ionic compounds

Properties of ionic compounds

Ionic compounds all have a giant ionic lattice structure with all the ions held in place by strong ionic bonds which prevent the ions from moving freely within the lattice structure. Ionic bonds are strong bonds. However most ionic compounds will dissolve in water; especially those containing a metal from group I and the bottom end of group II in the periodic table. When an ionic compound dissolves in water the water molecules pull apart the giant lattice structure and we end up with ions that are separated from each other and able to move freely within the solution formed. If an electric current is passed through the solution it will readily flow- solutions of ionic compounds conduct electricity. This is why for example you will get a large electric shock if you touch a live electrical wire with wet hands. Since you sweat salt water (a solution of sodium chloride) this solution will conduct the electricity and result in a severe electrical shock.

Model to show what happens to the sodium chloride lattice when it dissolves in water.

Solutions of ionic compounds

The image below shows the apparatus needed to demonstrate the fact that solutions of ionic compounds will conduct electricity. The two graphite electrodes are dipped into a solution of an ionic compound and the free moving ions will be attracted to the electrodes. If the ionic compound is heated and melts to form a liquid or melt then this liquid will also conduct electricity since the particles in a liquid are also free to move. These free moving ions enable an electrical current to flow. Solid ionic compounds DO NOT conduct electricity since the ions are held tightly in a 3d lattice structure and are not free to move.

Experimental set up to show how solutions of ionic compounds conduct electricity

Melting points

Since ionic compounds have a giant lattice structure with lots and lots of strong bonds between the ions it takes a lot of energy to break all these strong bonds and so ionic compounds have very high melting and boiling points. Sodium chloride (Na+Cl-) has a melting point of 8010C while aluminium oxide (Al2O3) which has ions with a much larger charge than in sodium chloride. The Al3+ and O2- ions are also much smaller than the ions in sodium chloride and this along with also the larger charges means that the ions can pack together very closely and the attraction between the ions is much larger; so aluminium oxide has a very high melting point of 20720C.

When these solid ionic compounds melt the resulting liquids or "melts" contain ions that are free to move. This means that the liquid will conduct electricity.

Ionic compounds have high melting points due to their giant structure and strong bonds

Solubility of ionic compounds

Model showing the polarity in a water molecule.  Water is a polar molecule which contains atoms with a partial charge.

Many ionic compounds are soluble in water; a phrase which is often used when discussing solubility is "like dissolves like". What this means is that ionic lattices obviously consist of charged ions, well water is a polar solvent it contains polar covalent bonds and also a dipole due to the fact that it contains atoms with partial positive and partial negative charges. Due to the differences between the electronegativity of oxygen (3.4) and hydrogen atoms (2.1) the O-H bond is a polar one with the hydrogen atoms having a partial positive charge (δ+) and the oxygen atom having a partial negative charge (δ-). Recall that molecules with charge ends are said to possess a dipole.

Ion-dipole forces are a type of attraction found between polar solvents such as water molecules and the ions present in an ionic solute which is dissolved in a polar solvent such as water. The partial negatively charged oxygen atoms in a water molecule will be attracted to any positively charged metal ions present in an ionic salt which is added to the water. While the partially positively charged hydrogen atoms will be attracted to any negatively charged anions present in the solute.

In the case of sodium chloride which contains positively charged sodium ions (Na+) that will attract the partially negatively charged oxygen atoms in the water molecules while the negatively charged chloride ions (Cl-) will attract the partially positively charged hydrogen atoms in the water molecules. This is outlined in the image below:

explanation of how ions in sodium chloride dissolve in water by forming solvation spheres.

The water molecules form what is called a solvation sphere around the dissolved metal cation and the non-metal anions. The solvation spheres shown in the image above involve six water molecules surrounding the metal cation and the non-metal anion, but in reality you can have a second and third or more solvation spheres surrounding the dissolved ions.

Ionic crystals are brittle

If an ionic lattice is subjected to any pushing or pulling forces which causes the layers of ions to move this will lead to widespread cracking within the lattice structure as ions of similar charge are brought in contact with each other. The ions will immediately repel each other and the lattice structure will break apart at this point. This means that ionic lattices are brittle and can easily break. Ionic lattice breaking apart, ionic lattices are brittle

Migration of ions during electrolysis

We can observe the movement of ions during the electrolysis of coloured compounds for example copper chromate dissolves to form a green coloured solution. If this green solution is added to a U-tube and electrolysed as shown in the diagram below we observe a pale blue colour at the cathode the cathode is also covered in a brown furry solid. At the anode a yellow colour and some bubbling is observed. electrolysis of copper chromate Copper chromate is an ionic compound containing blue copper ions (Cu2+) ions and yellow chromate ions (CrO42-). These two ions mix and the resulting solution is green. However when the solution is electrolysed the positively charged (Cu2+) ions are attracted to the negatively charged cathode and the yellow CrO42- ions are attracted to the positively charged anode. This simple demonstration is a good piece of evidence for the presence of ions in a solution.

Key Points

Practice questions

Check your understanding - Questions on properties of ionic compounds