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Period 3 Elements and their Oxides

2017 Paper One

Question Six
Part of a series looking at past AQA A-level Chemistry questions with a focus on better performance in examinations.

If you haven't gone through the question/paper, click the direct link below, and have a go...

AQA A-level CHEMISTRY Paper 1 Inorganic and Physical ChemistryTuesday 13 June 2017

Question 6 is about Period 3 Elements
A 13-mark, 7-part question


These are 2 Period 3 oxides - one a metal oxide and the other a non-metal oxide. The metal oxides are basic, non-metal oxides acidic, so we can test the pH.

Add water to each oxide then put a few drops of Universal Indicator in each.
The indicator will turn red in the phosphorus oxide solution and purple in the sodium oxide solution.

You could use a pH meter or even litmus paper

Silicon dioxide has a giant covalent structure and sulfur trioxide has a molecular structure (not simple covalent since this describes bonding)

Melting points are dependent on intermolecular forces in simple molecules - there are covalent bonds holding the crystal structure together in silicon dioxide.

Sulfur trioxide has weak van der Waals forces between the molecules.

It requires more energy to break the covalent bonds in the crystal structure of silicon dioxide than that needed to overcome the Van der Waals forces between sulfur trioxide molecules.


Other equations are possible but this is the easiest!

Many students had issues with this one.
Think of the phosphate ion as being analogous to the sulfate ion.

You can work out the charge on the phosphate ion...
O is always 2-. P is in Grp 5 and, since O is negatively charged, holds a charge of 5+.
4 x 2- = 8-, so the extra charge must be 8+ - 5 = 3+, enough for 3 x H+

Phosphoric acid's formula is

The reaction is a neutralisation, so you should have a magnesium salt and water as the products. But what is the salt's formula? You can use the cross-charge method...

Construct the equation and balance... done.
Lone pair is not required here.

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