2010 group ii metals_students

8/6/2019 2010 GROUP II Metals_students

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NYJC H2 Chemistry 9647 Group II Metals JC1/2010

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GROUP II Metals ( Mg, Ca, Sr, Ba )

Group II metals (also called alkaline earth metals) are s-block metals with a characteristicoutershell configuration, n s 2, since the only electrons in their outermost shell occupy a s orbital.

E l e m e n

t

P r o

t o n N o . Electronic Configuration

A t o m i c

r a d i u s

I o n i c

r a d i u s 1

st

IE / kJmol -1

E / V

Be 4 1s 2 2s 2 0.112 0.031 900 -1.85Mg 12 1s 2 2s 2 2p 6 3s 2 0.160 0.065 736 -2.38Ca 20 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 0.197 0.199 590 -2.87Sr 38 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 5s 2 0.215 0.113 548 -2.89Ba 56 [Xe] 6s 2 0.217 0.135 502 -2.90Ra 88 [Rn] 7s 2 0.220 0.140 - -

TREND DOWN THE GROUP More

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I PHYSICAL PROPERTIES of the Metals

ATOMIC and IONIC RADII increase down the Group from Mg to BaEach succeeding element has one more __________ of electrons .Hence, the outer electrons are further away from the nucleus.

MELTING / BOILING POINTS(i) are generally highThe metals have structure .

Strong electrostatic forces of attraction occurs between the. A lot of energy is required to break these bonds,

resulting in high melting points .

(ii) decrease down the Group from Mg to Ba From Mg to Ba, the atoms form M 2+ ions and ionic radii .Hence there is attractive force between the electron cloud and the cations.Strength of metallic bond .

(ii) Gp I < Gp II < TMGroup II metals have melting points which are than that of Group I metals(< 200 o C) and lower than for transition elements ( > 1000 o C)Reason: Group I metals contribute one electron per atom to the delocalized electron cloud, whiletransition elements can contribute variable electrons to the delocalized electron cloud.

GP II metals are good CONDUCTORS OF ELECTRICITYThe metals have giant metallic structures. The cations are surrounded by a sea of delocalizedmobile electrons. Each element contributes 2 electrons per atom to the sea of delocalizedelectrons which act as to carry the current.


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1 st IE decreases down the Group from Mg to BaIncrease in _______ outweigh the increase in

__________ . Hence, net attraction of outermost electron .energy required to remove the electron.

II CHEMICAL PROPERTIES of the Metals and some compounds

Group II metals tend to lose two electrons readily to form M 2+ ions, i.e. they are reducing agents.

Going down the group from Mg to Ba, IE decreases i.e. electrons are more easily lost.The REACTIVITY of the metal increases .

Reducing strength increases down the Group.

This can be deduced from the E values , which get more negative down the Group oxidation of metal becomes more favourable i.e. reducing strength increases.

2 2 M e M + + E is negative.

REACTIONS of the ELEMENTS with O 2 and H 2OReaction with Oxygen Reaction with Water All group II metals burn with a

bright flame to form basicoxides (except Be).

All group II metals (except Be) react with water toproduce oxide/hydroxides and H 2 (g), with increasingvigour down the Group.

Be 2 Be (s) + O 2 (g) 2 BeO (s) No reaction with either cold H 2O or steam

Mg2 Mg(s) + O 2 (g) 2 MgO (s)

Reacts very slowly with cold water but vigorously withsteam to form magnesium oxide.Mg (s) + H 2O ( l ) MgO (s) + H 2 (g)[ MgO is slightly soluble in H 2O to give an alkaline soln,

MgO (s) + H 2O ( l ) Mg(OH) 2 (aq) pH ~9 ]

CaSr Ba

2 Ca (s) + O 2 (g) 2 CaO (s)2 Sr (s) + O 2 (g) 2 SrO (s)2 Ba (s) + O 2 (g) 2 BaO (s)Burns with increasing vigour down the Group.

React with cold H 2O with increasing vigour downthe Group.Ca (s) + 2 H 2O ( l ) Ca(OH) 2 (aq) + H 2(g) pH ~ 10-13Sr (s) + 2 H 2O ( l ) Sr(OH) 2 (aq) + H 2(g) pH ~ 10-13Ba (s) + 2 H 2O ( l ) Ba(OH) 2 (aq) + H 2(g) pH ~ 10-13

REACTIONS of the OXIDES with H 2O

Reaction with water Nature of Oxide Dissolves to form hydroxides

Increasing solubility down theGroup.

BeO is amphoteric

Remaining oxides are basic (i.e. react with acids andCO 2 etc.)

