Group 2

Question 1

Elements?

Answer 1

Dominated by +2 oxidation state chichis normally in the form of the M2+ ions, ionic group 2 compound stabilised by relatively low values of the first and second ionisation enthalpies of the metals and the high values of the lattice enthalpies for group 2 salts, beryllium is exceptional as it forms a number of covalent compounds

Question 2

Properties?

Answer 2

Elements are metallic solids at room temperature and pressure, higher melting points, boiling points and enthalpy changes of atomisation than their group 1 neighbours, this indicates stronger metallic bonding in the metal lattices due to the presence of two rather than one delocalised valence electrons per atom

Question 3

Reactivity?

Answer 3

Group2 metals much less reactive than group 1 metals and with the exception of barium do not need to be stored under oil. Most group 2 metals have a coating of oxide on their surfaces which makes them much less reactive than might be expected. They are sufficiently reactive however the free metals do not occur naturally

Question 4

Preparation of magnesium?

Answer 4

Obtained from sea water, in the first stage of the process aqueous calcium hydroxide solution is added to the sea water causing Mg(OH)2 to precipitate
Mg2+ + Ca(OH)2 –> Mg(OH)2 + Ca2+
The solid magnesium hydride is treated with HCl to produce MgCl2 and the metal is liberated from molten MgCl2 using electrolysis
Reduction at cathode Mg2+ + 2e- —> Mg
Oxidation at anode 2Cl- —> Cl2 + 2e-

Question 5

Preparation of calcium?

Answer 5

Calcium metal is obtained from minerals such as limestone CaCO3, firstly the carbonate is converted into the oxide by heating to 900-1100 degrees
CaCO3 –> CaO + CO2
Calcium metal is obtained form the reaction of CaO (quicklime) with aluminium
6CaO + 2Al –> 3CaO.Al2O3 + 3Ca

Question 6

Group 2 oxides?

Answer 6

Group 2 metals burn in air reacting with oxygen to give the oxides
2M + O2 —> 2MO
Magnesium burns with a bright white flame with some of the nitride Mg3N2 produced alongside MgO
The reaction becomes more exothermic down the group and barium can ignite with moist air

Question 7

Barium oxide?

Answer 7

Barium can also form the peroxide with excess oxygen

Ba + O2 —> BaO2

Question 8

Nature of the oxides?

Answer 8

They are very basic and with the exception of BeO dissolve in water to give alkaline solutions of the hydroxides, this reaction is very exothermic fro CaO
CaO + H2O –> Ca(OH)2
but less so for MgO which only reacts slowly

Question 9

Group 2 hydroxides?

Answer 9

With the exception of beryllium the group 2 metals react with water to give the hydroxide liberating hydrogen gas in the process
Ba + 2H2O –> Ba(OH)2 + H2

Question 10

Reactivity of hydroxides?

Answer 10

As with group 1 metals the reaction become more vigorous as the group is descended. Magnesium requires hot water or steam to react whereas calcium reacts with cold water. The hydroxides are generally basic in solution

Question 11

Heating group 2 hydroxides?

Answer 11

The group 2 hydroxides decompose on heating to give oxides

M(OH)2 –> MO + H2O

Question 12

Nature of beryllium hydroxide?

Answer 12

Be(OH)2 is amphoteric, as well as reacting with acids in the manner expected for a hydroxide to dissolves in excess alkali to form a tetrahedral complex anion
Be(OH)2 + 2H3O+ –> Be2+ + 4H2O
Be(OH)2 + 2OH- –> [Be(OH)4]2-

Question 13

Group 2 halides?

Answer 13

The group 2 metals react with the halogens to give the halide. Eg magnesium burns in chlorine gas to form magnesium chloride
Mg + Cl2 —> MgCl2
Most of the group 2 halides are ionic but BeX2 compounds have polymeric structures with covalent bonds and bridging halides

Question 14

BeCl2 vs BeH2?

Answer 14

Both BeCl2 and BeH2 have structures containing bridging atoms but they differ with respect to the bonding, BeH2 is electron deficient and the Be-H-Be bridges involve 3 centre 2 electron bonding, BeCl2 is not electron deficient because the bridging chloride donates three electrons

Question 15

Why does MgCl not exist?

Answer 15

It is stable with respect to its elements but is unstable with respect to disproportionation

Question 16

Group 2 ethynides?

