ionic compounds

Foundation and higher tier

The 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 giant ionic lattice structure. The ions within the lattice structure are held in place by strong Ionic bonds . However most ionic compounds will dissolve in water, especially those containing a metal ion from group I and the bottom end of group II in the periodic table.

When an ionic compound dissolves in water, the water molecules will 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 can get a nasty electric shock if you touch a live electrical wire with wet hands. Since you sweat salt water (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. Here two graphite electrodes are dipped into a solution of an ionic compounds, the free moving ions present in the solution are attracted to the two electrodes. The positively charged metal ions will be attracted to the negatively charged electrode (the cathode) and the negatively charged non-metal ions in the solution will be attracted to the positively charged electrode (the anode). These free moving ions are able to conduct an electrical current, this explains why the bulb in the image lights up.

solutions of ionic compounds conduct electricity

Melts

If an ionic compound is heated and it melts to form a liquid, then this liquid will also conduct electricity, since the ions present in this liquid are free to move. These free moving ions, just as in the solution are able to conduct an elctric current. Since it is the presence of free moving ions that enable an electric current to flow in solutions and melts it makes sense that solid ionic compounds DO NOT conduct electricity since the ions are held tightly in a 3-d lattice structure and are not free to move.

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

Melting points

Ionic compounds have giant structures with lots and lots of strong ionic 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 metal 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 also much larger, so aluminium oxide has a very high melting point, 20720C.

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 positively charged metal ions and negatively charged non-metal ions, well water can use its lone pairs of electrons to form bonds with the positively charged metal ions in the lattice and effective pull the metal ions out of the ionic lattice and into solution. The water molecules can also form intermolecular bonds to the chloride ions (Cl-) and pull these from the ionic lattice and into solution as well. The ions are said to be solvated or hydrated, this process is shown in the image below:

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

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. This will cause the ions to 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. This is shown in the image below where ions of similar charge sit on top of one another, this will cause repulsion and the ionic lattice will fracture at these points. ionic lattice breaking apart, ionic lattices are brittle

Migration of ions during electrolysis

We can observe the movement of ion 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 (has an electrical current passed through it), 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

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