Cambridge International Examinations
Cambridge International Advanced Subsidiary and Advanced Level
9701/41
CHEMISTRY
Paper 4 A Level Structured Questions
2 hours
Candidates answer on the Question Paper
ENERGETICS
Date------/---------/-------------------
READ THESE INSTRUCTIONS FIRST
Write your Centre number, candidate number and name on all the work you hand in.
Write in dark blue or black pen.
You may use an HB pencil for any diagrams or graphs.
Do not use staples, paper clips, glue or correction fluid.
DO NOT WRITE IN ANY BARCODES.
Answer all questions.
Electronic calculators may be used.
You may lose marks if you do not show your working or if you do not use appropriate units.
At the end of the examination, fasten all your work securely together.
The number of marks is given in brackets [ ] at the end of each question or part question.
.
Answer all the questions in the spaces provided.
1
(a) Complete the table using ticks (s) to indicate whether the sign of each type of energy
change, under standard conditions, is always positive, always negative or could be either
positive or negative.
always
positive
energy change
always
negative
either positive
or negative
electron affinity
enthalpy change of atomisation
ionisation energy
lattice energy
[2]
(b) The Born-Haber cycle for magnesium chloride is shown.
Mg2+(g) + 2Cl (g) + 2e–
H4
H5
Mg2+(g) + 2Cl –(g)
Mg+(g) + 2Cl (g) + e–
H3
Mg(g) + 2Cl (g)
Mg(g) + Cl 2(g)
Mg(s) + Cl 2(g)
H2
H6
H1
H7
MgCl 2(s)
(i)
Explain why H4 is greater than H3.
.............................................................................................................................................
....................................................................................................................................... [1]
(ii)
What names are given to the enthalpy changes H6 and H7?
H6 ......................................................................................................................................
H7 ......................................................................................................................................
[1]
(c) Chlorine is in Group 17.
Suggest the trend in the first electron affinity of the elements in Group 17. Explain your answer.
....................................................................................................................................................
....................................................................................................................................................
.............................................................................................................................................. [2]
(d) The equation for the formation of magnesium oxide from its elements is shown.
Mg(s) + O2(g)
MgO(s) ΔHo = – 602kJmol–1
S o / J K–1 mol–1
substance
Mg(s)
32.7
O2(g)
205
MgO(s)
26.9
Use the equation and the data given in the table to calculate G o for the reaction at 25 C.
G o = .............................. units ..............................
[4]
[Total: 10]
2 (a) (i) Describe and explain the variation in the thermal stabilities of the carbonates of the Group 2
elements.
.............................................................................................................................................….……......
............................................................................................................................................................
…........................................................................................................................................................
............................................................................................................................................................
…................................................................................................................................................. [3]
ii) Suggest and explain a reason why sodium carbonate is more stable to heat than magnesium
carbonate.
……….................................................................................................................................................
……….................................................................................................................................................
....................................................................................................................................................... [2]
(b) Sodium hydrogen carbonate, NaHCO3, and potassium hydrogen carbonate, KHCO3,
decompose on heating to produce gases and the solid metal carbonate.
(i) Write an equation for the decomposition of KHCO3.
...........................................................................................................................................................[2]
(ii) Predict which of NaHCO3 or KHCO3 will decompose at the lower temperature. Explain your
answer.
…………………………............................................................................................................................
................................................................................................................................................................
.............................................................................................................................................................…
………………………………………………………………………………………………………............... [2]
(c) (i) Use the data in given, to calculate the lattice energy of potassium oxide, K2O(s).
Energy change value/kJmol–1
enthalpy change of atomisation of potassium,
K(s) +89
electron affinity of oxygen
O-(g) –141
electron affinity of oxygen
O2– (g) +798
enthalpy change of formation of potassium oxide,
K2O(s) –361
bond energy of oxygen molecule
(O=O)
496
+
419
ionisation energy of Potassium
K
ΔH0Latt = .............................. kJmol–1 [4]
(ii) State whether the lattice energy of Na2O would be more negative, less negative or the same as
that of K2O. Give reasons for your answer.
.............................................................................................................................................
.............................................................................................................................................