Answer 16

Group 2 metals form binary compounds with carbon, although sometimes called carbides these groups are really ethynides since they contain the C22- anion. These compound react with water to give ethyne
CaC2 + 2H2O —> Ca(OH)2 + C2H2
Ethyne is burned to produce light

Question 17

Group 2 Nitrides?

Answer 17

The group 2 metals all react with nitrogen on heating to give nitrides
3Mg + N2 –> Mg3N2
The nitrides decompose in water to form the metal hydroxide and ammonia
Mg3N2 + 6H2O –> 3Mg(OH)2 + 2NH3

Question 18

Group 2 compounds with oxyanions?

Answer 18

Group 2 carbonates are not very soluble in water and decompose on heating to the oxides and CO2
MCO3 –> MO + CO2
The decomposition temperature in degrees C of the carbonates increase as the group is descended

Question 19

What does decomposition depend on?

Answer 19

The decomposition temperature is related to the temperature at which a reaction becomes spontaneous that is when Gibbs energy becomes negative, for decomposition of a carbonate the entropy change is positive and largely independent of M this means that any changes in Gibbs energy are caused by changes in enthalpy. The enthalpy change is related to difference between the lattice enthalpies of MCO3 and MO

Question 20

Group 2 nitrates and sulfates?

Answer 20

These also decompose on heating to give the oxides
2M(NO3)2 –> 2MO + 4NO + 3O2
MSO4 –> MO + SO3

Question 21

Building materials?

Answer 21

Limestone CaCo3 is a very common minerals that is used directly as a building material, when its heated to 900 - 1100 degrees C it loses CO2 forming calcium oxide known as quicklime
CaCO3 –> CaO + CO2

Question 22

What makes up cement?

Answer 22

Tricalcium silicates (Ca3SiO5) is a typical component of comment. It hardens by absorbing water to form Ca3SiO5.2H2O long needle like crystals of this hydrated calcium silicate are formed which bon the cement particles together. Adding gypsum (CaSO4.2H2O) slows the solidification this mixture is called Portland cement
Concretes typically contain a cement together with an aggregate concrete is the most common building material in the world by mass

Question 23

Solubility?

Answer 23

The trend in solubility of group 2 compounds are similar to those for group 1 but as the lattice enthalpies are higher the compounds are generally less soluble. Compounds with large cations and large anions tend to be insoluble eg MgSO4 is very soluble, CaSO4 much less so and BaSO4 very poorly soluble.

Question 24

Poor solubility of barium sulfate used for…?

Answer 24

Test for sulfate ions as their presence gives a dense white precipitate
Ba2+ SO42- —> BaSO4

Question 25

Group 2 coordination chemistry?

Answer 25

M2+ ions are smaller than group one M+ ions and have twice the charge as a result the higher charge densities on the group 2 cations, group 2 coordination chemistry is more extensive than that of group 1, the increasing size of the M2+ ions down the group influences the coordination number which is the number of atoms jay are coordinate to the metal ion. Be2+ typically has a coordination number of 4, Mg2+ has a coordination number of 6 whereas the coordination numbers of Ca2+ Sr2+ and Ba2+ can be higher

Question 26

Complex formed with aqua ligands?

Answer 26

The size of the ions dictates the nature of the complex formed with aqua ligands, the high charge density of Be2+ leads to hydrolysis so aqueous solutions of Be2+ are acidic
[Be(H2O)4]2+ + H2O —> [Be(H2O)3(OH)]+ + H3O+
In contrast solutions of the other group 2 ions are generally neutral, in the presence of ion such as acetate hydrolysis eventually leads to compounds such as [Be4O(O2CMe)6]

Question 27

Chlorophylls?

Answer 27

Pigment molecules that give leaves their green colour the absorption of light by chlorophylls is vitally important for light as it provides the energy for photosynthesis chlorophylls are magnesium coordination complexes that contain Mg2+ ions lying in the centre of the chlorin ring

Question 28

Why is Li an exceptional element for group 1?

Answer 28

Lithium is the only group 1 metal that reacts with N2 to give a nitride
On burning in air lithium gives the oxide Li2O heres the other group 1 metals give peroxides of superoxides
Li2CO3 and LiOH both decompose on heating to give Li2O whereas the other group 1 carbonates and hydroxides are generally stable to heating
LiNO3 decomposes on heating to form Li2O whereas the other group 1 nitrates decompose to the nitrites MNO2
The carbonate, fluoride and hydroxides are all far less soluble in water for lithium than hey are for other group 1 metals

Question 29

Why is the anomalous behaviour of lithium expected?

