AN INTRODUCTION TO
GROUP II
Alkaline earths
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s23p64s2
KNOCKHARDY PUBLISHING
2015
SPECIFICATIONS
KNOCKHARDY PUBLISHING
GROUP II (Alkaline Earths)
INTRODUCTION
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©HOPTON
GROUP II
CONTENTS
• General properties
• Trends in electronic configuration
• Trends in atomic and ionic radius
• Trends in melting point
• Trends in ionisation energy
• Reaction with oxygen and water
• Oxides and hydroxides
• Carbonates
• Sulfates
©HOPTON
GROUP PROPERTIES
GENERAL
• metals
• all have the electronic configuration ... ns2
TRENDS
•
•
•
•
•
melting point
electronic configuration
electronegativity
atomic size
ionic size
©HOPTON
THE s-BLOCK ELEMENTS
Elements in Group I (alkali metals) and Group II (alkaline earths) are known as
s-block elements because their valence (bonding) electrons are in s orbitals.
©HOPTON
THE s-BLOCK ELEMENTS
Elements in Group I (alkali metals) and Group II (alkaline earths) are known as
s-block elements because their valence (bonding) electrons are in s orbitals.
ALKALI METALS
Gp I
1s2 2s1
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s23p64s1
Li
Na
K
… 5s1
Rb
… 6s1
Cs
Fr
©HOPTON
THE s-BLOCK ELEMENTS
Elements in Group I (alkali metals) and Group II (alkaline earths) are known as
s-block elements because their valence (bonding) electrons are in s orbitals.
ALKALI METALS
ALKALINE EARTHS
Gp I
Gp II
Li
Be
1s2 2s2
Na
Mg
1s2 2s2 2p6 3s2
K
Ca
1s2 2s2 2p6 3s23p64s2
… 5s1
Rb
Sr
… 6s1
Cs
Ba
Fr
Rn
1s2 2s1
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s23p64s1
©HOPTON
… 5s2
… 6s2
THE s-BLOCK ELEMENTS
Elements in Group I (alkali metals) and Group II (alkaline earths) are known as
s-block elements because their valence (bonding) electrons are in s orbitals.
ALKALI METALS
ALKALINE EARTHS
Gp I
Gp II
Li
Be
1s2 2s2
Na
Mg
1s2 2s2 2p6 3s2
K
Ca
1s2 2s2 2p6 3s23p64s2
… 5s1
Rb
Sr
… 6s1
Cs
Ba
Fr
Rn
1s2 2s1
1s2 2s2 2p6 3s1
1s2 2s2 2p6 3s23p64s1
Francium and radium are both
short-lived radioactive elements
©HOPTON
… 5s2
… 6s2
GROUP TRENDS
ELECTRONIC CONFIGURATION
Be
Mg
Ca
Sr
Ba
Atomic Number
4
12
20
38
56
Old e/c
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
New e/c
1s2 2s2
…3s2
… 4s2
… 5s2
… 6s2
©HOPTON
GROUP TRENDS
ELECTRONIC CONFIGURATION
Be
Mg
Ca
Sr
Ba
Atomic Number
4
12
20
38
56
Old e/c
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
New e/c
1s2 2s2
…3s2
… 4s2
… 5s2
… 6s2
As the nuclear charge increases, the electrons go into shells further
from the nucleus.
©HOPTON
GROUP TRENDS
ELECTRONIC CONFIGURATION
Be
Mg
Ca
Sr
Ba
Atomic Number
4
12
20
38
56
Old e/c
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
New e/c
1s2 2s2
…3s2
… 4s2
… 5s2
… 6s2
As the nuclear charge increases, the electrons go into shells further
from the nucleus.
