2.2.2 (c) Physical Properties of Ionic Substances

Syllabus

(c) explanation of the effect of structure and bonding on the physical properties of ionic compounds, including melting and boiling points, solubility and electrical conductivity in solid, liquid and aqueous states

What does this mean?

Melting and Boiling Points.

Melting and boiling points are high compared to most covalent substances.

This is because it is necessary to break a great many Ionic Bonds to allow the ions to move.

And because the Ionic Bonds are strong a great deal of energy is required, which requires a high temperature.

You might be asked to account for differences between melting/boiling points of substances like the ones in the table. But you have to compare like with like.

So, we can compare NaI with NaBr and NaCl and suggest that the higher temperature needed for NaCl is because the Chloride ions are smaller than the equally charged Bromide ions and are much smaller than the equally charged Iodide ions - allowing the ions to get closer together and making them stronger.

Or we can compare KBr with NaBr where the only difference is the smaller size of Sodium ions compared with Potassium ions.

Again, the smaller ions pack more closely and this makes stronger bonds.

It's harder to compare CaCl₂ with NaCl because Calcium ions are larger but more highly charged than Sodium ions.

And the ratio of anion to cation is 1:2 rather than 1:1

So, on the bright side, you won't asked to do this.

Solubility

Many Ionic Compounds dissolve easily in water, just like Sodium Chloride.

This is because water is polar and has a partially positive end (δ+) which can form attractions to the Chloride ions.

And it has a partially negative end (δ-) which can form attractions to the Sodium ions.

So although energy has to be put in to break the ionic bonds, more energy is released forming the new attractions.

This is referred to as hydration which you'll find out more about in the following year.

If the Ionic Bonds are stronger than the attractions of the ions to water then generally the substance is insoluble like BaSO₄, AgCl or CaSO₄.

Typically solubility increases with temperature because there's more energy around to help break the ionic bonds.

Although the graph suggests that it's not always quite that simple!

Electrical Conductivity.

Ions in the solid are held in place by strong bonds.

They can't move and there are no free electrons to move around either.

So solid ionic compounds don't conduct - they are insulators.

But if enough energy is put into melt the solid then the ions can move and it becomes a conductor as a liquid.

Equally, if the substance dissolves in water then the ions are free to move and the solution will be a conductor.

Typically, conductivity increases with concentration.

This is because there are more charge-carriers (free-ions) to carry current through the water.

And a 1 moldm-3 solution of CaCl2 would conduct better than a 1 moldm-3 solution of NaCl because there would be more dissolved ions in the Calcium Chloride solution

Malleability.

Ionic substances are brittle.

Any disruption of the arrangement of (+) and (-) ions is almost guaranteed to end up with repulsions between similarly charged ions that end up next to each other.

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Why are some Ionic Compounds not fully Ionic?

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