CHM 122 - 2016 - grp1-4 PDF
CHM 122 - 2016 - grp1-4 PDF
CHM 122 - 2016 - grp1-4 PDF
ELEMENT Li Na K Rb Cs
1st I.E. 521 492 415 405 376
2nd I.E. 7300 4560 3070 2370 2420
Chemical Properties
The alkali metals are so reactive that in nature, they occur a
s their compounds i.e. they are always found combined wit
h other elements. They exist as chlorides, nitrates, sulphate
s, carbonates, silicates, etc.
Chemical Properties contd.
• Their oxides and hydroxides are basic and they release h
ydrogen with acids.
• They rapidly tarnish in air and react violently with water-
for both reasons they are kept under a layer of oil or in a
vacuum -sealed ampoule to prevent atmospheric oxidati
on.
Reactions:
With Hydrogen:
• Alkali-metals react with hydrogen gas to form hydrides. T
he ease of formation of the hydrides decreases down th
e group.
2M (s) H 2 ( g ) 2MH (s)
(M = Group IA metal)
Reactions With Hydrogen contd.
• All the hydrides of the Group IA are white solids and ioni
c in nature. M+ H-. This is one of the reactions where hydr
ogen furnishes the negative ion.
• However, the reaction is slow at room temperature -as h
eating is required to melt the metals before reaction take
s place. Temperatures of between 300◦C and 500◦Care re
quired for such reactions.
.
• The most important alkali-metal hydride is lithium hydrid
e. On treatment with aluminum chloride in a dry solution
of ether, lithium aluminum hydride, (LiAlH), is formed.
4LiH AlCl3 Li ( AlH 4 ) 3LiCl
dry ether solution
Reactions With Hydrogen contd.
• LiAlH 4 is one of the most useful reducing agents in orga
nic chemistry.
• The alkali-metal hydrides react immediately with water, gi
ving off hydrogen:
LiH (s) H 2O(l ) LiOH (aq) H (2) ( g )
The stability of the hydrides decreases down the group wh
ich also implies that the reactivity of the metal hydrides in
creases down the group.
Reaction With Oxygen
Group IA elements react rapidly with oxygen to form three
types of oxides namely: monoxides (normal oxides), peroxi
des and superoxides. Lithium forms lithium oxide, Li O (i.e.
lithium monoxide); sodium forms mainly sodium peroxide.
Reaction With Oxygen contd.
Na though it can also form sodium oxide, NaO, while the
other metals- K, Rb, Cs, form mainly the superoxides as s
hown in the table below:
Lithium oxide
4Li(s) O( 2) ( g ) 2Li2O(s)
2 Na(s) O2 ( g ) Na2 O2 (s) Sodium peroxide
(M = Group IA metal)
Reaction With Halogens
The Group 1A elements react with halogens to form halides
6M (s) N 2( g ) 2M 3 N (s)
(M = Group IA metal)
Reaction With Nitrogen contd.
The alkali-metal nitrides decompose at low
temperatures and react with water vapour to form
ammonia and the metal hydroxide:
Na3 N 3H 2O NH3 3NaOH
Reaction with Ammonia
Alkali-metals dissolve in ammonia to form ionic amides
2Na 2NH3 2NaNH 2 H 2
Reaction with Alkyl halides
Group IA metals react with alkyl halides to form the metal a
2Li CH 3Cl CH 3 Li LiCl
lkyl. e.g.
THE CARBONATES/HYDROGEN CARBONATES
• The carbonates and hydrogen carbonates of the alkali-m
etals are white solids and have the general formula,
M2 CO3 and Mrespectively (M = Group IA metal).
• They are all soluble in water except lithium carbonate.
• Note that lithium does not give a hydrogen carbonate, ie
LiHCO3 does not exist.
THE SULPHATES:
• All the sulphates and hydrogen sulphates of the group
are soluble in water but thermally stable (except at very
high temperatures). Na is the major compound in
the group.
• Generally, sulphates are usually more
thermally stable than nitrates and carbonates
THE SULPHATES contd.
resonance structure of
sulphate
Anomalous Properties of Lithium
• Lithium stands at the head of Group IA metals. Many of i
ts properties quite differ from those of the other membe
rs of the group.
• The main reason for this anomalous behaviour is the sm
all size of lithium ion. This causes the behaviour of lithiu
m compounds to show differences from the compounds
of the other elements in the group.
• Due to the small size of lithium ion, its compounds have
a greater tendency towards covalency.
