the s-block elements

Chapter 9 The s-Block Elements

s-Block elements refer to elements having their outermost electrons occupying s-orbital. So, s-block elements also refer to group 1 and group 2 elements.

m.p. density crystal structure

Down a group : Li 181 0.53 b.c.c Be 1277 1.85 h.c.p
1. Increasing metallic character Na 98 0.97 b.c.c Mg 650 1.74 h.c.p
2. Weakening metallic bond K 64 0.86 b.c.c Ca 838 1.55 f.c.c
3. Increasing atomic size Rb 39 1.53 b.c.c Sr 768 2.6 f.c.c
4. Decreasing m.p. Cs 29 1.90 b.c.c Ba 714 3.5 b.c.c


9.1 Characteristic properties of the s-block elements

s-block elements are mainly metallic elements.
They have low electronegativities.
They form basic oxides and hydroxides.
They have fixed oxidation state in their compounds.
They do not have low lying d-orbitals to form complex.
Some of them have specific flame colour.
Li is anomalous to group 1 metal. Its chemistry resembles Mg rather than
a group 1 metal. This is part of the diagonal relationship.

Exercise 1. Why do s-block elements have low electronegativities?
2. Why do s-block elements have fixed oxidation states?


9.1.1 Metallic character
Group 1 and group 2 elements are metals.
- group 1 metals are named as "alkali metals".
- group 2 metals are named as "alkaline earth metals".
They are silvery and shiny but tarnish in air rapidly.
- freshly cut surface will expose metal to air and the metal
is oxidized.
They are low melting and can be cut by a knife.
- because each atom can only have 1 to 2 valence electrons to
form metallic bond. Of course, group 2 metals have
stronger metallic bond than group 1 metals.
Group 1 metals crystallize in body-centred cubic lattice.
- this explains the low density of group 1 metals


9.1.2 Electronegativity
The electronegativity of element
- decreases down a group and
- increases across a period
Explanation :



9.1.3 Formation of basic oxides and hydroxides
s-block elements have very low electronegativities and lose their
outermost electrons readily. Thus, they form ionic oxides and
ionic hydroxides which are necessarily basic in nature.




9.1.4 Standard electrode potential Eo /V Mn+(aq) + ne- → M(s)

Li Na K Rb Cs
-3.05 -2.71 -2.93 -2.99 -3.20

The abnormal value of Li is due to the exceptional small size of Li+
which is strongly hydrated to give the most negative Eo value.

Be Mg Ca Sr Ba
-1.85 -2.38 -2.87 -2.89 -2.90



9.1.5 Flame colour of compounds

Flame colour originates from the emission of visible light when an
excited electron de-excites(an electron returns to a lower energy level
from a higher energy level).
Before carrying out a flame test, the platinum wire(or silica rod) should
be washed with concentrated HCl by dipping the platinum wire into the
concentrated HCl and heat it in a non-luminous flame. As chlorides
are generally more volatile, the contaminant on the platinum wire
would be carried away. The cleaning process is completed when no
specific flame colour can be seen.

Flame colour of cations:
K+ lilac, viewed through a cobalt glass
Na+ golden yellow , persistent
Li+ crimson red, deep red
Ca2+ brick red
Sr2+ blood red
Ba2+ apple green












9.2 Variation in properties of the s-block elements and their compounds

9.2.1 Variation in atomic radii
Atomic radii increase down a group as the number of electron shell
is increasing and the outermost shell electrons are screened by core
electrons and experience a lower nuclear attraction.
Atomic radii decrease across a period because the addition electron
is put into the same quantum shell and the increase in nuclear charge
would attract the additional electron stronger to give a smaller atomic size.
atomic radius / nm





period number

9.2.2 Ionization Enthalpies
I.E. decreases down a group as the number of electron shell
is increasing and the outermost shell electrons are screened by core
electrons and experience a lower nuclear attraction.
First I.E. increases across a period because the addition electron
is put into the same quantum shell and the increase in nuclear charge
would attract the additional electron stronger to give a smaller atomic size.
The 2nd I.E. is always much greater than the 1st I.E. as it is more
difficult to remove a negatively charged electron from a positive ion.








9.2.3 Melting point
Plot the melting points of s-block elements in the following diagram.
m.p.(℃)








Period number
The melting point decreases down a group as the metallic bond is
weaker as the atomic size is larger.
The melting point of group 2 element is much larger than its group 1
counterpart as each group 2 atom has two electrons for bonding but
group 1 atom has one. The stronger metallic bond in group 2 metal
gives rise to higher melting point.
9.2.4 Hydration Enthalpies

The reaction of hydration is Mn+(g) + aq → Mn+(aq).
For an ion, hydration enthalpy is always negative.
The higher the charge density of an ion, the higher(more negative) the
hydration enthalpy. This is because the high electric potential holds
the water(polar solvent) molecules stronger.


hydration enthalpy kJ mol-1

-500 -2000

-200 -1500

Li+ Be2+

9.2.5 Reactions with oxygen

s-block elements reacts with oxygen gas readily.
K, Rb, Cs form superoxide, peroxide and normal oxide
Na, and Ba form peroxide and normal oxide
Others form normal oxides only.
K(s) + O2(g) → K2O(s) a normal oxide
K(s) + O2(g) → K2O2(s) a peroxide
K(s) + O2(g) → KO2(s) a superoxide

2-

potassium oxide K2O K+ O K+

2-

potassium peroxide K2O2 K+ O O K+

-

potassium superoxide KO2 K+ O O


-

potassium hydroxide KOH K+ O H


Soluble ionic oxide gives OH- and makes the solution alkaline.
2KO2(s) + 2H2O(l) → 2KOH(aq) + H2O2(aq) + 3O2(g)
2K2O2(s) + 2H2O(l) → 4KOH(aq) + O2(g)
K2O(s) + H2O(l) → KOH(aq)


9.2.6 Reactions with water

Sodium melts and moves rapidly on the surface of water. Hydrogen gas
is evolved and a strong alkaline solution is produced.