The extra distance of the outer shell from the nucleus affects…
Atomic radius
Ionisation energy
Chemical reactivity
Ionic radius
Melting point
©HOPTON
GROUP TRENDS
ATOMIC & IONIC RADIUS
Be
Mg
Ca
Sr
Ba
Atomic radius / nm
0.106
0.140
0.174
0.191
0.198
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
©HOPTON
GROUP TRENDS
ATOMIC & IONIC RADIUS
Be
Mg
Ca
Sr
Ba
Atomic radius / nm
0.106
0.140
0.174
0.191
0.198
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
ATOMIC RADIUS
INCREASES down Group
• the greater the atomic number
the more electrons there are;
these go into shells increasingly
further from the nucleus
©HOPTON
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s23p64s2
GROUP TRENDS
ATOMIC & IONIC RADIUS
Be
Mg
Ca
Sr
Ba
Atomic radius / nm
0.106
0.140
0.174
0.191
0.198
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
ATOMIC RADIUS
INCREASES down Group
• the greater the atomic number
the more electrons there are;
these go into shells increasingly
further from the nucleus
1s2 2s2 2p6 3s2
1s2 2s2 2p6 3s23p64s2
• atoms of Group II are smaller than
the equivalent Group I atom
the extra proton exerts a greater
attraction on the electrons
©HOPTON
11 protons
1s2 2s2 2p6 3s1
12 protons
1s2 2s2 2p6 3s2
GROUP TRENDS
ATOMIC & IONIC RADIUS
Be
Mg
Ca
Sr
Ba
Atomic radius / nm
0.106
0.140
0.174
0.191
0.198
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
Be2+
Mg2+
Ca2+
Sr2+
Ba2+
Ionic radius / nm
0.030
0.064
0.094
0.110
0.134
Electronic config.
2
2,8
2,8,8
2,8,18,8
2,8,18,18,8
©HOPTON
GROUP TRENDS
ATOMIC & IONIC RADIUS
Be
Mg
Ca
Sr
Ba
Atomic radius / nm
0.106
0.140
0.174
0.191
0.198
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
Be2+
Mg2+
Ca2+
Sr2+
Ba2+
Ionic radius / nm
0.030
0.064
0.094
0.110
0.134
Electronic config.
2
2,8
2,8,8
2,8,18,8
2,8,18,18,8
IONIC RADIUS
INCREASES down Group
• ions are smaller than atoms – on removing the outer shell
electrons, the remaining electrons are now in fewer shells
©HOPTON
GROUP TRENDS
ATOMIC & IONIC RADIUS
Be
Mg
Ca
Sr
Ba
Atomic radius / nm
0.106
0.140
0.174
0.191
0.198
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
Be2+
Mg2+
Ca2+
Sr2+
Ba2+
Ionic radius / nm
0.030
0.064
0.094
0.110
0.134
Electronic config.
2
2,8
2,8,8
2,8,18,8
2,8,18,18,8
IONIC RADIUS
INCREASES down Group
• ions are smaller than atoms – on removing the outer shell
electrons, the remaining electrons are now in fewer shells
1s2 2s2 2p6 3s2
1s2 2s2 2p6
©HOPTON
GROUP TRENDS
ATOMIC & IONIC RADIUS
Be
Mg
Ca
Sr
Ba
Atomic radius / nm
0.106
0.140
0.174
0.191
0.198
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
Be2+
Mg2+
Ca2+
Sr2+
Ba2+
Ionic radius / nm
0.030
0.064
0.094
0.110
0.134
Electronic config.
2
2,8
2,8,8
2,8,18,8
2,8,18,18,8
IONIC RADIUS
INCREASES down Group
• ions are smaller than atoms – on removing the outer shell
electrons, the remaining electrons are now in fewer shells
©HOPTON
1s2 2s2 2p6 3s2
1s2 2s2 2p6
1s2 2s2 2p6 3s23p64s2
1s2 2s2 2p6 3s23p6
GROUP TRENDS
MELTING POINT
Be
Mg
Ca
Sr
Ba
Melting point / ºC
1283
650
850
770
710
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
©HOPTON
GROUP TRENDS
MELTING POINT
Be
Mg
Ca
Sr
Ba
Melting point / ºC
1283
650
850
770
710
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
DECREASES down Group
©HOPTON
GROUP TRENDS
MELTING POINT
Be
Mg
Ca
Sr
Ba
Melting point / ºC
1283
650
850
770
710
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
DECREASES down Group
• each atom contributes two electrons to the delocalised cloud
• metallic bonding gets weaker due to increased size of ion
Larger ions mean
that the electron
cloud doesn’t bind
them as strongly
©HOPTON
GROUP TRENDS
MELTING POINT
Be
Mg
Ca
Sr
Ba
Melting point / ºC
1283
650
850
770
710
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
DECREASES down Group
• each atom contributes two electrons to the delocalised cloud
• metallic bonding gets weaker due to increased size of ion
Larger ions mean
that the electron
cloud doesn’t bind
them as strongly
• Group I metals have lower melting points than the equivalent Group II
metal because each metal only contributes one electron to the cloud
©HOPTON
GROUP TRENDS
MELTING POINT
©HOPTON
Be
Mg
Ca
Sr
Ba
Melting point / ºC
1283
650
850
770
710
Electronic config.
