Comparing the MP/BP of Ionic and Covalent Compouds

This question is by far my favourite and it seems like most students have difficulty giving a decent answer for this straight forward question. Lets look at how a sample question would look like.

Question normally ask you to state which substance has a higher MP or BP and explain why.

For our answer, it will be definitely be the ionic compound of magnesium oxide. For those who are lost, this is because ionic compounds have high MP and BP while simple covalent compounds have low MP and BP. If you have no clue how to differentiate between ionic and covalent compounds refer to this post.

Now the next step is to explain why does magnesium oxide has high MP and BP.

This is the suggested template.

a)Start by identify the type of compounds for the substances.

For our question magnesium oxide is the ionic compound and the oxide of phophorous is a simple covalent molecule.

b) Next, tell the marker the kinds of bonds broken when the substances undergo change of state.

This is the general rule of selection when it comes to identifying the kinds of bond broken.

Ionic Compounds --- break strong ionic bonds during change of state (aka melting and boiling)
Giant Covalent Compounds --- break strong covalent bonds during change of state (aka melting and boiling) Simple Covalent Molecules --- break weak intermolecular force of attraction or van da waal's forces of attraction during change of state (aka melting and boiling)

c)Last part sum up your answers by telling the marker the amount of energy needed to carry out the change of state.

That's all. I am gonna end this post by writing a sample answer for this type of question.

Qn: Which of the 2 substances, magnesium oxide and phophorous oxide has a higher melting point. Explain.

Ans: Magnesium oxide is an ionic compound and phosphorous oxide is a simple covalent compound. During change of state, magenesium oxide breaks strong ionic bonds while phosphorous oxide breaks weak intermolecular force of attraction between the molecules. Hence a lot of energy is needed to break the strong ionic bonds therefore MP is high, while lesser energy is needed to break the weak intermolecular force of attraction hence MP of simple covalent molescules is low.

Enjoy!

Entry by

Json Lim

http://www.oleveltuition.com/