A chemical equation is a symbolic representation of all of the substances involved in a chemical reaction. We use the chemical formulas of substance to represent each chemical species involved in the reaction. We also use the notation (g), (l), (s), or (aq) following the chemical formula to identify the phase of the substances in the equation.

How do we tell if a chemical reaction occurs? The best evidence is a difference in the physical and chemical properties of the reactants and the products. This can be obvious as in the case of the reaction between iron and sulfur. Other characteristics to watch for when mixing two or more substances;

The form of a chemical equation involves writing the formulas of the reactants (the substances that are mixed together) on the left, using '+' when more than one substance is involved and the formula(s) of the product(s) on the right. The reactants and the products are separated by an arrow '--->'. Sometimes additional information about the reaction is placed above or below the arrow which separates the reactants and products. Such information include;

Precipitation Reactions

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, that is, they 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 how 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 will be testing the conductivity of soluble ionic compounds in laboratory later this semester.

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.

In laboratory you will investigated the solubility of a large collection of ionic compounds. For those ionic compounds that dissolved in water you were expected to write the formula of ions formed. In the post-laboratory you were to write a chemical equation describing what happens when a soluble ionic compound dissolves in water.

For example, sodium chloride dissolves in water. The chemical equation that can be written to describe the solubility of sodium chloride is,

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

We can write the equation to describe what happens when copper(II) chloride dissolves in water,

CuCl2(s) ---H2O--> Cu2+(aq) + 2Cl-(aq)

You must be able to write the ions formed and the chemical equation which describe the behavior of soluble ionic compounds.

The goal of Experiment #2 is to organize all the experimental observations into something meaningful. This data can be organized into a Solubility Table. The Solubility Table summarizes the solubility behavior of ionic compounds in terms of anions and cations.

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.

The solubility of each of these compounds can be determined by using the Solubility Table.

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

For those compounds that are soluble write a chemical equation which describes what happens when the solid is added to water.

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

NaC2H3O2(s) ---H2O--> Na+(aq) + C2H3O2-(aq)

 

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;