Topic 15 past paper questions
1. (M01)
(a) (i) Define the term standard enthalpy of formation.
[2]
(ii) Write an equation, including state symbols, to show the formation of propane.
(b) (i) State what is meant by the term average bond enthalpy. Use the average bond enthalpies,
provided in the Data Booklet (Table 10), to calculate the enthalpy change for the following
reaction:
C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O(g)
[1]
[5]
(ii) Suggest, with a reason, whether the entropy change (∆S ÷ ) for the reaction would be positive or
negative.
[2]
2. (M00) The enthalpy diagram for a reaction between two aqueous solutions at room temperature is
shown below
products
enthalpy
reactants
(a) (i) Give the sign of H for this reaction and indicate whether it is endothermic or exothermic. State the
relative strengths of the bonds in the products and reactants.
[3]
(ii) This reaction is spontaneous at room temperature. Use this information along with that in (i) to
give signs of G and S, outlining your reasoning in each case. Comment on the meaning of
the sign of S in terms of the relative disorder of the reactants and products.
[5]
(iii) Describe an experiment that could be conducted in a school laboratory to determine the value of
H (in kJ mol-1) for this reaction. Show the calculations that would need to be carried out to obtain
a value of H.
[6]
(iv) Outline two sources of error in the experimental procedure that could result in a calculated H
value that is smaller than the accepted value and suggest a way that one of these could be
minimized.
[3]
(v) Discuss how the spontaneity of this reaction would change as the temperature is increased from
room temperature, and also as the temperature is decreased from room temperature. Outline
your reasoning in each case.
[4]
(b) Assume the reaction in (a) can be represented as
2A2 + B2  2 A2B
Derive an expression that could be used to find H reaction from the bond energies involved.
Topic 15 past paper questions
[2]
1
3. (N00) When solid blue copper(II) sulfate pentahydrate, CuSO4. 5H2 , loses water the white solid,
copper(II) sulfate monohydrate, CuSO4.H2O , is produced as represented by the following equation:
CuSO4. 5H2O (s) 
CuSO4.H2O (s) + 4H2O (g)
The thermodynamic data for the substances involved in the reversible process are:
∆Hf Ö / kJmol-1
CuSO4. 5H2O (s)
CuSO4.H2O (s)
H2O (g)
−2278
−1084
−242
S Ö / J K-1 mol-1
305
150
189
(a) (i) Name and define the terms ∆HfÖ and S Ö and explain the symbol Ö.
[5]
(ii) Explain why, in the case of S Ö, the symbol ‘∆’ is not included.
[1]
(iii) What is the ∆HfÖ value of elemental copper?
[1]
(b) (i) Calculate the value ∆HÖ of for the above reaction and state what information the sign of ∆HÖ
provides about this reaction.
[4]
(ii) Calculate ∆SÖ for the reaction and state the meaning of the sign of ∆SÖ obtained.
[4]
(iii) Identify a thermodynamic function that can be used to predict reaction spontaneity and state its
units.
[2]
(c) (i) Use the values obtained in (b) above to determine if the following reaction is spontaneous or nonspontaneous at 25C:
CuSO4. 5H2O (s) 
CuSO4.H2O (s) + 4H2O (g)
Identify which compound CuSO4. 5H2O (s) or CuSO4.H2O (s) is more stable at 25C.
[ 5]
(ii) Use the values obtained in (b) to determine the Centigrade temperature above which the other
compound in (c) (i) is more stable.
[3]
(iii) Suggest, with a reason, whether the entropy, S, change for the reaction would be positive or
negative.
[2]
4. (M04)
(a) Explain in terms of G, why a reaction for which both H and S are positive is sometimes
spontaneous and sometimes not.
(b) Consider the following reaction.
[4]
N2 (g) + 3H2 (g)  2NH (g)
(i) Using the average bond enthalpy values in Table 10 of the Data Booklet, calculate the standard
enthalpy change for this reaction.
[4]
(ii) The absolute entropy values, S, at 300 K for N2 (g), H2 (g) and NH3 (g) are 193, 131 and 192
respectively. Calculate S for the reaction and explain the sign of S.
(iii) Calculate G for the reaction at 300 K.
Topic 15 past paper questions
[3]
[1]
2
(iv) If the ammonia were produced as a liquid and not as a gas, state and explain the effect this would
have on the value of H for the reaction.
[2]
(c) Define the term standard enthalpy of formation, and write the equation for the standard enthalpy of
formation of ethanol.
