific example of what happens when sodium chloride is added to water can be used for the general case of any ionic compound.

Remember we talked about the conductivity of solutions containing dissolved ionic compounds. You are testing the conductivity of soluble ionic compounds in laboratory this week.

To begin we looked at the conductivity of a sample of deionized water (water with little or no ions) which could be viewed as pure water. Looking at the image on the left the light bulb is not glowing. This can be explained in terms of the nature of the species in the sample of water. That the light does not glow suggests there are no ions in the sample. This is an important measurement to establish subsequent measurements in deionized water. Any observed conductivity will due to the presence of the solute.

Next we measured the conductivity of an aqueous solution of sodium chloride. Here the light bulb glows brightly indicating the presence of ions in the solution. The ability of an aqueous solution of sodium chloride to conduct electricity is characteristic of a strong electrolyte. How do we express this observation using a chemical equation?

NaCl(s) ---H2O--> Na+(aq) + Cl-(aq)

Soluble ionic compounds are also strong electrolytes, i.e., behave the same way as sodium chloride...forming ions when dissolved in water. How do we know whether an ionic compound dissolves in water? That is determined by experiment. A Solubility Table summarizes the experimental observations of the solubility behavior of a large group of ionic compounds. We'll discuss a Solubility Table shortly.

When we tested the conductivity of an aqueous solution of sucrose the light bulb did not glow. This type of behavior is characteristic of a nonelectrolyte. No ions in this solution, yet the sucrose dissolved. How do we write a chemical equation in this case?

C12H22O11(s) ---H2O--> C12H22O11(aq)

In general soluble covalent compounds do not dissociate into ions when dissolved in water.

Mentioned earlier was the Solubility Table. A Solubility Table summarizes the solubility behavior of a large group of ionic substances. How to interpret a Solubility Table?

We can use the Solubility Table to determine whether an ionic compound exist as ions in aqueous solution (soluble) or as a solid (insoluble). Once we know the compound we use the Solubility Table to determine its solubility.

For example, consider the following compounds; NaCl, BaSO4, NaC2H3O2, and CaS. Determine the solubility in water for these ionic substances.

NaCl (all chlorides are soluble except...) SOLUBLE;

BaSO4 (all sulfates are soluble except...) INSOLUBLE;

NaC2H3O2 (all sodium compounds are soluble) SOLUBLE;

CaS (all sulfides are insoluble...) INSOLUBLE

 

We'll also use the information in a Solubility Table to help identify the phase of ionic substance in a chemical equation. The chemical reaction types where the Solubility Table is important are;

 

Lets consider a double displacement reaction problem;

Write the formula and identify the phase for the product(s) and balance the following reaction.

Na2SO4(aq) + CaCl2(aq) --->

Since this is a double replacement reaction we can write the formulas of the products by exchanging the cations and anions. When you do this remember to keep track of the individual charges on the cations and anions.

Na2SO4(aq) + CaCl2(aq) ---> CaSO4(?) + 2NaCl(?)

Now we'll use the Solubility Table to predict the phases of the products. According to the table CaSO4 is INSOLUBLE and NaCl is SOLUBLE.

Na2SO4(aq) + CaCl2(aq) ---> CaSO4(s) + 2NaCl(aq)

Let's consider another example.

Write the formula and identify the phase for the product(s) and balance the following reaction.

AgNO3(aq) + Na2CO3(aq) --->

Since this is a double replacement reaction we can write the formulas of the products by exchanging the cations and anions. When you do this remember to keep track of the individual charges on the cations and anions.

2AgNO3(aq) + Na2CO3(aq) --->Ag2CO3(?) + 2NaNO3(?)

Now we'll use the Solubility Table to predict the phases of the products. According to the table Ag2CO3 is INSOLUBLE and NaNO3 is SOLUBLE.

2AgNO3(aq) + Na2CO3(aq) ---> Ag2CO3(s) + 2NaNO3(aq)

Here is a Shockwave animation using the Solubility Table.

So lets write the ionic and net ionic equations for the two equations above. The first equation well convert is;

Na2SO4(aq) + CaCl2(aq) ---> CaSO4(s) + 2NaCl(aq)

To change the above equation to the ionic equation the aqueous ionic substances must be written as ions;

2Na+(aq) + SO42-(aq) + Ca2+(aq) + 2Cl-(aq) ---> CaSO4(s) + 2Na+(aq) + 2Cl-(aq)

Now we'll cancel all species common to both sides of the equation;

2Na+(aq) + SO42-(aq) + Ca2+(aq) + 2Cl-(aq) ---> CaSO4(s) + 2Na+(aq) + 2Cl-(aq)

The final net ionic equation is;

SO42-(aq) + Ca2+(aq) ---> CaSO4(s)


The second equation we'll convert is;

2AgNO3(aq) + Na2CO3(aq) ---> Ag2CO3(s) + 2NaNO3(aq)

To change the above equation to the ionic equation the aqueous ionic substances must be written as ions;

2Ag+(aq) + 2NO3-(aq) + 2Na+(aq) + CO32-(aq) ---> Ag2CO3(s) + 2Na+(aq) + 2NO3-(aq)

The net ionic equation is;

2Ag+(aq) + CO32-(aq) ---> Ag2CO3(s)