Chemistry of period 3 elements

This chapter compares the Chemistry of the elements in period 3 of the periodic table. Each element in a period provides representative Chemistry of the group into which it exists.

The elements of period 3 are Na, Mg, Al, Si, P, S, Cl and Ar.

Electronic structure

For each of these elements, the total number of electrons in the outermost shells is equal to the group number in which it belongs (table 6.1).

Table 1 The electron configurations of period 3 elements

Physical properties

Sodium, magnesium and aluminium are metals whose melting points increase in that order due to increase in the number of electrons contributed to form metallic bonds. Silicon, phosphorus, sulphur, chlorine and argon are nonmetals. Silicon, phosphorus and sulphur are solids while chlorine and argon are gases at room temperature.

Reactions of the elements

a) Reaction with water

Reactivity with water decreases across the period because there is a decrease in electropositivity of these elements across the period.

(i)  Sodium reacts vigorously with cold water to form sodium hydroxide and hydrogen gas.

       2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g)

ii) Magnesium slowly with cold water to form magnesium hydroxide but reacts with steam to form magnesium oxide and hydrogen gas.

Mg (s) + 2H₂O (l) → Mg(OH)₂ (aq) + H₂ (g)

   Mg (s) + H₂O (g) → MgO (aq) + H₂ (g)

iii)  Aluminium reacts with steam very slowly to form aluminium hydroxide and hydrogen gas.

       2Al (s) + 6H₂O (l) → 2Al(OH)₃ (s) + 3H₂ (g)

iv) Silicon, phosphorus, sulphur and argon have no reaction with water.

v) Chlorine reacts with water to form chloric (I) acid and hydrochloric acid.

      Cl₂ (g) + H₂O (l) → HOCl (aq) + HCl (aq)

b.   Reaction with NaOH

(i)  Sodium and magnesium have no reaction with sodium hydroxide because they are metals and magnesium falls below sodium in the reactivity series.

(ii)    Aluminium reacts with aqueous NaOH to produce hydrogen and a complex salt -sodium aluminate.

       2Al (s) + 2NaOH (aq) + 6H₂O (l) → 2NaAl(OH)₄ (aq) + 3H₂ (g)

This is because aluminium is amphoteric, i.e., possesses both acidic and basic properties.

ii)  Chlorine reacts with cold dilute sodium hydroxide to form sodium chlorate (I), sodium chloride and water.

      Cl₂ (g) + 2NaOH (aq) → NaOCl (aq) + NaCl (aq) + H₂O (l)

     Sodium hypochlorate (I) is used as a bleaching and antiseptic agent .

Chlorine reacts with concentrated sodium hydroxide to form sodium chlorate (V), sodium chloride and water.

3Cl₂ (g) + 6NaOH (aq) → NaClO3 (aq) + 5NaCl (aq) + 3H₂O (l)

iii) Phosphorus reacts with hot concentrated NaOH to produce phosphine

       P₄ (s) + 3OH^– (aq) + 3H₂O (l) → PH₃ (g) + 3H₂PO₂^– (aq)

iv)  Na, Mg, Si, S have no reaction with sodium hydroxide.

c.    Reaction with HCl.

Reactivity decreases across the period due to decrease in metallic properties.

i)    Na, Mg, Al react to form H₂ and metal salts.

          2Na (s) + 2HCl (aq) → 2NaCl (aq) + H₂ (g)

          Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)     

          2Al (s) + 6HCl (aq) → 2AlCl₃ (aq) + 3H₂ (g)

ii)   Si, P, S, Cl have no reaction with hydrochloric acid.

d)   Oxides

They are formed by reacting the elements with oxygen and the formulae of oxides are given in table 6.2.