• This is demonstrated by the lower solubility of lithium flu
oride in aqueous solution and by the fact that some lithi
um compounds are soluble in organic solvents e.g. lithiu
m chloride is soluble in ethoxy ethane.
Anomalous Properties of Lithium contd.
• From Fajan’s rule, an ionic compound will have a high degr
ee of covalency if the positive ion is small and highly charg
ed- i.e. covalency is promoted by small cations and large a
nions.
• Lithium rather shows similarities to magnesium with
which it exhibits a diagonal relationship. This
relationship refers to the resemblance of each of the
first three elements of the second period of the
Periodic Table to the elements located diagonally to it.
Period 2: Li Be B
Period 3: Mg Al Si
USES OF SOME COMPOUNDS OF GROUP IA ELEMENTS
Compounds of the Group IA elements have wide
industrial applications:
• Sodium carbonate: this is also known as soda ash and m
anufactured through the Solvay process. It is a fine whit
e powder used for the manufacture of glass. It is widely
used in the paper industry and in the manufacture of so
ap and detergents. It is used as a water softener and in
the laboratory; it is used to standardize acids and also us
ed as an analytical reagent. It is by far the most commer
cially important Group IA carbonate.
• Sodium hydroxide: this is used in soap and detergent m
anufacture. Also used for rayon and in paper industries.
In the laboratory, it is used as a strong alkali and analytic
al reagent.
USES OF SOME COMPOUNDS OF GROUP IA ELEMENTS
contd.
Physical properties:
• They are all metals and always occur in nature as compou
nds because they are too electropositive to occur free in
nature.
• Just as the alkali metals the metallic character increases
down the group.
• The table 4 below shows some of their properties:
TABLE 4: PROPERTIES OF GROUP IIA ELEMENTS
SYMBOL ELECT. ATOMIC IONIC MP BP DENSITY 1ST I.E. E.N.
CONFIG. RADIUS (Ả) RADIUS ( 0C) ( 0C) (g/ ) kJ mol-1
(Ả)
ELEMENT Be Mg Ca Sr Ba
3M (s) N 2 ( g ) M 3 N 2 (s)
(M = Group IIA metal)
• The nitrides dissolve in water to form ammonia and the
metal hydroxide.
M 3 N 2 (s) 6H 2O(l ) 3M (OH )2 (aq) 2 NH 3 ( g )
(M = Group IIA metal)
Reaction with Halogens
• The alkaline-earth metals react with halogens to form the
halides.
M (s) X 2 ( g ) MX 2 ( g )
(M = Group IIA metal X = Cl, F, Br or I)
I 2 Br2 Cl2 F2 ; Be Mg Ca Sr Ba
Apart from beryllium halides, the Group IIA halides are
predominantly ionic. Reactivity increases in the order:
CARBONATES of Group IIA:
• The Group IIA carbonates have the general formula,
MCO3 and are slightly soluble in water.
• They all decompose on heating to give the metal oxide
and CO2
MCO3 (s)
MO( s ) CO2 ( g )
THE NITRATES:
• The nitrates of the alkaline-earth metals are of the form,
M(NO3)2 they decompose on heating as follows:
2M + N2 2MN Al only
2M+6H+ 2M3++3H2 Tl also gives Tl+
2M + 2OH- + 6H2O With Al and Ga only
2M(OH)-4 + 3H2
CHEMICAL PROPERTIES CONTD.
• The sulfates, nitrates and the halides are water
soluble, but M3+ ions hydrolyse readily.
• Oxides and hydroxides exhibit the usual trend in decr
easing acidity with increasing atomic number .
• B2O3 and B(OH)3 are acidic.
B2O3 > Al2O3 >Ga2O3 > In2O3>Tl2O
• The oxides and hydroxides are insoluble in H2O
• while those of aluminium and gallium are amphoteric.
3
Al2O3 (s) 6H (aq) 2 Al (aq) 3H 2O(l )
3
Al2O3 (s) 6OH (aq) 3H 2O(l ) 2 Al (OH )6 (aq)
- In2O3, Tl2O3 are basic, they have no acid character at
all
CHEMICAL PROPERTIES contd.
B Al
Occurrence As borax, Na2 B4O7 .10 H 2O AsBauxite : Al2O3.2H2O
As Kernite, Na2 B4O7 .4 H 2O
Extraction Borax HCl Boricacid Electrolysis of the fused
H 3 BO3 salt
H 2O
H3 BO3 B2O3 3 2
.boricoxide 4 Al O 3C( s )
B2O3 3Mg(s)
2 B(s) 3MgO( s )
electrolysis
4 Al( s ) 3CO2( g )
2 BX 3( s ) 3H 2( g )
W
1500o C
2 B( s ) 6 HX ( g ) (X = Cl, Br)
(W = tungsten)
CHEMICAL PROPERTIES contd.
