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Instruction 3-3

Covalent, Metallic or Ionic Bonds | Chemical Bonds in Molecular Atoms | Salt Crystals | How Atoms and Molecules Move in Liquid? | Lewis Dot Structure and Molecular Shape | Electronegativity, Ionization, and Bond Formation | Van Der Waals Forces

Salt Crystals
CCSTD HS Chemistry 2.c.

A Salt Crystal is the solid form of an ionic compound.

Technically,

  • a Salt is a chemical compound formed by replacing all or part of the hydrogen ions of an acid with metal ions, and
     
  • a Crystal is a homogeneous body with the repeating shape of a polyhedron.

Most of us think of table salt, or Sodium Chloride (NaCl), when we hear the word ‘salt’, but in chemistry there are many salts. Additionally, we think of glassware or prisms when we hear the word ‘crystal’. However, when we speak of Salt Crystals in chemistry, we are referring to the solid form of an ionic compound.

The repeating geometric pattern of a salt crystal is called the Crystal Lattice, and the smallest unit of the lattice is known as a Unit Cell.

In comparing patterned wallpaper to a crystal lattice; just like the wallpaper has a repeating pattern, the Crystal Lattice has a repeating Unit Cell.

The three assumptions of the ionic Crystal Lattice model are;

  1. the ions are charged, incompressible, nonpolarizable spheres,
     
  2. the ions will surround themselves with as many ions of opposite charge that are packed as closely as possible. Usually the cation is just large enough to allow the anions to surround it without touching one another, and
     
  3. the cation to anion ratio must reflect the stoichiometry of the compound. For example, with MgCl2 as the unit cell, the lattice must be an arrangement of chloride anions with only half that number of magnesium ions.

There are seven types of Unit Cells, and these cells give rise to fourteen Crystal Lattices.

A listing of the fourteen Crystal Lattice types can be seen at: http://cst-www.nrl.navy.mil/lattice/ .

Using the example of common table salt (NaCl), the packing of the positive cations (Na+) and the negative anions (Cl-) leads to a Unit Cell that repeats into a lattice type called Face Centered Cubic (FCC) structure. You can see the FCC unit cell at: http://wb.chem.lsu.edu/htdocs/people/sfwatkins/MERLOT/lattice/04fcc.html
Figure 3.3.1 gives a 3-D representation of the FCC crystal lattice structure of NaCl.

The electrostatic forces that hold ions together in salt crystals are very strong. These ionic bonds are quite difficult to break, which translates to great amounts of energy contained in the bond. The strength of the ionic bond leads to relatively high melting points and extremely high boiling points for ionic Salt Crystals.

At the same time, drop this incredibly tough compound into a glass of water (H2O), and it will easily dissolve. What gives?
It turns out that water has partial charges, with a partial negative (δ-) charge on the Oxygen atom and partial positive (δ+) charges on the Hydrogen atoms. This makes sense if you consider that the Electronegativities of these two atoms are quite different, with Oxygen having the higher value of 3.5 and Hydrogen having a value of 2.1. Because of this difference, the electron densities in the water molecule are pulled toward the Oxygen and away from the Hydrogen.

In the event that NaCl dissolves in water, the partial positive charges of the water will attract the salt anions (Cl-), and the partial negative charge will attract the cations (Na+), resulting in a breaking apart of the ionic solid.

Salt solubility, or how well the salt dissolves, in water, is an important characteristic of any ionic compound. We will discuss salt solubility in Lesson 7.

Other Crystal Salts

Sodium Chloride is not the only crystal salt. There are other ions that come together to form crystal salts. Some of those salts have multiple charges on their cations, such as in the case of Calcium Fluorite (CaF2) , which is composed of one Ca++ cation and two F- anions. Others have multiple charges on their anions, such as Sodium Phosphate (Na3PO4), where there are three Sodium cations (Na+) for each Phosphate anion (PO4-3).

The salts that arise from ionic bonding have a number of characteristics in common, such as;

  • the ionic bonding is electrostatic and rather strong,
     
  • most ionic salts are readily soluble in water,
     
  • the crystals can be described as a collection ensemble of hard spheres that attempt to occupy a minimum volume while minimizing total electrostatic charge,
     
  • there are no free electrons in ionic salt crystals, so they make good insulators,
     
  • solid crystal forms of these salts do not conduct electricity, while fused (melted) forms do,
     
  • salt crystals have high melting temperatures, and extremely high boiling temperatures,
     
  • salt crystals are hard and brittle at room temperature, and
     
  • ionic crystals come in many repeating patterns other than the cube-like lattice structure of NaCl.

 

Types of Salts

There are two types of salts: acid salts and normal salts. In order to understand the difference between these two types of salts, we will first need to briefly discuss acids and bases.

An acid can be thought of as any compound that is willing to give up a Hydrogen cation (H+). This cation is often called a Proton since the Hydrogen cation has no electrons, but it does have one proton in its nucleus. A base is any compound that is willing to accept a Proton (the H+ kind).

With these definitions in mind, let’s first consider the neutralization of the strong acid, Hydrochloric Acid (HCl or H+ and Cl-) by a strong base, Sodium Hydroxide (NaOH or Na+ and OH-).

You can see that water and the salt crystal, Sodium Chloride, are products of the neutralization reaction. This reaction produces water and a normal salt, or a salt that contains no Hydrogen in its molecular structure.

Another reaction example is Phosphoric Acid (H3PO4) being neutralized with Sodium Hydroxide. Phosphoric Acid has three cations (H+) and one anion (PO4-3). Let’s say we react a given amount of H3PO4 with the same amount of basic Sodium Hydroxide (NaOH). We will get;

If we add more NaOH, we get;


and when we add still more NaOH, we then get;

With this sequence of reactions, we end up with a total of three molecules of water; two acid salts, and one normal salt. When a salt has no Protons (H+) or Hydroxyl ions (OH-), it is called a normal salt. Can you guess which product of reaction (3) is the normal salt?
Answer: The normal salt is the salt produced in reaction (3), which is Na2PO4. The salts produced in reactions (1) and (2) are called acid salts, because they contain one or more Protons. (A basic salt includes one or more Hydroxyl ions.)
 

Reading List

 
  Robert Gardner: Chemistry Science Fair Projects: Using Acids, Bases, Metals, Salts, and Inorganic Stuff

 

for Students, Parents and Teachers

Now let's do Practice Exercise 3-3 (top).

 

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