Soal IChO Ke-26

THEORETICAL PROBLEM No. 4

(21 blue points, 8 red points)

Platinum(IV) oxide is not found in the nature, but can be prepared in a laboratory. Solid platinum(IV) oxide is in equilibrium with platinum metal and oxygen gas of 1 atm (= 1.01325 ^. 10⁵ Pa) at 650 ^oC.

a) This suggests that the conditions on the Earth, when the minerals we know were formed, were:

[ 1 ] pO₂ = 1 atm, t= 650 deg C; [ 2 ] pO₂ < 1 atm, t < 650 deg.C; [ 3 ] pO₂ > 1 atm, t < 650 deg.C; [ 4 ] pO₂ < 1 atm, t > 650 deg C; [ 5 ] pO₂ > 1 atm, t > 650 deg.C

Mark the most probable alternative [ 1 ] – [ 5 ] on the answer sheet. Please note, the marking of only one alternative will be accepted.

b) What are [[Delta]]G and Kp for the formation of platinum(IV) oxide at 1 atm oxygen pressure and 650 deg.C?

The preparation of platinum(IV) oxide involves boiling of a solution which contains hexachloroplatinate(IV) ions with sodium carbonate. In this process PtO₂^. nH₂O is formed and this is in turn converted to platinum(IV) oxide upon subsequent filtering and heat treatment. In the following we assume n = 4.

PtO₂^. 4H₂O or Pt(OH)₄ ^. 2H₂O can be dissolved in acids and strong bases.

c) Write the balanced equations for the preparation of platinum(IV) oxide according to the procedure given above.

d) Write the balanced equations for the dissolution of PtO₂^. 4H₂O in both hydrochloric acid and sodium hydroxide.

Platinum is mainly found in the nature as the metal (in mixture or in alloying with other precious metals). Platinum is dissolved in aqua regia under the formation of hexachloroplatinate(IV) ions. Aqua regia is a mixture of concentrated hydrochloric and nitric acids in proportion 3 : 1, and of the nitrosylchloride (NOCl) and the atomic chlorine which are formed upon the mixing. The latter is believed to be the active dissolving component.

The hexachloroplatinate(IV) ions can be precipitated as diammonium hexachloroplatinate(IV) and by thermal decomposition of this compound, finely powdered platinum and gaseous products are formed.

e) Write the balanced equations for the formation of aqua regia and its reaction with platinum.

f) Write the balanced equation of the thermal decomposition of diammonium hexachloroplatinate(IV) at elevated temperature.

From diammonium hexachloroplatinate(IV) we can prepare Pt(NH₃)₂ Cl₂ which occurs in cis ([[Delta]]HO(f;^o) = -467.4 kJ/mol, [[Delta]]GO(f;^o) = -228.7 kJ/mol) and trans

([[Delta]]HO(f;^o) = -480.3 kJ/mol, [[Delta]]GO(f;^o) = -222.8 kJ/mol) form.

g) The occurrence of the isomers shows that Pt(NH3)₂Cl₂ has

[ 1 ] linear [ 2 ] planar [ 3 ] tetrahedral [ 4 ] octahedral geometry.

Mark the correct alternative of [ 1 ] – [ 4 ] on the answer sheet.

h) Is the [ 1 ] cis or [ 2 ] trans form the thermodynamic more stable?

Platinum is used as a catalyst in modern automobiles. In the catalyst carbon monoxide ([[Delta]]HO(f;^o) = -110.5 kJ/mol, [[Delta]]GO(f;^o) = -137.3 kJ/mol) reacts with oxygen to carbon dioxide ([[Delta]]HO(f;^o) = -393.5 kJ/mol, [[Delta]]GO(f;^o) = -394.4 kJ/mol).

i) Is the reaction spontaneous at 25 deg.C? [ 1 ] yes or [ 2 ] no.

Is the reaction [ 1 ] endo- or [ 2 ] exothermic?

Calculate [[Delta]]Sdeg. for the reaction. Establish whether the entropy of the reaction system [ 1 ] increases or [ 2 ] decreases.

j) Establish an expression for the temperature dependence of the equilibrium constant in this case.

The overall catalytic reaction is simple, whereas the reaction mechanism in the homogeneous phase is very complicated with a large number of reaction steps, and the course is difficult to control owing to a distinct chain character. With platinum as catalyst the significant reaction steps are: (i) Adsorption of CO and adsorption/dissociation of O₂ ([[Delta]]H = -259 kJ per mol CO + O), (ii) their activation (105 kJ per mol CO + O) and (iii) the reaction to and the desorption of CO₂ ([[Delta]]H = 21 kJ per mol CO₂).

A one-dimensional energy-diagram for the platinum catalyzed oxidation of carbon monoxide to dioxide can be represented as:

k) Mark the correct alternative of [ 1 ] – [ 4 ] on the answer sheet.