Table 2 The oxides of period 3 elements

Trial 1

(a) Complete the table below                                                           ( marks)

Element	Formulae of oxides	Type of bonding in oxide	Structures of oxides
Al
Si
P

(b) Write an equation for the reaction between the oxide of aluminium and sodium hydroxide (1½)

a.    Physical properties of oxides of period 3 elements

i)  Across the period there is a gradual change in the character of oxides from strongly basic to strongly acidic. Al₂O₃ is an amphoteric oxide, i.e., possesses both basic and acidic properties.

ii) – Na₂O, MgO and Al₂O₃ have high melting points due to the strength of ionic bonding.

• The melting point of magnesium is higher than that of Na₂O because MgO, stronger electrostatic forces since Mg²⁺ has 2 positive charges, secondary, it has higher molecular mass.
• The melting point of Al₂O₃ is lower than that of MgO because Al₂O₃ has bigger covalent character due to high polarizing power of Al³⁺ ions.
• SiO₂, although covalent, has a high melting point because the crystal of SiO₂ is a giant structure in which silicon and oxygen atoms are bonded together by strong single covalent bonds. 
• P₂O₅ (white solid) consists of P₄O₁₀ molecules; these contain electric dipoles due to electron displacements in their bonds and are quite strongly attracted to each other.  This is why its melting point is high compared with that of P₄O₁₀ which consists of simple SO₃ molecules.
• Cl₂O₇ has a low melting point because the molecules are held together by the weak van der Waals forces.

b.   Reaction

I)   The reaction of oxides of period 3 elements with water.

• Na₂O and MgO react with water to form hydroxides.

          Na₂O (s) + H₂O (l) → 2NaOH (aq)

           MgO(s) + H₂O (l) → Mg(OH)₂ (aq)

• Al₂O₃ and SiO₂ have no reaction with water because they are insoluble oxides.
• P₂O₅, SO₃, Cl₂O₇ are acid anhydrides, i.e., react with water to form phosphoric, sulpuric and perchloric acids respectively.

           P₂O₅ (s) + 3H₂O (l) → 2H₃PO₄ (aq)  

          SO₃ (s) + H₂O (l) → H₂SO₄ (aq)

          Cl₂O₇ (g) + H₂O (l) → 2HClO4(aq)

II) The reactions of period 3 oxides with NaOH.

• Na₂O, MgO are basic, therefore, have no reaction with water.
• Al₂O₃ being amphoteric reacts with dilute NaOH to produce sodium aluminate and water.

      Al₂O₃ (g) + 2NaOH (aq) →  2NaAlO₂(aq) + H₂O (l)

• SiO₂ reacts to form sodium silicate and water.    

           SiO₂ (s) + 2NaOH (aq)  → Na₂SiO₃ (aq)  + H₂O (l)

• P₂O₅, SO₃ and Cl₂O₇ react to form salts and water

       SO₃ (g) + 2NaOH (aq) → Na₂SO₄ (aq) + H₂O (l)

       P₂O₅ (s) + 2NaOH (aq) → 2NaPO₃ (aq) + H₂O (l)

       Cl₂O₇ (g) + 2NaOH (aq) → 2NaClO₄ (aq) + H₂O (l)

Trial 2

 Write ionic equations for the reactions between sodium hydroxide and (1½ mark each)

• Silicon (IV) oxide.
• Lead (II) oxide.
• Aluminium oxide.

e.   Chlorides of period 3 elements

These are formed by reacting the elements with chlorine gas and the physical properties of their chlorides are shown in table 3.

Table 3 The chlorides of period 3 elements

 

I.  Physical properties of period 3 chlorides.

NaCl and MgCl₂ have high melting points due to the strength of the ionic bonding.  AlCl₃ has a fairly high melting point because, in the solid-state, it consists of Al₂Cl₆ molecules and not simple AlCl₃. These molecules are produced through dative bonding between Al and Cl in the Al₂Cl₆ molecules.

SiCl₄ and S₂Cl₂ are liquids and consist of respective simple SiCl₄ and S₂Cl₂ molecules and in the solid-state the molecules are held by weak van der Waals force which explains their very low melting points.  PCl₅ a pale yellow solid has a fairly high melting point because the solid undergoes partial ionisation.  