Answer 29

It fits the expected pattern of changes that occur with a decrease in the size of the cation and consequent increase in the charge density of the ion. The small cation size of Li+ leads directly to increases in lattice enthalpies and Gibbs energy changes of hydration which are the key factors in determining the thermal stabilities and solubilities of the group 1 compounds

Question 30

Why is beryllium in group 2 anomalous?

Answer 30

Beryllium metal does not react with water in contrast to the other group 2 metals
Beryllium is amphoteric it reacts with acids as do other group 2 metals but it also reacts with alkalis to form [Be(OH)4]2-
Beryllium compounds have a much greater covalent character than those of the other group 2 metals. For example BeCl2 has a polymeric covalently bonded structure whereas the other chlorides are ionic solids
Be2+ is acidic in aqueous solution unlike the other group 2 cations which are neutral
BeO does not react with water in contrast to the other group 2 oxides

Question 31

Why is the anomalous behaviour of beryllium expected?

Answer 31

The high charge density of Be2+ makes the lattice enthalpies of its compounds very high. This high charge density means that the free Be2+ ion is strongly polarising, it draws the electrons of neighbouring ions towards itself which gives these bonds a much higher degree of covalent character than equivalent bonds of other group 2 cations
The increase in electronegativity of the group 2 element as the group is descended also contributes to the decrease in covalent character

Question 32

Organometallic compounds?

Answer 32

Compounds containing one or more bonds between a metal and carbons are known as organometallic. Organometallic compounds are known for all of the group 1 and group2 metals but those of lithium and magnesium are the most common and most important synthetically, since ions of these metals have high charge densities there is a large degree of covalent character in the bonding of the organometallic compounds, the carbon atoms are polarised so the compounds are nucleophilic and used extensively in organic synthesis

Question 33

Organolithium compounds?

Answer 33

RLi are prepared by reacting lithium with a halogenoalkane
RI + 2Li –> RLi + Lil
Organolithium compounds are both air and water sensitive so must be prepared under an inert atmosphere and use a solvent that has been rigorously dried. Two of the most important organolithium compounds are methylithium and n-butylithium
Although normally expressed by the simple formula RLi organolithium compounds exist as oligomers both in solution and the solid state, eg solid methyl lithium contain [MeLi]4 units the bonding here is manly ionic though with significant covalent character

Question 34

Organomagnesium compounds?

Answer 34

Dimethylmagnesium MgM2 is a polymeric solid containing bridging methyl groups, contains a 3 centre 2 electron bond. Organomagnesium compounds with the general formula RMgX where X is a halide are known as Grignard regents, Grignard regents formed from the reactions of a halogenoalkane with magnesium using ethoxyethane or tetrahydrofuran as the solvent for example
EtBr + Mg —> EtMgBr

Question 35

Grignard reagents?

Answer 35

Air and moisture sensitive must be prepared under inert atmosphere using a dry solvent. Magnesium metal normally has a coating of insoluble oxide on its surface and this acts as a kinetic barrier to reaction, this problem is overcome by adding a trace amount of iodine to activate the magnesium. Although represented by the simple formula RMgX the actuations structures of Grignard reagents are more complex, int he solid magnesium centres are normally tetrahedral with solvent molecules occurring some of the coordination sites

Question 36

Compounds present in solution?

Answer 36

Depend on the number of factors including the nature of the solvent and the concentration, the following reactions called the Schlenk redistribution reaction occurs
2MgMgBr —> MgMe2 + MgBr2

Question 37

Diagonal relationships between magnesium and lithium ?

Answer 37

Lithium and magnesium have many properties in common, both metals react with N2 to give nitrides, both metals burn in air to form the normal oxides and not peroxides, the carbonates and nitrates both decompose to the oxides on heating, both ions are more heavily hydrated than others in their respective groups, both forms an extensive number of organometallic compounds that have a large degree of covalent character in the M-C bonds, these similarities form the basis of a diagonal relationships between two elements

Question 38

Why do diagonal relationships occur?

Answer 38

Because of the different ways in which many atomic properties vary down groups and across periods of the periodic table, eg atomic and ionic radii increase don group 1 due to the increasing size of the outermost occupied atomic orbital but decrease across the second period from lithium to neon due to infect shielding and the consequent increase in effect nuclear charge, moving diagonally on element down and one across to the right these two trends largely cancel out