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Trends in chemical properties (chemical periodicity)
Eg : elements in period 3
( Na to Ar )
Reaction with water
1. 2Na + 2H2O 2NaOH + H2
2. a. Mg +2H2O Mg(OH)2 +H2 (g)
b. Mg + H2O MgO + H2(g)
slow
cold
steam
fast
Reaction with oxygen ( Na to S )
1. 4Na + O2 2Na2O
Observation : burns with yellow flame to form white solid
2. 2Mg (s) + O2(g) 2MgO (s)
white
Observation : burns with intense white flame to form white solid
3. 4Al + 3O2 2Al2O3
4. Si + O2 SiO2
5. P4 + 5O2 P4O10 white solid at rtp
(or 4P + 5O2 P4O10 )
Observation: burns with white/yellow flame.
Another possible compound : P4O6
6. S + O2 SO2
Observation: burns with a blue flame/ choking or pungent gas produced
Further oxidation: 2SO2 + O2 2SO3
Reaction with chlorine ( Na to P )
1. 2Na + Cl2 2NaCl
white solid
Observation : burns with yellow flame /
forms white solid /colour of Cl2 disappears
2. Mg + Cl2 MgCl2
Observation : burns with intense white
flame / forms white solid
3. 2Al + 3Cl2 2AlCl3 Al2Cl6
covalent
simple molecular
ions formed in aq soln
Al3+(aq) or [Al(H2O)6]3+ and Cl-(aq)
Note : Al2O3 is ionic as difference in electronegativity between Al and O is larger
1800 C
Method :
Pass dry chlorine gas over heated aluminium foil in a tube.
4. Si + 2Cl2 SiCl4 (l)
5. P4 + 10Cl2 4PCl5 (s)
white solid
( or 4P + 10Cl2 4PCl5 )
Observation : burns with white flame / forms white solid
Another possible compound : PCl3 , colourless liquid
Oxides of elements in Period 3
I) Trend in acidity of oxides :
Na2O MgO Al2O3 SiO2 P4O10 SO3
Ionic oxide
Basic oxide
Mainly ionic
Oxide
Amphoteric oxide
Covalent oxide
Acidic oxide
Oxides becomes more acidic across the period
1. Na2O : basic oxide
a. dissolves in water strongly alkaline solution
Na2O + H2O 2NaOH (pH=13)
NaOH + HCl NaCl + H2O
(alkali)
b. able to neutralize acids
Na2O + 2HCl 2NaCl + H2O
2. MgO : basic oxide
a. dissolves slightly in water weakly alkaline solution
MgO + H2O Mg(OH)2 Mg2+ + 2OH-
pH = 9
Mg(OH)2 + 2HCl MgCl2 + 2H2O
(alkali)
b. able to neutralize acids :
MgO + 2HCl MgCl2 + H2O
3. Al2O3 :
a. stronger ionic bonds not broken by water
therefore insoluble in water
b. nature of oxide : amphoteric
i) Al2O3 + 6H+ 2Al3+ + 3H2O
acid
ii) Al2O3 + 2OH- + 3H2O 2Al(OH)4-
alkali aluminate ion
c. nature of hydroxide : amphoteric
i) Al(OH)3 + 3H+ Al3+ + 3H2O
ii) Al(OH)3 + OH- Al(OH)4-
4. SiO2 :
a. giant molecular structure with strong
Si – O bonds not broken by water ,
therefore insoluble in water
b. nature of oxide : acidic oxide
SiO2 + 2OH- SiO32- + H2O
hot , conc silicates
Notes :
(1) no reaction with aqueous alkali
(2) SiO2 + O2- SiO32-
acidic basic silicates
5. Oxides of P and S: P4O6 / P4O10*, SO2 /SO3
a. nature of oxide : acidic oxide
b. hydrolyses in water acidic solutions
i) P4O10 + 6H2O 4H3PO4 (phosphoric
acid)*
ii) P4O6 + 6H2O 4H3PO3 (phosphonic
acid)
Observation : white solid dissolves
iii) SO2 + H2O H2SO3 (sulfurous
acid)
iv) SO3 + H2O H2SO4
c. able to neutralize alkali :
i) P4O10 + 12OH- 4PO43- + 6H2O
phosphate
ii) SO2 + 2OH- SO32- + H2O
sulfite
iii) SO3 + 2OH- SO42- + H2O
II) Trends in physical properties of the oxides
Related to structure and bonding
Na2O MgO Al2O3 SiO2 P4O10 SO3
Ionic oxides
Giant ionic structure
Covalent oxide
Giant molecular
structure
Covalent oxide
Simple molecular
structure
1. Na2O , MgO , Al2O3 :
a. ionic with giant ionic structure
b. strong ionic bonds , therefore high
m.p
( m.p MgO Al2O3 Na2O )
c. presence of mobile ions,
good electrical conductor in aqueous
solution or molten state
2. SiO2 :
a. covalent with giant molecular structure
