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© HOPTON. AN INTRODUCTION TO GROUP II Alkaline earths. 2008 SPECIFICATIONS. KNOCKHARDY PUBLISHING. © HOPTON. KNOCKHARDY PUBLISHING. GROUP II (Alkaline Earths). INTRODUCTION

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slide1

©HOPTON

AN INTRODUCTION TO

GROUP II

Alkaline earths

2008 SPECIFICATIONS

KNOCKHARDY PUBLISHING

slide2

©HOPTON

KNOCKHARDY PUBLISHING

GROUP II (Alkaline Earths)

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected topics at AS and A2 level Chemistry. It is based on the requirements of the AQA and OCR specifications but is suitable for other examination boards.

Individual students may use the material at home for revision purposes or it may be used for classroom teaching with an interactive white board.

Accompanying notes on this, and the full range of AS and A2 topics, are available from the KNOCKHARDY SCIENCE WEBSITE at...

www.knockhardy.org.uk/sci.htm

Navigation is achieved by...

either clicking on the grey arrows at the foot of each page

or using the left and right arrow keys on the keyboard

slide3

©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
  • Sulphates
slide4

©HOPTON

GROUP PROPERTIES

GENERAL• metals

• all have the electronic configuration ... ns2

TRENDS• melting point

• electronic configuration

• electronegativity

• atomic size

• ionic size

slide5

©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.

slide6

©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

Li

1s22s1

Na

1s2 2s2 2p63s1

K

1s2 2s2 2p63s23p64s1

Rb

… 5s1

Cs

… 6s1

Fr

slide7

©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

1s22s1

1s22s2

Na

Mg

1s2 2s2 2p63s1

1s2 2s2 2p63s2

K

Ca

1s2 2s2 2p63s23p64s1

1s2 2s2 2p63s23p64s2

Rb

Sr

… 5s1

… 5s2

Cs

Ba

… 6s1

… 6s2

Fr

Rn

slide8

©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

1s22s1

1s22s2

Na

Mg

1s2 2s2 2p63s1

1s2 2s2 2p63s2

K

Ca

1s2 2s2 2p63s23p64s1

1s2 2s2 2p63s23p64s2

Rb

Sr

… 5s1

… 5s2

Cs

Ba

… 6s1

… 6s2

Fr

Rn

Francium and radium are both short-lived radioactive elements

slide9

©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

1s22s2

…3s2

… 4s2

… 5s2

… 6s2

slide10

©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

1s22s2

…3s2

… 4s2

… 5s2

… 6s2

As the nuclear charge increases, the electrons go into shells further from the nucleus.

slide11

©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

1s22s2

…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 Ionic radius

Ionisation energy Melting point

Chemical reactivity

slide12

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

slide13

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

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 2p63s2

1s2 2s2 2p63s23p64s2

slide14

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

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 2p63s2

1s2 2s2 2p63s23p64s2

• atoms of Group II are smaller than

the equivalent Group I atom

the extra proton exerts a greater

attraction on the electrons

11 protons

1s2 2s2 2p63s1

12 protons

1s2 2s2 2p63s2

slide15

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

slide16

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

IONIC RADIUS INCREASES down Group

• ions are smaller than atoms – on removing the outer shell

electrons, the remaining electrons are now in fewer shells

slide17

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

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 2p63s2

1s2 2s2 2p6

slide18

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

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 2p63s2

1s2 2s2 2p6

1s2 2s2 2p63s23p64s2

1s2 2s2 2p63s23p6

slide19

©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

slide20

©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

slide21

©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

slide22

©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

• Group I metals have lower melting points than the equivalent Group II

metal because each metal only contributes one electron to the cloud

Larger ions mean that the electron cloud doesn’t bind them as strongly

slide23

©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

• 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

Larger ions mean that the electron cloud doesn’t bind them as strongly

slide24

©HOPTON

FIRST IONISATION ENERGY

slide25

©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

slide26

©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

DECREASESdown the Group

Despite the increasing nuclear charge the values decrease due to the

extra shielding provided by additional filled inner energy levels

slide27

4+

©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

DECREASESdown the Group

Despite the increasing nuclear charge the values decrease due to the

extra shielding provided by additional filled inner energy levels

BERYLLIUM

There are 4 protons pulling on the outer shell electrons

1st I.E. = 899 kJ mol-1

slide28

4+

12+

©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

DECREASESdown the Group

Despite the increasing nuclear charge the values decrease due to the

extra shielding provided by additional filled inner energy levels

MAGNESIUM

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

BERYLLIUM

There are 4 protons pulling on the outer shell electrons

1st I.E. = 899 kJ mol-1

slide29

4+

12+

©HOPTON

©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

DECREASESdown the Group

Despite the increasing nuclear charge the values decrease due to the

extra shielding provided by additional filled inner energy levels

MAGNESIUM

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

BERYLLIUM

There are 4 protons pulling on the outer shell electrons

1st I.E. = 899 kJ mol-1

slide30

©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

slide31

©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

slide32

©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.

slide33

©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+

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.

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.

slide34

©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+

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.