[2]
[Total: 15]
3 (a) Draw an energy cycle to show the dissolving of magnesium iodide in water.
[5]
b)The table shows the values for all but one of the enthalpy changes relevant to this cycle.
Value/kJmol-
Enthalpy change
−2327
attice energy
enthalpy change of hydration of Mg
2+
enthalpy change of hydration of I− ion
ion
−1920
−314
i) Define enthalpy change of hydration.
…………………………………………………………………………………………………………………
………………………………………………………………………………………………………………[2]
ii) Use the values in the table to calculate the value for the enthalpy change of solution of
magnesium iodide.
ΔHθsol =…………kJmol-[3]
C. Draw a diagram to show how water molecules are arranged around a magnesium ion.
[2]
d. Explain why the enthalpy change of hydration of a magnesium ion is more exothermic than the
enthalpy change of hydration of a sodium ion.
…………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………
…………………………………………………………………………………………………………………
……................................................................................................................................................[3]
[Total 15]
4. Iron (II) oxide can be reduced to iron both by carbon, which is oxidised to carbon monoxide,
and by carbon monoxide, which is oxidised to carbon dioxide, as shown in the equations below.
FeO(s) + C(s) → Fe(s) + CO (g)
FeO(s) + CO (g) → Fe(s) + CO2 (g)
(a) Use the data given below to deduce the free energy change, ΔGθ , measured at 450 K, for each of
these reduction processes.
(Assume that the enthalpy change and the entropy change in each of these reactions remain
the same if the temperature is raised from 298 K to 450 K.)
FeO(s)
Fe(s)
C(s)
CO(g)
CO2(g)
ΔH /kJ mol–1
–271.9
0
0
–110.5
-393.5
S /J K–1 mol–1
58.5
27.3
5.7
197.6
213.6
ΔGθ =………….units………
[10]
(b) Deduce how an increase in temperature will affect the feasibility of each of the processes in which
iron (II) oxide is reduced to iron.
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
……………………………………………………………………………………………………………
………………………………………………………………………………………………………..[5]
[Total15]
5(a) Calcium metal reacts with oxygen gas to form calcium oxide, CaO.
(i) Write an equation, including state symbols, to represent the lattice energy of calcium chloride, CaO.
................................................................................................................................................................ [2]
(ii) Draw a fully labelled Born-Haber cycle that could be used to calculate the lattice energy for calcium
oxide.
[8]
(b) Entropy is a measure of the disorder of a system. Describe and explain what happens to the entropy
of a gas when the temperature is increased.
....................................................................................................................................................................
....................................................................................................................................................................
................................................................................................................................................................ [2]
c) The table shows four reactions.
(i) For each reaction, predict the sign of the entropy change, ∆So . If you predict no entropy
change, write ‘no change’ in the table below. The first one has been done for you.
sign of ∆So
reaction
CO(g) + O2(g) → CO2(g)
negative
Mg(s) + O2(g) → MgO(s)
CuSO4(s) + 5H2O(l) → CuSO4.5H5O(s)
NaHCO3(s) + H+(aq) → Na+(aq) + CO2(g) + H2O(l)
[3]
(ii) Explain why the entropy change for the first process is negative.
..................................................................................................................................................................
….………………....................................................................................................................................... [1]
d) Calculate the standard entropy change, ∆So , for this reaction.
N2(g) + 3H2(g) → 2NH3(g)
Standard entropies, So , in JK–1mol–1 are given.
N2(g)
H2(g)
NH3(g)
+192
+131
+193
∆So ............ JK–1mol–1 [2]
(e)Whether or not a chemical reaction is spontaneous (feasible) can be deduced by calculating the
change in free energy, ∆Go , at a given temperature.
MgCO3(s) → MgO(s) + CO2(g)
∆Ho = +117kJmol–1
∆So = +175JK–1mol–1
(i) Calculate the value of ∆Go at 298K for the above reaction.
[2]
(iii) Use your answer to (i) to explain whether or not this reaction is spontaneous at 298K.
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........................................................................................................................................................
....................................................................................................................................................[1]
[Total 21]
BEST OF LUCK
AAOS CHEMISTRY TOPICAL TEST
(Seif-llah)