2,2
2,8,2
2,8,8,2
2,8,18,8,2
2,8,18,18,8,2
DECREASES down Group
• each atom contributes two electrons to the delocalised cloud
• metallic bonding gets weaker due to increased size of ion
Larger ions mean
that the electron
cloud doesn’t bind
them as strongly
• Group I metals have lower melting points than the equivalent Group II
metal because each metal only contributes one electron to the cloud
NOTE
Magnesium doesn’t fit the trend because crystalline
structure can also affect the melting point of a metal
FIRST IONISATION ENERGY
©HOPTON
FIRST IONISATION ENERGY
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
©HOPTON
FIRST IONISATION ENERGY
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
DECREASES down the Group
Despite the increasing nuclear charge the values decrease due to the
extra shielding provided by additional filled inner energy levels
©HOPTON
FIRST IONISATION ENERGY
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
DECREASES down the Group
Despite the increasing nuclear charge the values decrease due to the
extra shielding provided by additional filled inner energy levels
4+
BERYLLIUM
There are 4 protons pulling
on the outer shell electrons
1st I.E. = 899 kJ mol-1
©HOPTON
FIRST IONISATION ENERGY
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
DECREASES down the Group
Despite the increasing nuclear charge the values decrease due to the
extra shielding provided by additional filled inner energy levels
MAGNESIUM
4+
12+
BERYLLIUM
There are 4 protons pulling
on the outer shell electrons
There are now 12 protons
pulling on the outer shell
electrons. However, the extra
filled inner shell shields the
nucleus from the outer shell
electrons. The effective nuclear
charge is less and the
electrons are easier to remove.
1st I.E. = 738 kJ mol-1
1st I.E. = 899 kJ mol-1
©HOPTON
FIRST IONISATION ENERGY
©HOPTON
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
DECREASES down the Group
Despite the increasing nuclear charge the values decrease due to the
extra shielding provided by additional filled inner energy levels
MAGNESIUM
4+
12+
BERYLLIUM
There are 4 protons pulling
on the outer shell electrons
There are now 12 protons
pulling on the outer shell
electrons. However, the extra
filled inner shell shield the
nucleus from the outer shell
electrons. The effective nuclear
charge is less and the
electrons are easier to remove.
1st I.E. = 738 kJ mol-1
1st I.E. = 899 kJ mol-1
©HOPTON
SUCCESSIVE IONISATION ENERGIES
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
Successive Ionisation Energy values get larger
©HOPTON
SUCCESSIVE IONISATION ENERGIES
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
Successive Ionisation Energy values get larger
12+
1st I.E. = 738 kJ mol-1
©HOPTON
SUCCESSIVE IONISATION ENERGIES
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
Successive Ionisation Energy values get larger
12+
12+
1st I.E. = 738 kJ mol-1
2nd I.E. = 1500 kJ mol-1
There are now 12 protons and
only 11 electrons. The
increased ratio of protons to
electrons means that it is
harder to pull an electron out.
©HOPTON
SUCCESSIVE IONISATION ENERGIES
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
Successive Ionisation Energy values get larger
12+
12+
1st I.E. = 738 kJ mol-1
2nd I.E. = 1500 kJ mol-1
There are now 12 protons and
only 11 electrons. The
increased ratio of protons to
electrons means that it is
harder to pull an electron out.
©HOPTON
12+
3rd I.E. = 7733 kJ mol-1
There is a big jump in IE because
the electron being removed is
from a shell nearer the nucleus;
there is less shielding.
