Chemical Properties Scandium subgroup elements, iiib

Sc subgroup elements are strong reducing agents in accordance with standard potential values. Moreover, chemical activity is increased with the growth of atomic radius R_а in the series Sc, Y, La, Ac. Sc does not react with water, аlthough La and Ac displace hydrogen from water at STP:

2La + 6H₂O = 2La(OH)₃ + 3H₂

These metals react actively with diluted HNO₃ and nitrogen is reduced to N^-3:

8Sc + 30HNO₃ = 8Sc(NO₃)₃ + 3NH₄NO₃ + 9H₂O

They react also with nonmetals:

4M + 3O₂ = 2М₂О₃;

2M + N₂ = 2MN;

M + 2C = МС₂;

and with hydrogen form hydrides with limiting cases of composition MH₂ and МН₃.

Compounds

Hydrides.Their composition depends on the ratio of components ( — the lack of hydrogen, H₂) and like hydrides of s-elements they can be decomposed by water and oxidised by O₂:

2 + 6 = 2 + 5

+ 3 = + 3

Oxides. The examples of their preparation are shown below:

4Sc + 3O₂ = 2Sc₂О₃

4Y(NO₃)₃ = 2Y₂О₃ + 12NO₂ + 3O₂

They react actively with water. In the series Sc —Y— La—Ac the basic properties of oxides are increased, as well as the rate of their reaction with water, ? exothermic heat effect of the reaction.

Hydroxides. In the series Sc(OH)₃—Y(OH)₃—La(OH)₃—Ac(OH)₃ basic properties are also increased. This fact is explained by the M³⁺ size growth and weakening the bond of these ions with hydroxide groups ОН^-. Sc(OH)₃ is an amphoteric hydroxide:

Sc(OH)₃ + 3NaOH = Na₃[Sc(OH)₆]

Oxoscandates are formed when melting Sc₂O₃ or Sc(OH)₃ with alkalis:

Sc₂O₃ + 2NaOH = 2NaScO₂ + H₂O

Y(OH)₃, La(OH)₃ have low solubility in water, although their solutions form weak alkaline medium. For instance, saturated solution of La(OH)₃ absorbs CO₂ from air:

2La(OH)₃ + 3CO₂  La₂(CO₃)₃ + 3H₂O

Salts. Solubility of them is close to alkali-earth metals salts solubility. Fluorides MF₃, carbonates М₂(СО₃)₃, phosphates МРО₄ and sulfates M₂(SO₄)₃ are soluble in water.

Scandium forms the most stable coordination compounds with coordination number 6. After scandium it becomes larger and stability of a complex ion decreases. For instance, there are the following aquacomplexes of the Sc subgroup elements: [Sc(H₂O)₆]³⁺, [Y(H₂O)₉]³⁺, [La(H₂O)₉]³⁺. ScF₃ can react with fluorides of alkali metals and form soluble complexes:

ScF₃ + 3KF = K₃[ScF₆]

Carbonates M₂(СO₃)₃ dissolve in saturated carbonates of alkali metals (or ammonium) due to the double salt formation:

K₂CO₃ + La₂(CO₃)₃  2K[La(CO₃)₂]

Halides can be crystalhydrates. Losing water after thermal decomposition, they are transformed to oxohalides:

ScCl₃^.6H₂O ScCl₃^.3H₂O Sc(OH)₂Cl ScOCl

- H₂O - H₂O,-HCl - H₂O

Titanium subgroup elements, ivb

Ti, Zr, and Hf have higly negative values of standard electrode potential, Е⁰, that characterises equilibrium between metallic and ionic forms of the element:

Ox/Red	Ti3+/Ti	Zr4+/Zr	Hf4+/Hf
Е0, V	-1.76	-1.43	-1.57

However, despite the strong reducing properties elements of titanium subgroup are chemically stable at normal conditions due to protective properties of oxide film ЕО₂.

Reactions with nonmetals require heating:

E + О₂ = EО₂

E + 2Hal₂ = EHal₄

Ti(Zr) + 2S = Ti(Zr)S₂

Fine powders of metals of titanium subgroup are pyrophoric. Ti, Zr, and Hf are stable in some corrosive media (moisten air, sea water, solutions of salts), and also in most of acids except HF, hot H₃PO₄, and alkalis. For example, cold HNO₃ passivates Ti, Zr, and Hf due to increase of oxide film thickness. Zr and Hf don’t react with HCl and diluted H₂SO₄. Hot HCl and diluted H₂SO₄ interaction with Ti is shown below:

2Ti + 6HCl = 2TiCl₃ + 3H₂

2Ti + 3H₂SO₄ = Ti₂(SO₄)₃ +3H₂

Aquacomplex [Ti(H₂O)₆]³⁺ that appears is lilac.

