Lecture 12. S-elements of the ІІ А group. Alkaline earth metals.

р-Elements of the ІІІА group. Boron and Aluminium

Be

Mg

Ca

Sr

Ba

Ra

PhD Halina Falfushynska

Members of the s-Block Elements

Li Be

Na

K

Rb

Cs

Fr

Mg

Ca

Sr

Ra

Ba

IA IIA

IA Alkali metals

IIA Alkaline Earth metals

• High tendency to lose e- to form positive ions

• Metallic character increases down both groups

• Low nuclear attraction for outer electrons

• Highly electropositive• Small electronegativity

Group II

Be 1.5

Mg 1.2

Ca 1.0

Sr 1.0

Ba 0.9

Ra 0.9

Occurrence and Extraction These elements are widely distributed in rock structures. The main minerals in which magnesium is found are carnellite, magnesite and dolomite. Calcium is found in chalk, limestone, gypsum and anhydrite.Magnesium is the eighth most abundant element in the Earth’s crust, and calcium is the fifth. It is extracted from sea-water by the addition of calcium hydroxide, which precipitates out the less soluble magnesium hydroxide. This hydroxide is then converted to the chloride, which is electrolysed in a Downs cell to extract magnesium metal. MgCl2 → Ca + Cl2; cathode: Mg2+ + 2e → Mg0 ; anode: 2Cl- – 2e → Cl0

2

Beryllium: BeF2 + Mg MgF2 + Be.

Beryllium's low aqueous solubility means it is rarely available to biological systems, is usually highly toxic.

Magnesium and calcium are ubiquitous and essential to all known living organisms. They are involved in Mg/Ca ion pumps, magnesium functioning as the active center in some enzymes, and calcium salts taking a structural role (e.g. bones).

Strontium and barium have a lower availability in the biosphere. Strontium plays an important role in marine aquatic life, especially hard corals. They use strontium to build their exoskeleton. These elements have some uses in medicine, for example "barium meals" in radiographic imaging, whilst strontium compounds are employed in some toothpastes.

Biological occurrence of elements of group IIA

Flame test

HCl(aq) sample

Mg brilliant white Ca brick redSr blood redBa apple green

Basic oxides, hydroxidesOxide Hydroxides

BeO Be(OH)2

MgO Mg(OH)2

CaO Ca(OH)2

SrO Sr(OH)2

BaO, Ba2O2 Ba(OH)2

Reaction with water:

Oxide: O2- + H2O 2OH-

Predominantly ionic with fixed oxidation state

Group I: Most electropositive metals. Low first I.E. and extremely high second I.E.

Group II: Electropositive metals. Low first and second I.E. but very high third I.E.. Have a fixed oxidation state of +2. Be and Mg compounds possess some degree of covalent character.

Compare to

Atomic radii (nm)

Li 0.152 Be 0.112

Na 0.186 Mg 0.160

K 0.231 Ca 0.197

Rb 0.244 Sr 0.215

Cs 0.262 Ba 0.217

Fr 0.270 Ra 0.220

Li

Fr

Be

Ra

Ionization Enthalpy

LiNa

KRb

Cs

1st I.E.

300

400

500

600

500

1000

1500

2000

Be

CaBa

Be+

Ca+

Ba+

1st IE

2nd IE

Variation in Melting Points

10 20 30 40 50 60

250

500

750

1000

1250Be

Mg

CaSr

Ba

LiNa K Rb

Cs

Reactions with oxygen

Normal Oxide

Peroxide Superoxide

Formed by Be, Mg, Ca, Sr

Ba None

S-block elements reacts readily with oxygen. they have to be stored under liquid paraffin to prevent contact with the atmosphere.

S-block elements are strong reducing agents.Their reducing power increases down both groups.(As the atomic size increases, it becomes easier toremove the outermost electron)

Reaction with waterM(s) M+(aq) + e-

H2O(l) + e- OH-(aq) + ½ H2(g)

Reaction with hydrogen

All the II A elements except Be react directly withhydrogen.Ca(s) + H2(g) CaH2(s)The reactivity increases down the group.

Only BeH2 and MgH2 are covalent, others are ionic.

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

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

Reaction with chlorine

All the s-block metals react directly with chlorine to produce chloride.Ba (s) + Cl2(g) BaCl2 (s)BeCl2 is essentially covalentThe lower members in group II form essentially ionicchlorides, with Mg having intermediate properties.

Reactions of chlorides

Group II chlorides show some degree of covalent character. BeCl2 hydrolysis to

form Be(OH)2(s) and HCl(aq).

