Periodicity of chemical properties of the elements in Period 3

Describe the acid–base nature of the solutions obtained when the following compounds are added to water.

Use equations to illustrate your answers.

a. sulfur trioxide

b. sodium oxide

a. Sodium oxide reacts with water to produce sodium hydroxide (NaOH), which is a strong base. The balanced chemical equation for this reaction is:

`Na _2O(s) + H_2O(l) → 2NaOH(aq)`

In this reaction, `Na₂O` acts as a base because it forms hydroxide ions in solution.

b. Sulfur trioxide reacts with water to form sulfuric acid (`H₂SO₄`), which is a strong acid. The balanced chemical equation is:

`SO _3(g) + H_2O(l) → H_2SO_4(aq)`

Here, SO₃ acts as an acid because it forms proton (`H⁺`) ions in solution.

A. Write an equation for the reaction of magnesium with cold water.

B. Predict and explain the pH of the resulting solution.

A. The reaction of magnesium with cold water is

`Mg(s) + 2H_2O(l) → Mg(OH)_2(aq) + H_2(g)`

B. Magnesium reacts extremely slowly with cold water, making the solution formed very weakly alkaline, having a pH around 10 - 12 because magnesium hydroxide formed is only slightly soluble in water. Therefore, a lower concentration of `OH^-` (aq) ions enters the solution.

When magnesium is added to water, the reaction is very slow. In contrast, phosphorus(III) chloride (PCl₃) is a liquid that reacts vigorously with water. One of the products of this reaction is H₃PO₃, which forms in solution.

A. Write an equation, including state symbols, showing the reaction of phosphorus(III) chloride with water.

B. Predict the pH of the solution obtained.

C. State one observation a student watching the reaction would make.

A. The reaction of phosphorus(III) chloride with water is

`PCl _3(l) + 3H_2O(l) → H _3PO_3(aq) + 3HCl(g) or (aq)`

B. In the products, we have H₃PO₃ and probably HCl (aq), making a pH around 1-3.

C. Phosphorus(III) chloride reacts violently with water. You get phosphorous acid, H₃PO₃, and white fumes of hydrogen chloride (or a solution containing hydrochloric acid if lots of water is used).

A. Describe how the atomic radius varies down each group of the Periodic Table.

B. Explain this trend

A. It increases down the group

B. The structure gains electron shells as the group descends.

Give the formula of a Period 3 oxide includes MgO, Al₂O₃, SiO₂, P₄O₁₀, SO₂

A. Which one reacts with water to produce a hydroxide?

B. Which one reacts with water to produce an acid?

C. Which one has no reaction at all with water

A. The one reacts with water to produce a hydroxide is MgO

B. The ones react with water to produce an acid are P₄O₁₀ and SO₂

C. The ones have no reaction at all with water include Al₂O₃ and SiO₂

The oxides of the elements of the third Period behave differently with NaOH (aq) and HCl (aq). In some cases, no reaction occurs. 

Complete the table below by writing 'reaction' for any reaction that occurs between the Period 3 oxide and HCl (aq) or NaOH (aq), with heating if necessary. If you think no reaction takes place, write ‘no reaction’.

`MgO + 2HCl -> MgCl_2 + H_2O`

`Al_2O_3 + 2NaOH + 3H_2O -> 2 NaAl(OH)_4`

`Al_2O_3 + 6HCl -> 2 AlCl_3 + 3H_2O` or  `Al_2O_3 + 6HCl -> Al_2Cl_6 + 3H_2O`

`SO_2 + 2NaOH -> Na_2SO_3 + H_2O`

Liquid silicon chloride, SiCl₄ , and solid phosphorus(V) chloride, PCl₅ , can be hydrolysed in water, both reactions produce acidic solutions and release white fumes. 

Give the balanced symbol equation for the reaction of phosphorus(V) chloride with water. Include state symbols.

The balanced symbol equation for the reaction of phosphorus(V) chloride with water, including state symbol

`PCl_5 (s) + 4H_2O -> H_3PO_4 (aq) + 5HCl (g)`

Oxides are compounds which usually contain oxygen combined with one other element. Oxides are classified as follows.

Complete the table to describe the oxides of the elements of the third period of the Periodic Table from sodium to sulfur.

What happens to the pH of water when magnesium oxide is dissolved into it?

A. pH increases, because MgO is acidic.

B. pH decreases, because MgO is basic.

C. pH increases, because MgO is basic.

D. Nothing happens because MgO is neutral.

The answer is C

The oxides of Period 3 are basic on the left-hand side (sodium and magnesium oxides) and acidic on the right-hand side (silicon, phosphorus, sulfur and chloride oxides) and aluminium oxide is amphoteric.

`MgO + H_2O -> Mg(OH)_2`

Thus, MgO is basic and produce hydroxide ions (OH^-) and pH of water increases from 7 to around 9

Which chlorides of Period 3 elements will form a neutral solution when added to water?

A. 1 only

B. 1 and 2

C. 2 and 3

D. 1, 2 and 3

The answer is B

MgCl₂ and NaCl are ionic compounds with giant ionic structures.

Thus, both dissolve in water to form neutral solutions 

`MgCl_2 (s) + H_2O(l) -> H_2O(l) + Mg^(2+) (aq) + 2Cl^(-) (aq)`

`NaCl (s) + H_2O(l) -> H_2O(l) + Na^(+) (aq) + Cl^(- )(aq)`

A is incorrect because it does not take into account for NaCl

C and D are incorrect because SiCl₄ is a covalent molecule

`SiCl_4 + 2H_2O -> SiO_2 + 4HCl`