RG S C
M BE
HO Stemberg HEXAGON
S TE
OL
8
8
R
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UN
G HO
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1 Tut # 13 A
1. Solution Y is hydrochloric acid, HCl(aq). Solution Z is aqueous 4-chlorobutanoic acid, Cl(CH2 )3 CO2 H(aq).
The pKa of Cl(CH2 )3 CO2 H(aq) is 4.52. The pH of both solutions is 4.00.
(a) i. (1 point) Write an expression for the Ka of Cl(CH2 )3 CO2 H(aq).
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ii. (1 point) Write a mathematical expression to describe the relationship between Ka and
pKa .
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iii. (1 point) Calculate [H+ ] in solutions Y and Z.
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iv. (2 points) Calculate the ratio
[HCl] dissolved in solution Y
[Cl(CH2 )3 CO2 H] dissolved in solution Z
.
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(b) (3 points) Abuffer solution of pH 5.00 is produced by adding sodium propanoate to 5.00g of
propanoic acid in 100cm3 of distilled water.
Calculate the mass of sodium propanoate that must be used to produce this buffer solution.
The Ka of propanoic acid is 1.35 × 10−5 moldm−3 .
[Mr : propanoic acid, 74.0; sodium propanoate, 96.0]
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(c) (2 points) Some dilute sulfuric acid is mixed with a small sample of the buffer solution
described in (b). The final pH of the mixture is close to 1.
Explain this observation.
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2. Hydrogen peroxide is a liquid at 298 K. It is moderately stable under room conditions but will
decompose quickly if a catalyst is added.
reaction 1: 2 H2 O2 (l) −−→ 2 H2 O(l) + O2 (g)
(a) i. (1 point) Define entropy.
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ii. (1 point) Predict the sign of the standard entropy change of reaction 1.
Explain your answer
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(b) (1 point) Some bond energy data are shown in Table 3.1.
Table 3.1
type of bond bond energy / kJmol−1
O−O 150
O−H 460
−
O O 496
Use the data in Table 3.1 to show that the enthalpy change of the following reaction is −196
kJmol−1 .
2 H2 O2 (g) −−→ 2 H2 O(g) + O2 (g)
(c) (3 points) Some standard entropies, So , are shown in Table 3.2.
Table 3.2
substance So / JK−1 mol−1
H2 O2 (l) +102
H2 O(l) +70
The enthalpy change and Gibbs free energy change for the following reaction are shown.
2 H2 O2 (l) −−→ 2 H2 O(l) + O2 (g)
o
∆H = −196 kJmol −1
∆Go = −238 kJmol−1
Use the data given to calculate the standard entropy of oxygen, So , O2 (g).
(d) (1 point) The decomposition of H2 O2 (aq) is catalysed by aqueous iron(III) chloride and by
silver metal.
Identify which of these two catalysts is acting as a homogeneous catalyst.
Explain your answer.
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(e) The Eo values for two electrode reactions are given.
H2 O2 + 2 H+ + 2 e− ↽ −−⇀
−− 2 H2 O Eo = + 1.77 V
Cr3+ + e− − ↽− − Cr2+
−⇀ Eo = − 0.41 V
i. (1 point) An electrochemical cell is constructed with the following half-cells (electrodes):
• an acidified solution of H2 O2 , a platinum wire
• Cr2+ mixed with Cr3+ , a platinum wire.
Identify the positive half-cell and calculate the standard cell potential, Eocell .
ii. (2 points) Calculate the value of ∆Go for the cell reaction that occurs, per mole of H2 O2 .
(f) (2 points) The Eo values for two electrode reactions are given.
H2 O2 + 2 H+ + 2 e− ↽−−−⇀
− 2 H2 O Eo = + 1.77 V
Co3+ + e− − −
↽−− Co2+ Eo = + 1.82 V
⇀
An electrochemical cell is constructed with the following half-cells.
half-cell 1: an acidified solution of H2 O2 under standard conditions, a platinum wire
half-cell 2: a solution containing 0.020 moldm−3 Co3+ and 2.0 moldm−3 Co2+ , a platinum
wire
i. (2 points) Use the Nernst equation to calculate the value of E, the electrode potential
of half-cell 2 under these conditions.
ii. (1 point) Write an equation for the cell reaction that occurs in this cell under these
conditions.
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(g) i. (1 point) Define enthalpy change of hydration, ∆Hhyd .
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ii. (2 points) Aluminium fluoride, AlF3 , is an ionic solid.
Complete and label the energy cycle to show the relationship between:
• the enthalpy change of solution of AlF3 , ∆Hsol
o
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• the lattice energy of AlF3 , ∆Hlatt
o
• the enthalpy changes of hydration of Al3+ and F− , ∆Hhyd
o .
Include state symbols for all substances and ions.
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∆H o .................. ∆H o ..................
∆H o ..................
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iii. (1 point) Relevant data for this question are given.
∆Hsolo AlF = -209 kJmol−1
3
∆Hhyd Al3+ = -4690 kJmol−1
o
∆Hhydo F− = -506 kJmol−1
o
Use these data and your energy cycle in (g)(ii) to calculate the ∆Hlatt of AlF3 .
