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	Ti^3+/Ti	Zr^4+/Zr	Hf^4+/Hf
Е^0, 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.

Дата добавления: 2015-07-25