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.

slide35

©HOPTON

CHEMICAL PROPERTIES OF THE ELEMENTS

Reactivity increases down the Group due to the ease of cation formation

slide36

©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)

slide37

©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

Mg —> Mg2+ + 2e¯

O + 2e¯ —> O2-

slide38

©HOPTON

CHEMICAL PROPERTIES OF THE ELEMENTS

Reactivity increases down the Group due to the ease of cation formation

slide39

©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

slide40

©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) + H2(g)

slide41

©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) + H2(g)

Ba reacts vigorously with cold water

Ba(s) + 2H2O(l) —> Ba(OH)2(aq) + H2(g)

slide42

©HOPTON

OXIDES OF GROUP II

Bonding • ionic solids; EXCEPT BeO which has covalent character

• BeO (beryllium oxide) MgO (magnesium oxide)

CaO (calcium oxide) SrO (strontium oxide)

BaO (barium oxide)

slide43

Be

Mg

Ca

Sr

Ba

Reactivity with water

NONE

reacts

reacts

reacts

reacts

Solubility of hydroxide g/100cm3 of water

Insoluble

Sparingly

soluble

Slightly

soluble

Quite

soluble

Very

soluble

pH of solution

-

9-10

©HOPTON

OXIDES OF GROUP II

Bonding • ionic solids; EXCEPT BeO which has covalent character

• BeO (beryllium oxide) MgO (magnesium oxide)

CaO (calcium oxide) SrO (strontium oxide)

BaO (barium oxide)

Reaction

with water

slide44

Be

Mg

Ca

Sr

Ba

Reactivity with water

NONE

reacts

reacts

reacts

reacts

Solubility of hydroxide g/100cm3 of water

Insoluble

Sparingly

soluble

Slightly

soluble

Quite

soluble

Very

soluble

pH of solution

-

9-10

©HOPTON

OXIDES OF GROUP II

Bonding • ionic solids; EXCEPT BeO which has covalent character

• BeO (beryllium oxide) MgO (magnesium oxide)

CaO (calcium oxide) SrO (strontium oxide)

BaO (barium oxide)

Reaction

with water

React with water to produce the hydroxide (not Be)

e.g. CaO(s) + H2O(l) —> Ca(OH)2(s)

slide45

©HOPTON

HYDROXIDES OF GROUP II

Properties basic strength also increases down group

slide46

©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

slide47

Be

Mg

Ca

Sr

Ba

Reactivity with water

NONE

reacts

reacts

reacts

reacts

Solubility of hydroxide in water

Insoluble

Sparingly

soluble

Slightly

soluble

Quite

soluble

Very

soluble

pH of solution

-

9-10

©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

slide48

Be

Mg

Ca

Sr

Ba

Reactivity with water

NONE

reacts

reacts

reacts

reacts

Solubility of hydroxide in water

Insoluble

Sparingly

soluble

Slightly

soluble

Quite

soluble

Very

soluble

pH of solution

-

9-10

©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

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.

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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

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CARBONATES OF GROUP II

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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

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CARBONATES OF GROUP II

Properties

• insoluble in water

• undergo thermal decomposition to oxide and carbon dioxide

e.g. MgCO3(s) —> MgO(s) + CO2(g)

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

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CARBONATES OF GROUP II

Properties

• insoluble in water

• undergo thermal decomposition to oxide and carbon dioxide

e.g. MgCO3(s) —> MgO(s) + CO2(g)

• the ease of decomposition decreases down the group

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

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CARBONATES OF GROUP II

Properties

• insoluble in water

• undergo thermal decomposition to oxide and carbon dioxide

e.g. MgCO3(s) —> MgO(s) + CO2(g)

• the ease of decomposition decreases down the group

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

EASIER

HARDER

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©HOPTON

CARBONATES OF GROUP II

Properties

• insoluble in water

• undergo thermal decomposition to oxide and carbon dioxide

e.g. MgCO3(s) —> MgO(s) + CO2(g)

• the ease of decomposition decreases down the group

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

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.

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CARBONATES OF GROUP II

Properties

• insoluble in water

• undergo thermal decomposition to oxide and carbon dioxide

e.g. MgCO3(s) —> MgO(s) + CO2(g)

• the ease of decomposition decreases down the group

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

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.

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MgSO4

CaSO4

SrSO4

BaSO4

Solubility g/100cm3 of water

3.6 x 10-1

1.1 x 10-3

6.2 x 10-5

9.0 x 10-7

©HOPTON

GROUP TRENDS

SULPHATES

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MgSO4

CaSO4

SrSO4

BaSO4

Solubility g/100cm3 of water

3.6 x 10-1

1.1 x 10-3

6.2 x 10-5

9.0 x 10-7

©HOPTON

GROUP TRENDS

SULPHATES

SOLUBILITY DECREASESdown the Group

• as the cation gets larger it has a lower charge density

• it becomes less attracted to the polar water molecules

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MgSO4

CaSO4

SrSO4

BaSO4

Solubility g/100cm3 of water

3.6 x 10-1

1.1 x 10-3

6.2 x 10-5

9.0 x 10-7

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

SULPHATES

SOLUBILITY DECREASESdown the Group

• as the cation gets larger it has a lower charge density

• it becomes less attracted to the polar water molecules

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MgSO4

CaSO4

SrSO4

BaSO4

Solubility g/100cm3 of water

3.6 x 10-1

1.1 x 10-3

6.2 x 10-5

9.0 x 10-7

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

SULPHATES

SOLUBILITY DECREASESdown the Group

• as the cation gets larger it has a lower charge density

• it becomes less attracted to the polar water molecules

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MgSO4

CaSO4

SrSO4

BaSO4

Solubility g/100cm3 of water

3.6 x 10-1

1.1 x 10-3

6.2 x 10-5

9.0 x 10-7

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

SULPHATES

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

USE barium sulphate’s insolubility is used as a test for sulphates

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AN INTRODUCTION TO

GROUP II

Alkaline earths

THE END

©2011 JONATHAN HOPTON & KNOCKHARDY PUBLISHING