 2PCl₅      ↔     PCl₄⁺ + PCl₆^–

Trial 3

Sodium chloride melts at 800⁰C whereas aluminium chloride sublimes at 180⁰C. Explain the following observations: (3marks)

Boiling points of chlorides of period 3 elements

The table below shows the boiling points of the chlorides of period 3 elements.

Element	Na	Mg	Al	Si	P	S	Cl
Atomic number	11	12	13	14	15	16	17
Formula of the chloride	NaCl	MgCl2	AlCl3	SiCl4	PCl3	S2Cl2	Cl2
Boiling point (oC)	1465	1418	180	57	76	136	-35

 

A graph of boiling points of the chlorides against atomic number of the elements.                                                                                                  

 

Explanation

NaCl and MgCl2 have high boiling points due to strong the ionic bonding. 

Aluminium chloride, silicon chloride, phosphorus chloride, disulphur dichloride and chlorine have low boiling points because their molecules are held by weak molecular forces. 

AlCl3 has a fairly high boiling point because in the liquid state, it consists of Al2Cl6 molecules and not simple AlCl3. These molecules are produced through dative bonding between Al and Cl in the Al2Cl6 molecules.

Disulphur dichloride has relatively high boiling point due to its high molecular mass that increases the strength of the molecular forces

The reaction of chlorides of period 3 with water.

      NaCl and MgCl₂ have no reaction with water but simply dissociate into ions.

      NaCl (s) + (aq)   →   Na⁺ (aq) + Cl^– (aq)

      MgCl₂ (s) + (aq)  →  Mg²⁺ (aq) + 2Cl^– (aq)

ii)   AlCl₃, SiCl₄, PCl₅, Cl₂ are hydrolyzed by water.  However, the extent of the hydrolysis of these chlorides varies across the period.

 AlCl₃ is slightly hydrolyzed liberating HCl and Al(OH)₃.

AlCl₃ (s) + 3H₂O (l)  → Al(OH)₃(s) +  3HCl (aq)

However, when aluminium chloride is added to water, it reacts exothermically to give hydrated aluminium ions, [ Al (H₂O)₆]³⁺ and chloride ions, Cl^–. The energy needed to break the Al-Cl covalent bond is derived from the high enthalpy of hydration of small highly charged Al³⁺ ions.

Al₂Cl₆ (s) + 12 H₂O (l) →  2[Al(H₂O)₆]³⁺(Cl^–)₃ (aq)

covalent                                      ionic 

Hydrated aluminium chloride is readily soluble in water yielding

[Al(H₂O)₆]³⁺ (aq)  +  3Cl^– (aq) ions

SiCl₄, PCl₅, and S₂Cl₂ are completely hydrolyzed liberating hydrogen chloride.

PCl₅ (s)  + 4H₂O (l) → 5HCl (aq) + H₃PO₄ (aq) [phosphoric acid]

SiCl₄ (l) + 3H₂O (l) →  4HCl (aq) + H₂SiO₃ (aq) [silicic (IV) acid]

2S₂Cl₂   + 3H₂O (l) → 4HCl (aq) + H₂SO₃ (aq) + 3S (s)

Hydrogen chloride under moist conditions appears as white fumes. For this reason, SiCl₄, PCl₅, S₂Cl₂ fume in moist air since their hydrolysis leads to formation of HCl. 

Trial 4

The melting points of the chlorides of some elements are given in table 6.4 below.

Table 4  Melting points of chlorides

chlorides	MgCl2	FeCl3	PCl5
Mp/0C	712	282	-112
Type bonding

(a) State the type of bonding that exists in each of the chlorides in the table above.              (1 ½ marks)

(b) State what would be observed and write an equation for the reaction that takes place when water is added to each of the chlorides in the table above.                                                                  (2½ marks each)

f, The hydrides of period 3 elements.

The hydrides formed by period 3 elements are NaH, MgH₂, AlH₃, SiH₄, PH₃, H₂S, HCl. These hydrides are formed by action of hydrogen on the elements.