___ K(s) + ____H2O(l) →
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
____Ca(s) + ____H2O(l) →
____Mg(s) + ____H2O(g) →
____Al(s) + ____H2O(l) →
rocksil soaked
in water

heat heat magnesium

Exercise : Describe what you can see when a small piece of
(a) potassium
(b) magnesium
is dropped into water.




9.2.7 Reactions with hydrogen

Except Be , all s-block elements react with hydrogen.
Magnesium reacts under high pressure.

2K(s) + H2(g) 2KH(s) potassium hydride
Ca(s) + H2(g) CaH2(s)

The metal hydrides are ionic hydrides.
The hydride ion H- reacts with water vigorously.

KH(s) + H2O(l) → KOH(aq) + H2(g)

H-(s) + H2O(l) → OH-(aq) + H2(g)

9.2.8 Reactions with chlorine

All s-block elements react with chlorine.
2K(s) + Cl2(g) → 2KCl(s) potassium chloride
Ca(s) + Cl2(g) → CaCl2(s)

BeCl2(s) is covalent and has a linear polymeric structure.

9.2.9 Reactions of oxides, hydrides and chlorides with
water, acids and alkalis

metal oxide metal hydride metal chloride
water BeO(s) MgO(s), CaO(s)are insolubleOthers :K2O+H2O→2KOH(aq) CaH2+2H2O→Ca(OH)2+2H2 BeCl2+2H2O→Be(OH)2+2HClOthers no reaction
diluteacid NeutralizationMgO+2HCl→MgCl2+H2O Explosive BeCl2 reacts with waterOthers : no reaction
dilutealkali BeO+2NaOH+H2O→Na2Be(OH)4Others : no reaction Reacts with water in thealkaline solution. BeCl2+2NaOH→Be(OH)2+2NaClOthers : no reaction






9.2.10 Relative thermal stability of the carbonates and hydroxides

Stability of carbonates

carbonate


heat
lime water

Group 1 carbonates except Li2CO3 are stable to heat.

Group 2 carbonates decompose on heating.

Decomposition temperature:
BeCO3 MgCO3 CaCO3 SrCO3 BaCO3
~100℃ 540 900 1290 1360

Group 2 carbonates are becoming less stable to heat on going down the group.
BaCO3 is quite stable to heat.
MgCO3 is most unstable to heat because its cation is far more smaller than the
anion. The small cation has a high charge density and high polarizing power
and polarizes the large anion. The anion is distorted and can be decomposed
easily.


Stability of hydroxides
Li and group 2 metal hydroxides are decomposable at high temperature.
The ease of decomposition decreases down group 2. The trend again is
governed by the polarizing power of cation.
Mg(OH)2(s) MgO(s) + H2O(g)

9.2.11 Relative solubility of the sulphates(VI) and hydroxides

Sulphates

Group 1 sulphates are soluble.


Group 2 sulphates :
Solubility of sulphates : MgSO4 > CaSO4 > SrSO4 > BaSO4
soluble insoluble insoluble insoluble
Reason : Sulphate is a large anion. The size of the cations down the
group are progressively increasing. The decrease in
hydration enthalpy of the cation outweighs the decrease in
lattice enthalpy. So, sulphates become less soluble down the group.


MgSO4(s) + aq → MgSO4(aq) enthalpy of solution of MgSO4

-lattice energy hydration enthalpy of Mg2+(g)
+ hydration enthalpy of SO42-(g)

Mg2+(g) + SO42-(g)





Hydroxides

Group 1 hydroxides are strong alkalis.

Group 2 hydroxides :
Solubility of hydroxides :
Mg(OH)2 < Ca(OH)2 < Sr(OH)2 < Ba(OH)2
insoluble insoluble soluble soluble
Reason : __________ is a small anion. The size of the cations down
the group are progressively increasing. The decrease in
____________ enthalpy of the cation outweighs the decrease
in ___________ enthalpy. So, hydroxides become ________
soluble down the group.

Energy cycle :











9.2.12 Uses of s-block elements


1. Na2CO3 is useful in making soda glass.

CaCO3(s) + SiO2(s) CaSiO3(s) + CO2(g)

Na2CO3(s) + SiO2(s) Na2SiO3(s) + CO2(g)
sodium silicate

Soda glass is a mixture of calcium silicate and sodium silicate.


2. NaHCO3 as baking powder
2NaHCO3(s) Na2CO3(s) + H2O(g) + CO2(g)

The carbon dioxide evolved will rise the cake.


3. NaOH/KOH in making soap

Fat molecule can be hydrolysed(saponification) to form soap.
e.g. glyceryl stearate + alkali → glycerol + sodium stearate(soap)


4. Mg(OH)2 as anti-acid to neutralize excess acid in stomach
Mg(OH)2(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)


5. Slaked lime to neutralize acid in industrial effluent
Ca(OH)2(s) + 2HCl(aq) → CaCl2(aq) + H2O(l)


6. Strontium in firework
The burning of strontium compounds gives ___________ _________ colour glare.