SUCCESSIVE IONISATION ENERGIES
Be
Mg
Ca
Sr
Ba
1st I.E. / kJ mol-1
899
738
590
550
500
2nd I.E. / kJ mol-1
1800
1500
1100
1100
1000
3rd I.E. / kJ mol-1
14849
7733
4912
4120
3390
Successive Ionisation Energy values get larger
12+
12+
1st I.E. = 738 kJ mol-1
2nd I.E. = 1500 kJ mol-1
There are now 12 protons and
only 11 electrons. The
increased ratio of protons to
electrons means that it is
harder to pull an electron out.
©HOPTON
12+
3rd I.E. = 7733 kJ mol-1
There is a big jump in IE because
the electron being removed is
from a shell nearer the nucleus;
there is less shielding.
CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
©HOPTON
CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
OXYGEN
react with increasing vigour down the group
Mg
burns readily with a bright white flame
0
0
+2 -2
2Mg(s) + O2(g) —> 2MgO(s)
Ba
burns readily with an apple-green flame
2Ba(s) + O2(g) —> 2BaO(s)
©HOPTON
CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
OXYGEN
react with increasing vigour down the group
Mg
burns readily with a bright white flame
0
0
+2 -2
2Mg(s) + O2(g) —> 2MgO(s)
Ba
burns readily with an apple-green flame
2Ba(s) + O2(g) —> 2BaO(s)
In both cases…
the metal is oxidised
Oxidation No. increases from 0 to +2
oxygen is reduced
Oxidation No. decreases from 0 to -2
O
+
Mg
2e¯
—>
—>
Mg2+
O2-
©HOPTON
+
2e¯
CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
©HOPTON
CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
WATER
react with increasing vigour down the group
©HOPTON
CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
WATER
react with increasing vigour down the group
Mg
reacts very slowly with cold water
Mg(s) + 2H2O(l) —> Mg(OH)2(aq) + H2(g)
but reacts quickly with steam
Mg(s) + H2O(g) —> MgO(s) +
©HOPTON
H2(g)
CHEMICAL PROPERTIES OF THE ELEMENTS
Reactivity increases down the Group due to the ease of cation formation
WATER
react with increasing vigour down the group
Mg
reacts very slowly with cold water
Mg(s) + 2H2O(l) —> Mg(OH)2(aq) + H2(g)
but reacts quickly with steam
Mg(s) + H2O(g) —> MgO(s) +
Ba
H2(g)
reacts vigorously with cold water
Ba(s) + 2H2O(l) —> Ba(OH)2(aq) + H2(g)
©HOPTON
OXIDES OF GROUP II
Bonding
• ionic solids; EXCEPT BeO which has covalent character
• BeO
CaO
BaO
(beryllium oxide)
(calcium oxide)
(barium oxide)
©HOPTON
MgO
SrO
(magnesium oxide)
(strontium oxide)
OXIDES OF GROUP II
Bonding
• ionic solids; EXCEPT BeO which has covalent character
• BeO
CaO
BaO
Reaction
with water
(beryllium oxide)
(calcium oxide)
(barium oxide)
MgO
SrO
(magnesium oxide)
(strontium oxide)
BeO
MgO
CaO
SrO
BaO
Reactivity with water
NONE
reacts
reacts
reacts
reacts
Solubility of hydroxide
M(OH)2 in water
Insoluble
Sparingly
soluble
Slightly
soluble
Quite
soluble
Very
soluble
pH of 0.1M solution
-
10.4
9-10
12.5
13.0
13.1
©HOPTON
OXIDES OF GROUP II
Bonding
• ionic solids; EXCEPT BeO which has covalent character
• BeO
CaO
BaO
Reaction
with water
(beryllium oxide)
(calcium oxide)
(barium oxide)
MgO
SrO
(magnesium oxide)
(strontium oxide)
BeO
MgO
CaO
SrO
BaO
Reactivity with water
NONE
reacts
reacts
reacts
reacts
Solubility of hydroxide
M(OH)2 in water
Insoluble
Sparingly
soluble
Slightly
soluble
Quite
soluble
Very
soluble
pH of 0.1M solution
-
10.4
9-10
12.5
13.0
13.1
React with water to produce the hydroxide (not Be)
e.g.
