ionic compounds

Properties of ionic compounds

Ionic compounds all have a giant ionic lattice structure. The ions are held in fixed positions and cannot move freely within the lattice structure. Ionic bonds are strong bonds. However most ionic compounds dissolve in water, especially those containing a metal from group I and the bottom end of group II. When the ionic compound dissolves in water, the water molecules pull apart the giant lattice structure and we end up with ions which are separate 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 get a bad 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.

solution of sodium chloride

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 compounds and the free moving ions are 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 free to move also. These free moving ions enable an electric 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.

solutions of ionic compounds conduct electricity

Melting points

Since ionic compounds have giant structures 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. Aluminium oxide, Al2O3, has ions with a much larger charge than in sodium chloride. The Al3+ and O2- ions are 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, 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

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 (contains charged "ends", it has a dipole), it contains polar covalent bonds and also 2 lone pairs of electrons. The water molecules can form coordinate bonds to the metal ions in the ionic lattice and effective pull them into solution, similarly the polar water molecules can form hydrogen bonds to the chloride anion and pull these from the lattice and into solution as well. The ions are said to be solvated or hydrated if water is the solvent. You can use the above to explain why ionic compounds are insoluble in non-polar solvents. This is shown below:

explanation of how ions in sodium chloride dissolve in water, solvation

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 below involve only 6 water molecules surrounding the metal cation and the non-metal anion, but in reality you can have a second or more 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 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