MgCl2 is intermediate, it dissolves and

hydrolysis slightly. Other group II chlorides

just dissolve without hydrolysis.

Reaction with nitrogen

These reactions can not occur without extreme circumstances. A compound may be created via really high temperatures. 3Mg(s) + N2(g) -> Mg3N2(s)

Reactions with acid

All the IIA metals react directly with acid to produce salt and hydrogen.Mg (s) + H2SO4(l)→ MgSO4 (s) + H2 (g)Mg + 2HCl -----> MgCl2 + H2

Reactions with alkali. Only for Beryllium!!! Be (s) + 2NaOH(l) + 2H2O = Na2[Be(OH)4] (s) + H2

Reactions of oxides and hydroxides1. All group I oxides reacts with water to form hydroxides

Oxide: O2- + H2O 2OH-

Peroxide: O22- + 2H2O H2O2 + 2OH-

Superoxide: 2O2- + 2H2O 2OH- + H2O2 + O2

2. All group I oxides/hydroxides are basic and the basicity increases down the group.

Thermal Stability of carbonates

Li2CO3 Li2O + CO2 ( at 700oC)All other group I carbonates are stable at ~800oC

BeCO3 BeO + CO2 ( at 100oC)MgCO3 MgO + CO2 ( at 540oC)CaCO3 CaO + CO2 ( at 900oC)SrCO3 SrO + CO2 ( at 1290oC)BaCO3 BaO + CO2 ( at 1360oC)

Thermal Stability of hydroxides

All group I hydroxides are stable except LiOHat Bunsen temperature.

Be(OH)2(s) BeO(s) + H2O(g) H = +54 kJ/molMg(OH)2(s) MgO(s) + H2O(g) H = +81 kJ/molCa(OH)2(s) CaO(s) + H2O(g) H = +109 kJ/molSr(OH)2(s) SrO(s) + H2O(g) H = +127 kJ/molBa(OH)2(s) BaO(s) + H2O(g) H = +146 kJ/mol

Explanation of Thermal Stability

+

+

+

-

-

-

Decreasingpolarizingpower

Increasingstability

Relative solubility of Group II hydroxides

Compound

Solubility / mol per 100g water

Mg(OH)2 0.020 x 10-3

Ca(OH)2 1.5 x 10-3

Sr(OH)2 3.4 x 10-3

Ba(OH)2 15 x 10-3

Solubility of hydroxidesincreases down the group.

Solubility of Group II sulphates

Compound Solubility / mol per 100g water

MgSO4

epsom salt

3600 x 10-4

CaSO4 11 x 10-4

SrSO4 0.62 x 10-4

BaSO4 0.009 x 10-4

Solubility of sulphatesincreases up the group.

Explanation of solubility

1. Group I compounds are more soluble than Group II because the metal ions have smaller charges and larger sizes. H lattice is smaller, and H solution is more exothermic.

H solution -H lattice H hydration= +

2. For Group II sulphates, the cations are much smaller than the anions. The changing in size of cations does not cause a significant change in H lattice (proportional to 1/(r+ + r-). However, the changing in size of cations does cause H hydration (proportional to 1/r+ and 1/r-) to become less exothermic, and the solubility decreases when descending the Group.

SO42-

MgSO4

SO42-

SrSO4

3. For the smaller size anions, OH-. Down the Group, less enthalpy is required to break the lattice

as the size of cation increases. However the change in H solution is comparatively smaller due to the large value of 1/r- .

As a result, H solution becomes more exothermic and the solubility increases down the Group.

Mg(OH)2 Sr(OH)2

Comparison of the physical properties and chemical properties between alkaline earth and alkali metals---The main difference is the electron configuration, which is ns2 for alkaline earth metals and ns1 for alkali metals. For the alkaline earth metals, there are two electrons that are available to form a metallic bond, and the nucleus contains an additional positive charge. Also, the elements of group 2A (alkaline earth) have much higher melting points and boiling points compared to those of group 1A

---The alkaline earth metals are much harder and denser compare to alkali metals.

---The elements of the group 2A contain a smaller atomic radius and much higher ionization energy than the group 1A. Even though the group 2A contains much higher ionization energy, they still form an ionic compound with 2+ cations.

Uses of IIA group compounds• Magnesium

– Magnesium hydroxide - Milk of magnesia, an antacid– Magnesium can be obtained by eating food which is rich in

Magnesium, such as nuts and certain vegetables or by eating supplementary diet pills.