CaO(s) + H2O(l) —> Ca(OH)2(s)
©HOPTON
HYDROXIDES OF GROUP II
Properties
basic strength also increases down group
©HOPTON
HYDROXIDES OF GROUP II
Properties
•
•
•
•
•
basic strength also increases down group
this is because the solubility increases
the metal ions get larger so charge density decreases
get a lower attraction between the OH¯ ions and larger 2+ ions
the ions will split away from each other more easily
there will be a greater concentration of OH¯ ions in water
©HOPTON
HYDROXIDES OF GROUP II
Properties
•
•
•
•
•
basic strength also increases down group
this is because the solubility increases
the metal ions get larger so charge density decreases
get a lower attraction between the OH¯ ions and larger 2+ ions
the ions will split away from each other more easily
there will be a greater concentration of OH¯ ions in water
Be(OH)2
Mg(OH)2
Ca(OH)2
Sr(OH)2
Ba(OH)2
Solubility
in water
Insoluble
Sparingly
soluble
Slightly
soluble
Quite
soluble
Very
soluble
pH of 0.1M solution
-
10.4
9-10
12.5
13.0
13.1
©HOPTON
HYDROXIDES OF GROUP II
Properties
•
•
•
•
•
basic strength also increases down group
©HOPTON
this is because the solubility increases
the metal ions get larger so charge density decreases
get a lower attraction between the OH¯ ions and larger 2+ ions
the ions will split away from each other more easily
there will be a greater concentration of OH¯ ions in water
Be(OH)2
Mg(OH)2
Ca(OH)2
Sr(OH)2
Ba(OH)2
Solubility
in water
Insoluble
Sparingly
soluble
Slightly
soluble
Quite
soluble
Very
soluble
pH of 0.1M solution
-
10.4
9-10
12.5
13.0
13.1
Lower charge density of the larger Ca2+
ion means that it doesn’t hold onto the
OH¯ ions as strongly. More OH¯ get
released into the water. It is more soluble
and the solution has a larger pH.
HYDROXIDES OF GROUP II
Uses
Ca(OH)2
used in agriculture to neutralise acid soils
Ca(OH)2(s) + 2H+ (aq) —> Ca2+(aq) + 2H2O(l)
Mg(OH)2
used in toothpaste and indigestion tablets as an antacid
Mg(OH)2(s) + 2H+ (aq) —> Mg2+(aq) + 2H2O(l)
Both the above are weak alkalis and not as caustic as sodium hydroxide
©HOPTON
CARBONATES OF GROUP II
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CARBONATES OF GROUP II
Properties
• insoluble in water
Solubility g/100cm3 of water
MgCO3
CaCO3
SrCO3
BaCO3
1.5 x 10-4
1.3 x 10-5
7.4 x 10-6
9.1 x 10-6
©HOPTON
CARBONATES OF GROUP II
Properties
• insoluble in water
MgCO3
CaCO3
SrCO3
BaCO3
Solubility g/100cm3 of water
1.5 x 10-4
1.3 x 10-5
7.4 x 10-6
9.1 x 10-6
Decomposition temperature / ºC
400
980
1280
1360
• undergo thermal decomposition to oxide and carbon dioxide
e.g.
MgCO3(s) —> MgO(s) + CO2(g)
©HOPTON
CARBONATES OF GROUP II
Properties
• insoluble in water
MgCO3
CaCO3
SrCO3
BaCO3
Solubility g/100cm3 of water
1.5 x 10-4
1.3 x 10-5
7.4 x 10-6
9.1 x 10-6
Decomposition temperature / ºC
400
980
1280
1360
• undergo thermal decomposition to oxide and carbon dioxide
e.g.
MgCO3(s) —> MgO(s) + CO2(g)
• the ease of decomposition decreases down the group
©HOPTON
CARBONATES OF GROUP II
Properties
• insoluble in water
MgCO3
CaCO3
SrCO3
BaCO3
Solubility g/100cm3 of water
1.5 x 10-4
1.3 x 10-5
7.4 x 10-6
9.1 x 10-6
Decomposition temperature / ºC
400
980
1280
1360
• undergo thermal decomposition to oxide and carbon dioxide
e.g.