Alumina
Borax
Boric acid
Reactions B CHEMICAL PROPERTIES CONTD. Al
4M+3O2 At high temp; high heat of reaction At high temp.
2M2O3
2M+N2 At white heat Forms
2MN
2M+3S Passage of S vapour over B at 12000C At high temperature
M2S3
Al(OH)3 + NaOH
NaAlO2 + 2H2O
(Sodium aluminate)
Uses of Group IIIA Elements
1. Boron is used both as a deoxidiser in the
manufacture of some metals.
2. Boron is used as neutron absorber in the
production of nuclear energy.
3. Boron is used as an additive to the semi-
conductors, silicon and germanium.
4. Aluminium is used in making pans, kettles and
teapots because of its high electrical and thermal
conductivity.
5. Aluminium is used to replace copper for
conductors where lightness is of paramount
importance.
Explaining the Concept of Shielding effect
Why do we use the term “effective nuclear charge”
when describing nuclear charge?
• The answer to this question is very important
when discussing the chemistry of the elements.
Since we know that, the chemistry of an element
depends upon its electronic configuration and
thus on the relative energies of the electrons
within the atom.
• The energy of one electron within the atom
depends upon the energies of the other electrons
present
Shielding effect Contd.
• This is, in fact, why the orbital
energy diagram for hydrogen
is different from that of the
other elements.
• Hydrogen only has one electro
n and therefore there are no
MO diagram for dihydrogen.
electron-electron interactions. Here electrons are shown by dots
All of the other elements have electron-electron •
interactions and these interactions depend on the
shape of the orbital, l that each of the electrons is
You will also recall that the energy of the electrons in multi-electron •
atoms depends on both the quantum numbers n and l (l determines
the shape of the orbital).
Shielding effect Contd.
Classification
C is strictly nonmetallic, Si and Ge are metalloid. Sn and Pb
are metallic.
The electronic configuration and physical properties of the
elements are shown in the following Figure.
GROUP IV A ELEMENTS Carbon Silicon Germanium Tin Lead
Properties
C Si Ge Sn Pb
Ground State Electronic [Xe]6s26
[He]2s2p2 [Ne]3s23p2 [Ar]4s24p2 [Kr]5s25p2
Configuration p2
m. pt (oC) 3570 1420 959 232 327
b.pt (oC) 4200 2355 2700 2360 1755
D 3.51 W 7.31
Density g/cm-3 2.33 5.36 11.34
G 2.22
Covalent radius (Ao) 0.77 1.17 1.22 1.41 1.47
Ionization Potential (eV)
1st 11.3 8.1 8.09 7.30 7.38
2nd 24.4 16.3 15.86 14.5 14.96
3rd 47.9 33.5 34.10 30.5 31.9
4th 64.5 45.1 45.5 39.4 42.1
Electrode Potentials
*Eo(V)
M2+ + 2e M(s) -
Extraction
supply of air.
(Carbon black)
Sn and Pb are obtained from their ores in various ways, commonly by reduction of their oxides with
carbon.
Extraction of Pb
Example Pb from galena (PbS)
(i) PbS is roasted to the oxide
Carbon
α – Sn
18 C
β – Sn
16 1 C
Sn
232 C
Sn(l)
Carbon has the unique ability to form compounds in which many carbon atoms are bonded to each
other in chains or rings. This property is called CATENATION. This property is also exhibited by other
elements near carbon in the periodic table (B, N, P, S, O, Si, Ge, & Sn) but to a much lesser extent
than carbon. This property of carbon accounts for the large number of organic compounds.
In Group IVA, the tendency to catenation is in the following order C Si >Ge Sn>>Pb
Oxidation States (-4, 4+, 2+)
The elements exhibit variable oxidation state. Carbon exhibits +4 in most of its compounds –
tetravalency
-1 oxidation state is exhibited by carbon e.g. CaC2. They are essentially covalent compounds; the
covalent bond is usually tetrahedral involving sp3 hybridization.
Ge, Sn, & Pb show inert pair effect. Form more +2 ions (or divalent); the stability of which increases
down the group e.g. Ge2+ ions are very unstable but Pb2+ ions are stable. PbO is more stable than
PbO2.
The elements behave more like metals in the lower oxidation states.