ANONYMITY NUMBER:
Université Grenoble Alpes
May 2016
CHI 241-int, Final Exam
• Instructions:
-
This is a 2hrs. exam. Answers are to be given in spaces set aside for this purpose.
Calculator is allowed (no cell phone). Documents forbidden, except language dictionnaries.
Activities are taken equal to concentrations in mol.L−1 ; pKw = 14 ; T◦ = 298 K
Systematically check every approximation you make (i.e. write it!)
I- Precipitate of Zn(OH)2 (4 pts)
Data: pKsZn(OH)2 =16.35
2+ + 4OH− ; pKr = 15.35
Zn(OH)2−
4 Zn
I-1 Consider a solution containing some Zn2+ ions at the initial concentration of 0.1 mol.L−1 . At pH=1, the
solution is clear. The addition of a strong base, without changing the volume, leads to the formation of a
precipitate of Zn(OH)2 . Calculate the pH at which Zn(OH)2 starts to precipitate. (2pts)
I-2 Further addition of base, still at constant volume, leads to the redissolution of the precipitate until it totally
disappears, by formation of the soluble species Zn(OH)2−
4 . Calculate the pH at which the precipitate is
totally redissolved. (2pts)
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II- Solubility of SrSO4 3pts
Data: pKs(SrSO4 ) = 6.55 ; pKa1(H2 SO4 /HSO− ) < 0 (strong acid) ; pKa2(HSO− /SO2− ) = 1.9
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II-1 Give an expression for s, the solubility of SrSO4 in pure water, in function of the equilibrum constants and
the concentration of H3 O+ . (2pt)
II-2 Calculate the solubility of BaSO4 in a buffer solution of pH = pKa2 = 1.9. (1pt)
III- RedOx potential of Br (4pts)
Nota: The three species Br2 , HBrO and HBrO2 are considered stable in water.
Data: E0HBrO/Br2 = 1.574 V/SHE ; E0HBrO2 /HBrO = 1.455 V/SHE
RT
F
× ln(x) = 0.06 × log(x)
Solubility of Br2 : s = 0.214 mol.L−1 .
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III-1 Write the redox half-equation for the HBrO/Br2 pair. (0.5 pts)
III-2 Write the redox half-equation for the HBrO2 /HBrO pair. (0.5 pts)
III-3 Is an aqueous solution of hypobromous acid (HBrO) stable in standard conditions? Justify. (1 pt)
III-4 If the initial concentrations of HBrO and HBrO2 are both 0.1mol.L−1 and if the solution is a saturated in
Br2 , is the hypobromous acid (HBrO) stable? Justify. (2 pts)
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IV- Pourbaix diagram of nickel 9pts
Consider the simplified Pourbaix diagram of Ni, involving the following species: Ni(s) ; Ni(OH)2 (s) ; NiO2 (s) and
Ni2+ . All the soluble species have a similar concentration of 10−2 mol.L−1 .
Data: E0N i2+ /N i = -0.25 V/SHE
E0N iO2 /N i2+ = 1.59 V/SHE
E0O2 /H2 O = 1.23 V/SHE
RT
F
E0H + /H2 O = 0.0 V/SHE
× ln(x) = 0.06 × log(x)
Solubility product of Ni(OH)2 : Ks = 10−16 .
IV-1 Complete the following diagram of Ni by giving the prevalent species in each domain. (0.5 pt)
IV-2 Write the chemical equilibria corresponding to lines 1 to 5 of the Pourbaix diagram of Ni. (2 pts)
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IV-3 Calculate the pH at which Ni(OH)2 starts to precipitate. (0.5 pts)
IV-4 Write the equations for lines 1, 2, 4 and 5 of the diagram. (3 pts)
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IV-5 Calculate the limits of the stability domain of water in standard conditions (0.5 pts)
IV-6 On the following diagram of Ni, plot the stability domain of water in standard conditions. (0.5 pts)
IV-7 Is Ni stable in water in these conditions ? Use the diagram to justify your answer. (2 pts)
— End of questions —
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