Hot nitric acid reacts with titanium too:

3Ti + 4HNO₃ + H₂O = 3H₂TiO₃ + 4NO.

Generally, dissolution and simultaneous oxidation of Ti subgroup elements in acid medium depends on the fact whether their oxidation and formation of Е(IV) anions (as a rule anionic coordination compounds) are possible.

2Ti + 6HF  2TiF₃ + 3H₂

Zr(Hf) + 6HF = H₂[Zr(Hf)F₆] + 2H₂

3Е + 4HNO₃ + 18HF = 3H₂[ЕF₆] + 4NO + 8H₂O

Zr + 5H₂SO₄ = H₂[Zr(SO₄)₃] + 2SO₂ + 4H₂O

3Zr + 4HNO₃ + 18HCl = 3H₂[ZrCl₆] + 4NO + 8H₂O

Titanium subgroup elements are soluble in molten alkalis

Е + 4NaOH = Na₄ЕO₄ + 2H₂

Hf is soluble in molten KHF₂:

Hf + KHF₂ = K₂HfF₆ + 2KF + 2H₂

Ti, Zr і Hf form anionic coordination compounds. In the series Ti—Zr—Hf coordination number is increased: (М¹-cations of monovalent metals) М¹₂[TiF₆], М¹₂[ZrF₆], М¹₃[ZrF₇], М¹₄[ZrF₈]. Stability of coordination compounds of halides is decreased in the series F^- —Cl^- —Br^- — I^-.

The summary of chemical properties of free elements of titanium subgroup is shown in the table below:

Element	Conditions	Reagent	Product
Ti	Room temperature	HF(conc.)	TiF3
		HCl(conc.)	TiCl3
		Aqua regua	H2[TiCl6]
	Heating	Air or oxygen	TiO2
		Halogens	TiHal4
		S,N,C	TiS2, TiN, TiC
		Molten NaOH	Titanates
Zr	Room temperature	HF(conc.)	H2[ZrF6]
		Aqua regua	H2[ZrCl6]
	Heating	Air or oxygen	ZrO2
		Halogens	ZrHal4
		S,N,C	ZrS2, ZrN, ZrC
		Molten NaOH	zirconates
Hf	Room temperature	HF(conc.)	H2[HfF6]
		Aqua regua	H2[HfCl6]
	Heating	Air or oxygen	HfO2
		Halogens	HfHal4
		S,N,C	HfN, HfC
		Molten KHF2	K2[HfF6]

Compounds with metallic type of bond. Hydrogen, carbon, nitrogen and some other nonmetals react with IVB group elements forming solid solutions of varying composition that are nonstoichiometric compounds (see also dioxygen reaction below).

Hydrides. At the beginning of reaction solid solutions of insertion of E₂Н composition (mole fraction of Н₂ is 33%) appear. The limiting composition will be EН₂, when the temperature and hydrogen content are increased. The composition of the latter hydride can be varied from EH to EH₂. It is a powder of gray or black colour.

Carbides and nitrides. Their preparation methods are illustrated by reactions below:

2Е + N₂ = 2ЕN

2ЕО₂ + 4С + N₂ = 2ЕN + 4CO

Е + С = ЕС

ЕО₂ + 3С = ЕС + 2СО

Chemically inert TiN reacts with steam and alkalis when subject to strong heating only:

2TiN + 4H₂O = 2TiO₂ + 2NH₃ + H₂

2TiN + 4KOH + 2H₂O = 2K₂TiO₃ + 2NH₃ + H₂

Сarbides EС are metallike crystals that are good conductors, hard, and refractory (3000-4000 ^oC). They are chemically inert, although at high temperature they react with halogens, oxygen, nitrogen, for instance:

EС + 2Cl₂  ECl₄ + C

2TiC + N₂ + H₂  2TiN + C₂N₂

Оxides and hydroxides (IV). Reactions of IVB subgroup elements with dioxygen include the stage of a solid solution of insertion type with molar fraction of oxygen up to 30%. The intermediates of this transformation are mostly nonstoichiometric. Let’s take titanium:

Ti + O₂ Ti₆O Ti₃O TiO Ti₂O₃ TiO₂

Oxides ЕО₂ are refractory (t_m TiO₂ = 1870 ^oC, t_m ZrO₂ = 2850 ^oC, t_пл HfO₂ = 2900 ^oC), low active due to their polymeric structure. So, TiO₂ represents 3D-polymer where each titanium atom is linked with 6 oxygenones, in turn, each oxygen is linked with 3 titaniumones. Oxides EO₂ have no reaction with water, cold diluted acids (except HF) and alkalis. The long heating is the only method to realise reactions with acids. Displaying amphoteric properties, EO₂ react with the molten alkalis and form salts.