BeO Insoluble in water:High lattice energy makesdissolution difficult.

Amphoteric* reacts with acids and alkalisBeO (s)+ 2 HC l (aq) BeC l 2 (aq) + H 2O ( l )BeO (s) + 2 NaOH (aq) + H 2O ( l ) Na 2Be(OH) 4 (aq)

MgO Dissolve only very slightly inwater to form Mg(OH) 2 (pH ~ 9)MgO(s) + H 2O( l ) Mg(OH) 2 (aq)

Basic reacts with acids MgO (s) + 2 HC l (aq) MgC l 2 (aq) + H 2O ( l )


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CaOSrOBaO

These oxides reactexothermically with water to formsoluble hydroxides (pH ~ 10-13).CaO(s) + H 2O ( l ) Ca(OH) 2 (aq)SrO(s) + H 2O ( l ) Sr(OH) 2 (aq)BaO(s) + H 2O ( l ) Ba(OH) 2 (aq)

Basic reacts with acids with increasing vigour downthe Group

CaO (s) + 2 HC l (aq) CaC l 2 (aq) + H 2O ( l )SrO (s) + 2 HC l (aq) SrC l 2 (aq) + H 2O ( l )BaO (s) + 2 HC l (aq) BaC l 2 (aq) + H 2O ( l )

Thermal Stability of the Group II nitrates

General equation: 2 M(NO 3 ) 2 (s) 2 MO (s) + 4 NO 2 (g) + O 2 (g) Brown

T hermal stability increases down the group from Mg(NO 3 ) 2 to Ba(NO 3 ) 2 .

Thermal Decomposition of Gp II carbonates and hydroxides follow the same trend andexplanation as for the NO 3-.Equation: MgCO 3 (s) MgO (s) + CO 2 (g) Mg(OH) 2 (s) MgO (s) + H 2O (l)

Explanation: Thermal stability can be explained in terms of the charge density of the cation and the polarisabilityof the large anion.

The greater the charge density, the greater the polarizing power of the cation. The larger the anion, the more easily the electron cloud is polarized (distorted)

This causes the compound to be less stable i.e. it decomposes at a lower temp.

Down the Group,Radius of the cation increases but ionic charge remains the same (+2)The charge density of the cation decreases, and the polarizing power also decreases The electron cloud of the NO 3

- ion becomes less polarized (distorted)i.e. the thermal decomposition becomes more difficult down the Group.e.g. Ba(NO 3 ) 2 decomposes at a higher temp than Mg(NO 3 ) 2

Summary: Thermal decomposition decreases down Group II due to decreasingpolarization of the anion (e.g. NO 3-, CO 3-) by the cation (NOT ATOMS) of increasingsize.

Food for thought : How would you expect the thermal stability of the Group I nitrates to comparewith that of the Group II nitrates? Explain.

The electron cloud of the anion is pulled towards the cation.

This oxygen atom is well on the way to becoming an oxide ion.

This end of the ion is on its way to breaking away and

becoming carbon dioxide. M 2+


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NYJC H2 Chemistry 9647 Group II Metals (SUMMARY) JC1/2010

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I. Physical Properties

Atomic /Ionic radius

Trend:

Explanation:

Melting Point

Trend:

Explanation:

Trend:

Explanation:

Trend:

Explanation:

Electrical

Conductivity

Trend:

Explanation:

1 st IE

Trend:

Explanation:


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NYJC H2 Chemistry 9647 Group II Metals (SUMMARY) JC1/2010

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II. Chemical Properties

Reducing

Strength

Trend:

Explanation:

Relationship with E:

Mg Ca, Sr, Ba

Trend:

Reactionwith O 2

General Equation:

Trend

Equation

Reactionwith water Reactivity

Nature of oxide

Equation

[pH ______ except Mg(OH) 2: pH 9]Reaction of oxide withwater

Reaction With acid :

With CO 2:

ThermalStability of

Nitrates

Trend:

Explanation:


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Group II Tutorial*Reference to A-Level Chemistry TYS Nov 2000 2009 Edition

MCQ Structured and Free Response Section A Pgs 87-88

N 2001/III/13

N 2005/I/15

N 2006/I/15, 16, 17

N 2008/I/15

Section B Pgs 88-89

N 2003/I/34

N 2004/I/34

N 2005/I/35

N 2007/I/34

Section C Pg 89N 2004/II/4

Section D Pg 90

N 2005/III/5 Or N 2007/III/3(c) N 2008/III/5(b)

2010 group ii metals_students

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