• Calcium hydroxide– To neutralize acids in waste water treatment

• Strontium compound– Strontium ranelate is used in the treatment of osteoporosis. It is a

prescription drug in the EU, but not in the USA.– Strontium chloride is sometimes used in toothpastes for sensitive

teeth. One popular brand includes 10% total strontium chloride hexahydrate by weight.

BariumUses of barium sulfate include being a radiocontrast agent for X-ray imaging of the digestive system

Boron is a non-metal, whereas the other Group IIIA (13) elements are metals. • The most common oxidation state for Group 13 is +3. Gallium, indium, and thallium also often form 1+ ions by retaining their ns2 electrons; this is called the inert pair effect.• Aluminium and gallium are amphoteric, but indium and thallium show more metallic character and do not dissolve in alkalis.

Properties and Trendsin Group IIIA

• The Hall–Heroult process is used to isolate aluminum.

• Bauxite (aluminum ore) is first treated with NaOH to form aluminate ion, Al(OH)4

–.

• This solution is then diluted with water and acidified slightly, precipitating Al(OH)3. The Al(OH)3 is then heated to about 1200 oC and decomposes to pure Al2O3.

• The Al2O3 is then electrolyzed using cryolite, Na3AlF6, as the electrolyte and graphite (solid carbon) as the electrodes.

Production of Aluminum

Electrolysis Cell forAluminum Production

Carbon

aluminium

• Most of the chemistry of boron compounds is based on the lack of an octet of electrons about the central boron atom.

• These compounds are electron-deficient; the deficiency causes them to exhibit some unusual bonding features.

• Boron hydride (BH3) forms a coordinate covalent bond with another atom that has a lone pair of electrons to complete its octet; this is called an adduct.

Boron

• Borax, Na2B4O7·10 H2O, a hydrated borate, is the primary source of boron.

• Sodium perborate is used as a bleach; perborate’s structure is:

Structure of Diborane, B2H6

The “regular” B—H bond distance is shorter than the B—H distance in the three-

center bond; why?

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• The reduction of Al3+(aq) to Al(s) occurs with difficulty.• Thus, aluminum metal is a good reducing agent.• As an active metal, aluminum readily reacts with acids to

produce hydrogen gas.• Aluminum also dissolves in basic solutions.• Because its combustion is a highly exothermic reaction,

powdered aluminum is used as a component in rocket propellants and fireworks.

• Perhaps the most familiar use of aluminum is in beverage cans, cookware, and as a foil for wrapping foods.

Properties and Usesof Aluminum

Reactivity towards air

• Boron is unreactive in crystalline form. Aluminium forms a very thin oxide layer on the surface which protects the metal from further attack. Amorphous boron and aluminium metal on heating in air form B2O3 and Al2O3 respectively. With dinitrogen at high temperature they form nitrides.

Boron forms the oxide B2O3 and a large number of borate anions containing trigonal planar BO3 units and/or tetrahedral BO4 units. • Boric acid (B(OH)3) is a monobasic acid. It acts as a Lewis acid, interacting with water to form B(OH)3(OH2), which loses a proton to form [B(OH)4]–

• A12O3 and Ga2O3 are amphoteric, but In2O3 and T12O3 are basic. • [M(H2O)6]3+ salts are acidic in aqueous solution due to hydrolysis.

Oxides, hydroxides, acids

• The oxide layer on aluminum protects it from further corrosion.

• The oxide layer may be enhanced by making an aluminum object the anode in an electrolysis apparatus.

• The thick, hard oxide layer that results is porous and may be dyed.

• When the pores are sealed, the color is a permanent part of the object.

Anodized Aluminum

Reactivity towards acids and alkalies

• Boron does not react with acids and alkalies even at moderate temperature; but aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric character.Aluminium dissolves in dilute HCl and liberates dihydrogen.2Al(s) + 6HCl (aq) → 2Al3+ (aq) + 6Cl-(aq) + 3H2 (g)However, concentrated nitric acid renders aluminium passive by forming a protective oxide layer on the surface.Aluminium also reacts with aqueous alkali and liberates dihydrogen

Reaction with halogens

The IIIA elements react with halogens to form trihalides (except TlI3).2E(s) + 3 X2 (g) → 2EX3 (s) (X = F, Cl, Br, I)

Halides properties

The Lewis acidity of the boron halides increases in the order BF3 < BCl3 < BBr3 < BI3. This is because it takes less energy to distort the trigonal planar geometry with larger halides. This trend can be explained by weaker pπ–pπ bonding.

• Aluminium fluoride is a solid with high ionic character, but the other aluminium halides have structures containing covalent bonds.