MgCO3(s) —> MgO(s) + CO2(g)
• the ease of decomposition decreases down the group
EASIER
HARDER
©HOPTON
CARBONATES OF GROUP II
Properties
• insoluble in water
MgCO3
CaCO3
SrCO3
BaCO3
Solubility g/100cm3 of water
1.5 x 10-4
1.3 x 10-5
7.4 x 10-6
9.1 x 10-6
Decomposition temperature / ºC
400
980
1280
1360
• undergo thermal decomposition to oxide and carbon dioxide
e.g.
MgCO3(s) —> MgO(s) + CO2(g)
• the ease of decomposition decreases down the group
EASIER
HARDER
One might think that the greater charge density of the smaller Mg2+ would mean that it
would hold onto the CO32- ion more and the ions would be more difficult to separate.
©HOPTON
CARBONATES OF GROUP II
©HOPTON
Properties
• insoluble in water
MgCO3
CaCO3
SrCO3
BaCO3
Solubility g/100cm3 of water
1.5 x 10-4
1.3 x 10-5
7.4 x 10-6
9.1 x 10-6
Decomposition temperature / ºC
400
980
1280
1360
• undergo thermal decomposition to oxide and carbon dioxide
e.g.
MgCO3(s) —> MgO(s) + CO2(g)
• the ease of decomposition decreases down the group
EASIER
HARDER
One might think that the greater charge density of the smaller Mg2+ would mean that it
would hold onto the CO32- ion more and the ions would be more difficult to separate.
The driving force must be the formation of the oxide. The smaller ion with its greater
charge density holds onto the O2- ion to make a more stable compound.
GROUP TRENDS
SULFATES
Solubility g/100cm3 of water
MgSO4
CaSO4
SrSO4
BaSO4
3.6 x 10-1
1.1 x 10-3
6.2 x 10-5
9.0 x 10-7
©HOPTON
GROUP TRENDS
SULFATES
Solubility g/100cm3 of water
MgSO4
CaSO4
SrSO4
BaSO4
3.6 x 10-1
1.1 x 10-3
6.2 x 10-5
9.0 x 10-7
SOLUBILITY DECREASES down the Group
• as the cation gets larger it has a lower charge density
• it becomes less attracted to the polar water molecules
©HOPTON
GROUP TRENDS
SULFATES
Solubility g/100cm3 of water
MgSO4
CaSO4
SrSO4
BaSO4
3.6 x 10-1
1.1 x 10-3
6.2 x 10-5
9.0 x 10-7
SOLUBILITY DECREASES down the Group
• as the cation gets larger it has a lower charge density
• it becomes less attracted to the polar water molecules
Greater charge density of Mg2+ ion
means that it is more attracted to water
so the ionic lattice breaks up more easily
©HOPTON
GROUP TRENDS
SULFATES
Solubility g/100cm3 of water
MgSO4
CaSO4
SrSO4
BaSO4
3.6 x 10-1
1.1 x 10-3
6.2 x 10-5
9.0 x 10-7
SOLUBILITY DECREASES down the Group
• as the cation gets larger it has a lower charge density
• it becomes less attracted to the polar water molecules
Greater charge density of Mg2+ ion
means that it is more attracted to water
so the ionic lattice breaks up more easily
Lower charge density of larger Ca2+ means that it
is less attracted to water so the ionic lattice
breaks up less easily – IT IS LESS SOLUBLE
©HOPTON
GROUP TRENDS
SULFATES
Solubility g/100cm3 of water
MgSO4
CaSO4
SrSO4
BaSO4
3.6 x 10-1
1.1 x 10-3
6.2 x 10-5
9.0 x 10-7
SOLUBILITY DECREASES down the Group
• as the cation gets larger it has a lower charge density
• it becomes less attracted to the polar water molecules
Greater charge density of Mg2+ ion
means that it is more attracted to water
so the ionic lattice breaks up more easily
USE
Lower charge density of larger Ca2+ means that it
is less attracted to water so the ionic lattice
breaks up less easily – IT IS LESS SOLUBLE
barium sulfate’s insolubility is used as a test for sulfates
©HOPTON
AN INTRODUCTION TO
GROUP II
Alkaline earths
THE END
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©2015 JONATHAN HOPTON & KNOCKHARDY PUBLISHING