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RG S C
M BE
HO Stemberg HEXAGON
S TE
OL
8
8
R
T
UN
G HO
C
2 Tut # 13 B
1. Some electrode potentials are shown in Table 3.1.
electrode reaction Eo / V
V2+ + 2 e− −↽−⇀
−−V −1.20
3+ −
V +e ↽ −−−⇀− V2+ −0.26
2+ + −
VO + 2 H + e − ↽−⇀
−− V3+ + H2 O +0.34
VO2 + + 2 H+ + e− ↽−−
−⇀− VO2+ + H2 O +1.00
2+
Fe + 2 e −− −⇀ −0.44
↽ −− Fe
3+
Fe + 3 e ↽− −−⇀ −0.04
−− Fe
Fe3+ + e− −
↽−− Fe2+
−⇀ +0.77
2 H+ + 2 e− ↽
−−−⇀− H2 0.00
ClO + H2 O + 2 e− −
−
↽−− Cl− + 2 OH−
− ⇀ +0.89
(a) i. (1 point) Complete the diagram to show a standard hydrogen electrode.
Label your diagram. Identify all substances. You do not need to state standard condi-
tions.
ii. (1 point) An electrochemical cell is set up using an Fe3+ / Fe2+ electrode and a standard
hydrogen electrode.
Identify the positive electrode in the electrochemical cell and the direction of electron
flow in the external circuit.
positive electrode ..............................................................................................................
Electrons flow from the .............................. electrode to the ...............
(b) The vanadium-containing species in the electrode reactions given in Table 3.1 are V, V2+ ,
V3+ , VO2+ and VO2 + .
i. (1 point) Identify one vanadium-containing species that does not react with Fe2+ ions
under standard conditions.
Use data from Table 3.1 to explain your answer.
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ii. (1 point) Identify all the vanadium-containing species that will react with Fe2+ ions
under standard conditions.
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iii. (1 point) Write an equation for one of the possible reactions identified in (ii).
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(c) Another electrochemical cell is set up using an Fe3+ / Fe2+ electrode and an alkaline ClO−
/ Cl− electrode.
The concentration of Fe3+ is 1000 times greater than the concentration of Fe2+ in the
Fe3+ /Fe2+ electrode. All other conditions are standard.
i. (2 points) Use the Nernst equation to calculate the E value of the Fe3+ /Fe2+ electrode.
Show your working
ii. (1 point) Write an equation for the reaction that occurs in the cell, under these condi-
tions.
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(d) (3 points) Another electrochemical cell is set up using an Fe2+ /Fe electrode and an alkaline
ClO− /Cl− electrode under standard conditions.
Calculate the value of ∆Go for the cell.
(e) A solution of iron(II) sulfate, FeSO4 (aq) is electrolysed with iron electrodes. Under the
conditions used, no gas is evolved at the cathode.
A current of 0.640 A is passed for 17.0 minutes. The mass of the cathode increases by 0.185
g.
Use these results to calculate an experimental value for the Avogadro constant, L. Show your
working
(f) Iron(II) chloride, FeCl2 , is oxidised by chlorine to form iron(III) chloride, FeCl3 , under stan-
dard conditions.
2 FeCl2 (s) + Cl2 (g) −−→ 2 FeCl3 (s) ∆H o = −128 kJmol−1
Table 3.2
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species So / JK−1 mol−1
Cl2 (g) 223
FeCl2 (s) 120
FeCl3 (s) 142
i. (3 points) Use Table 3.2 and other data to calculate the Gibbs free energy change, ∆Go ,
for this reaction.
Show your working.
ii. (1 point) Predict whether this reaction becomes more or less feasible at a higher tem-
perature.
Explain your answer
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2. Pure water is a very poor conductor of electricity. However, when hydrogen chloride gas is
dissolved in water, ions are formed and a current flows during electrolysis.
HCl(g) + aq −−→ H+ (aq) + Cl− (aq)
The overall change after electrolysis is that hydrogen chloride gas is converted into hydrogen and
chlorine.
2 HCl(g) −−→ H2 (g) + Cl2 (g)
When a current of 3.10A is passed through the solution for Y minutes, 351cm3 of chlorine are
produced at the anode, measured under room conditions.
(a) (2 points) Calculate the number of chlorine molecules produced during the electrolysis.
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(b) (1 point) Calculate the total number of electrons transferred to produce this number of
chlorine molecules.
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(c) (1 point) Calculate the quantity of charge, in coulombs, of the total number of electrons
calculated in (b).
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(d) (1 point) Calculate the time, Y , in minutes, for which the current flows.
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(e) The standard entropies, S θ , of three species are given in the table.
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species S θ /JK−1 mol−1
HCl(g) +187
H2 (g) +131
Cl2 (g) +223
i. (1 point) Calculate ∆S θ for the reaction 2 HCl(g) −−→ H2 (g) + Cl2 (g).
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ii. (2 points) ∆H θ for the reaction 2 HCl(g) −−→ H2 (g) + Cl2 (g) is +185kJmol−1 .
Calculate ∆Gθ for this reaction at 298K.
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iii. (1 point) Predict the effect of increasing temperature on the spontaneity of this reaction.
Explain your answer.
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