b. numerous strong covalent bonds between atoms , therefore relatively high m.p
( lower than m.p of MgO and Al2O3 ,
but higher than Na2O )
c. molecular , all electrons paired in covalent bonds
no free electrons or ions non conductor
3. P4O10 , SO3 :
a. covalent with simple molecular structure
b. exists as discrete molecules held by weak VDW forces low m.p
m.p of P4O10 (more electrons) SO3
c. molecular , all electrons paired in covalent bonds ,
no free electrons or ions ,
therefore non conductor
Atomic no
m.p of
oxides
Na Mg Al Si P S
Chlorides of elements in Period 3
I)Reaction with water
1. NaCl : dissolve in water without chemical reaction neutral solution
NaCl (s) Na+ (aq) + Cl- (aq)
2. MgCl2 :
a. ionic with a little covalent character
b. relatively high charge density on Mg2+ causes polarisation of water
Undergoes slight hydrolysis in water to form slightly acidic solution (pH=6.5)
MgCl2 + H2O Mg(OH)Cl(s) + HCl(aq)
However only very slight reaction – observation : dissolves without reaction
3. AlCl3 :
a.covalent with a little ionic character
b.high charge density on Al3+ polarises water
Undergoes substantial hydrolysis in water to form acidic solution ( pH=3 )
AlCl3 + 6H2O [Al(H2O)6]3+ + 3Cl-
covalent ionic
[Al(H2O)6]3+ + H2O [Al(H2O)5OH]2+
+ H3O+
( first of 3 equations , polarisation continues )
Overall equation :
AlCl3 + 3H2O Al(OH)3 + 3HCl (*)
H=exothermic
4. SiCl4 ,PCl3/PCl5
a. covalent chlorides
b. hydrolysed in water strongly acidic solution ( pH = 0 – 3 )
SiCl4 + 2H2O SiO2 + 4HCl(g)
PCl3 + 3H2O H3PO3 + 3HCl(g)
PCl5 + 4H2O H3PO4 + 5HCl(g)
HCl(g):
Observation
white fumes
SiO2 white
solid
II)Trends in physical properties of the chlorides
Related to structure and bonding
NaCl MgCl2 AlCl3 SiCl4 PCl5
Ionic
Giant ionic
structure
Covalent
Layered
structure
Covalent
Simple
molecular
structure
1. NaCl , MgCl2 :
a. ionic with giant ionic structure
b. strong ionic bonds therefore high m.p/b.p
( m.p NaCl MgCl2 )
c. presence of free ions : good conductor when molten or in aqueous solution
2. AlCl3 :
a. predominantly covalent with layered structure in the solid state
b. in each layer , all AlCl3 molecules are joined by dative bonds and forms a giant structure
Layers are held together by weak VDW forces
c. when it melts , the VDW forces and dative bonds have to be broken
relatively high m.p / b.p but lower than that of ionic chlorides
d. molecular , all electrons paired in covalent bonds
no free electrons or ions
therefore non conductor
3. SiCl4 , PCl5 / PCl3 :
a. covalent with simple molecular structure
b. i) exist as discrete molecules held by
weak VDW forces
therefore low m.p / b.p
ii) m.p of PCl5 > m.p of SiCl4
Reason :
no of electrons in PCl5 > SiCl4
stronger VDW forces , higher b.p/m.p
PCl5 ( white solid ) ,
SiCl4 ( colourless liquid )
c. molecular , all electrons paired in covalent bonds
no free electrons or ions
therefore non conductor
Atomic no
b.p of
chlorides
Na Mg Al Si P
Patterns in the formula of compounds (oxides & chlorides)
1. Oxidation no of the element = no of electrons used for bonding
Eg Compound Oxidation no
SO2 +4
Na2O +1
PCl3 +3
2. Many of the oxidation no corresponds to loss or gain of enough electrons to obtain a stable octet in its outermost shell.
Eg Na - 1e Na+
1s22s22p63s1 1s22s22p6
P + 3e P3-
[Ne]3s23p3 [Ne]3s23p6
3. The maximum oxidation no = group no
= no of valence electrons
Egs: oxides
max oxidation no (in SO3 )= +6
max oxidation no in P4O10 = + 5
a. S (Group 16) : forms SO2 and SO3
b. P (Group 15) : forms P4O6 and P4O10
Trend in maximum oxidation:
Increases across the period
Na Si Cl
+1 +4 +7