NaH and MgH₂ have high melting and boiling points due to the strength of ionic bonds.  H₂S and HCl have relatively higher melting and boiling points than SiH₄ and PH₃ because their molecules are polar molecules and are held together by hydrogen bonds whereas the molecules of SiH₄ and PH₃ are non-polar and therefore, held together by the weak van der Waals forces.

Reactions

The reaction of hydrides of period 3 elements with water.

 Due to the high polarity of the bonds in the hydrides NaH and MgH₂, they react with water readily producing hydrogen and the metal hydroxides.

NaH (s) + H₂O (l) → NaOH (aq) + H₂ (g)

MgH₂ (s) + 2H₂O (l) → Mg(OH)₂ (aq) + 2H₂ (g)

AlH₃, SiH₄, and PH₃ have no reaction with water due to lack of polarity in their bonds.

H₂S and HCl dissociate in water producing acids.

H₂S (g) + 2H₂O (l)  ↔   H₃O⁺ (aq) + HS^– (aq) + H₂O (l) ↔ 2H₃O⁺ (aq) + S^2- (aq)

HCl (g) + H₂O (l) → H₃O⁺ (aq) + Cl^– (aq)

Reactions of hydrides of period 3 elements with hydroxides (NaOH)              

HCl and H₂ S react with hydroxides to form salts.

HCl (g) + OH^– (aq) → Cl^– (aq) + H₂O (l)

H₂S (g) + OH^– (aq) → HS^– (aq) + H₂O (l)

then HS^– (aq) + OH^– (aq) → S^2- (aq) + H₂O (l)

Reaction of period 3 element with HCl.

NaH and MgH₂ react to form hydrogen and metal salts.

NaH (s) + HCl (aq) → NaCl (aq) +  H₂ (g)

MgH₂ (s) + 2HCl (aq) → MgCl₂ + 2H₂ (g)

Trial 5

The elements contained in the third short period of the periodic table, given in alphabetic order are; aluminium, argon, chlorine, magnesium, phosphorus, silicon, sodium and sulphur.

(a) In the table below, write the formulae of the hydrides formed by the elements listed. State the oxidation states (or valences) of the elements in these hydrides and classify the bonding in the hydroxides as ionic or covalent. (6marks)

Elements	Formula of hydride	Oxidation(or valence) of the elements in these hydrides	Type of bonding
Aluminium
Chlorine
Magnesium
Phosporus
Silcon

(b) The hydrides formed by sodium and sulphur were separately shaken with water.

 Write the equations to show the reactions that took place, if any with;                              (3marks)

• sodium hydride.
• Sulphur hydride.                                                                                                             (3marks)

Trial 6

(a) Describe how sodium hydroxide can be prepared on industrial scale (Your answer should include equations for the relevant reaction)

(b) Write equation and state the conditions under which sodium hydroxide can react with

(i) Aluminium

(ii) Phosphorous

(iii) Chlorine

Trial 7

The atomic number and melting points of oxides of elements of period III of The Periodic Table are shown in the table below

Element	Na	Mg	Al	Si	P	S	Cl
Atomic number	11	12	13	14	15	16	17
Oxide	Na2O	MgO	Al2O3	SiO2	P2O5	SO3	Cl2O7
Melting point of oxide	1275	2327	2007	1607	560	30	-91

(a)(i) Plot a graph of melting point of oxide against atomic number. (04marks)

(ii) Explain the shape of the graph you have drawn in (a)(i) above

(b) Write equation to show the reaction between

(i) Water and P₂O₅ (1 ½ marks)

(ii) Sodium hydroxide and

Al₂O₃ (2 ½ marks)

SiO₂ (2 ½ marks)

SO₃ (1 ½ marks)

(iii) Hydrochloric acid and Al2O3 (1  ½ marks)

Trial 7

(a) Write an equation for the reaction between and the hydride of

Sodium

Silcon

Sulphur

(a) Write an equation for the reaction between sodium hydroxide solution

Aluminium oxide

Phosphorus pentoxide

Sulphur dioxide

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