ANONYMITY NUMBER:
Université Joseph Fourier
June 2015
CHI 241-int.
Final exam, 2nd session
• Indications:
- This is a 2 hrs. closed-book test. Personnal documents are forbidden with the exception of language
dictionnaries which are allowed. Any calculator may be used as long as it cannot connect to a network.
- 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
CO2 Pollution
(5 pts)
Consider an aqueous solution of NaHSO4 (strong electrolyte) at 5×10−2 mol.L−1 .
−
2−
Data: pKa(H2 SO4 /HSO−
4 ) < 0 ; pKa(HSO4 /SO4 ) = 1.9.
I-1 What are the prevalent species present in solution?
I-2 What acid-base reaction is dominant? Is it a total reaction?
I-3 Calculate the pH of the solution and give the concentrations of the prevalent species at the equilibrium
state
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ANONYMITY NUMBER:
Consider now a solution of NaHSO4 at 5.0×10−2 mol.L−1 , but prepared with water polluted by carbon
dioxide (CO2 ). The concentration of CO2 is 10−2 mol.L−1 .
−
2−
Data: pKa(H2 SO4 /HSO−
4 ) < 0 ; pKa(HSO4 /SO4 ) = 1.9.
−
2−
pKa(HCO2 /HCO−
3 ) = 6.4 ; pKa(HCO3 /CO3 ) = 10.3.
I-4 What are the prevalent species present in solution?
I-5 What acid-base reaction is dominant? Is it a total reaction?
I-6 Calculate the pH of the solution and give the concentrations of the prevalent species at the equilibrium
state
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ANONYMITY NUMBER:
II
Solubility of cadmium hydroxide Cd(OH)2
(2 pts)
Data: Solubility of Cd(OH)2 in pure water is 5.96 g.L−1 at 20◦ C
Molar mass of Cd(OH)2 is 146.4 mol.L−1
II-1 Determine the pH of a solution saturated with cadmium hydroxide.
II-2 Calculate the solubility produt of Cd(OH)2
III
Fe/Cu galvanic cell
(3 pts)
A galvanic cell is constituted with the following items:
• Half-cell (A) is made of an electrode of metallic copper plunged in a solution of cupric ions Cu2+ at
0.5 mol.L−1 .
• Half-cell (B) is made of a platinum wire plunged in a soluion of ferrous ions Fe2+ at 0.1 mol.L−1 and
ferric ions (Fe3+ ) at 0.2 mol.L−1 .
• The two half-cells are linked together with a salt bridge (containing potassium chloride).
Data: E0Cu2+ /Cu = 0.345V ; E0F e3+ /F e2+ = 0.77V ; RT/F ln(X) = 0.06 log(X)
III-1 Calculate the potential of half-cell (A).
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ANONYMITY NUMBER:
III-2 Calculate the potential of half-cell (B).
III-3 Make a skectch of the complete cell, with a voltmeter. Indicate the sign of each pole as well as the
direction of the current.
III-4 Write the redox half-reaction of each half-cell as well as the total redox equation of the galvanic cell.
III-5 Determine the electromotive force (emf) of the cell.
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ANONYMITY NUMBER:
IV
Pourbaix diagram of iron
(10 pts)
Data:
3+
• Species to consider are: Fe(solid) ; Fe2+
(aq.) ; Fe(aq.) ; Fe(OH)2,(solid) ; Fe(OH)3,(solid)
•
•
•
•
Concentrations of species in solution are all equal to C0 = 0.1mol.L−1
RT/F ln(X) = 0.06 log(X)
Standard potentials: E0F e2+ /F e = -0.44V; E0F e3+ /F e2+ = 0.77V
pKs(F e(OH)2 ) = 15; pKs(F e(OH)3 ) = 38
IV-1 Give the oxidation number of iron in all the species considered.
IV-2 Calculate at which pH Fe(OH)2 starts to precipitate.
IV-3 Calculate at which pH Fe(OH)3 starts to precipitate.
IV-4 Calculate the potentials of the following pairs: Fe2+ /Fe and Fe3+ /Fe2+ .
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ANONYMITY NUMBER:
IV-5 Complete the following E-pH diagram:
• Give the prevalent species in each domain
• Give the values of potential for the two horizontal limits
• Give the values of pH for the two vertical limits
IV-6 Write the chemical equilibria corresponding to lines 1, 2 and 3.
IV-7 Write the equations (E in function of pH) for lines 1, 2 and 3.
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ANONYMITY NUMBER:
IV-8 Calculate the limits of the stability domain of water under standard conditions.
Data: E0O2 /H2 O = 1.23 V
IV-9 On the following E-pH diagram of Fe, plot the the stability domain of water under standard conditions.
IV-10 Is iron stable in the following cases? justify your answers with the E-pH diagram.
a- In water
b- In aerated water (containing dissolved O2 )
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