[5]
(d) Bond enthalpies are tabulated as average bond enthalpies. Explain what this term means.
[2]
(e) Enthalpies of reactions, for example combustion, can be calculated using average bond enthalpies or
enthalpies of formation. The two methods give closer results for cyclohexane than they do for benzene.
Explain this difference.
[4]
5.
substance
Mg(NO3)2.6H2O (s)
Mg(NO3)2 (s)
MgO (s)
H2O (g)
NO (g)
NO2 (g)
substance
N2
O2
Hf in kJ mol-1
-2612
-790
- 602
-242
+ 90
+ 33
Bond enthalpy in kJ mol-1
944
496
(a) Magnesium nitrate hexahydrate, Mg(NO3)2.6H2O, can be decomposed by heating gently.
(i) Write a balanced equation for this reaction.
(ii) Calculate the enthalpy change for the reaction in (a) (i).
[1]
[3]
(b) When magnesium nitrate is heated strongly, it decomposes as follows:
2Mg(NO3)2(s)  2MgO (s) + 4NO2 (g) + O2 (g)
Calculate the enthalpy change for this reaction.
[5]
(c) (i) Calculate the enthalpy of the nitrogen-oxygen bond in the NO molecule. Use the bond enthalpies
for N2, O2 and the enthalpy of formation of NO.
[2]
(ii) What does the value calculated in (c) (i) imply about the nature and strength of the nitrogenoxygen bond?
[1]
Topic 15 past paper questions
3
6. (M95) The Born-Haber cycle below represents the enthalpy changes for the formation of an alkali metal
halide, MX, from an alkali metal, M, and a halogen, X2.
M+ (g) + e + X (g)
E
H3
H4
M+ (g) + X- (g)
M (g) + X (g)
M (g) + ½ X (g)
H2
H1
H5
M (s) + ½ X (g)
Hf
MX (s)
(a) (i) Identify the alkali metal that has the largest value for H3.
[1]
(ii) Identify the halogen that has the largest value for H2.
[1]
(b) (i) Identify the halogen that has the smallest value for H4.
[1]
(ii) Name the property represented by H5 and identify the alkali metal halide that has the smallest
value for this enthalpy change.
[2]
(c)
(i) Discuss the origin of H5.
[5]
(ii) Identify TWO factors that affect H5 and describe the effects of changes in magnitude of each
of these factors.
[4]
(d) Calculate the value of H5 for NaCl from the following data:
[2]
H (in kJ mol-1)
Na (s)  Na (g)
109
+
Na (g)  Na + e
494
Cl2 (g)  2 Cl (g)
242
Cl (g) + e  Cl- (g)
-360
Na (s) + ½ Cl2 (g)  NaCl (s)
-411
7.
Methane, CH4, the main constituent of natural gas burns by combining with oxygen tom produce
carbon dioxide and water according to equation (I)
CH4 (g) + 2O2 (g)  2H2O (l) + CO2 (g)
(I)
(a) Use the following information to calculate H in kJ mol-1 for the combustion of methane to form
carbon dioxide and water.
[2]
Topic 15 past paper questions
4
CH4 (g) + O2 (g)  CH2O (g) + H2O (g)
CH2O (g) + O2 (g)  CO2 (g) + H2O (g)
H2O (l)  H2O (g)
H = -284 kJ
H = -518 kJ
H = + 44.0 kJ
(b) The standard enthalpies of formation, Hf, for CH4 (g), O2 (g) and H2O (g) are –74.9, 0.0 and –
241.8 kJ mol-1 respectively.
[3]
(i) Explain why Hf for O2 (g) is 0.0.
(ii) Calculate Hf for CH2O (g)
(c) Predict the sign of H for the reaction in equation (I) and give your reasoning.
[2]
(d) Predict whether the reaction in equation (I) will become more or less spontaneous as the
temperature is increased. Explain.
[2]
8. (N04) The equation for the decomposition of calcium carbonate is given below.
CaCO3 (s)  CaO (s) + CO2 (g)
At 500 K, H for this reaction is +177 kJ mol-1 and S is + 161 J K-1 mol-1.
(a) Explain why H for the reaction above cannot be described as Hf .
[2]
(b) State the meaning of the term S.
[1]
(a) Calculate the value of G at 500 K and determine, giving a reason, whether or not the reaction will be
spontaneous.
[3]
9. (M99)
substance
C(s)
CO2(g)
H2(g)
H2O (l)
O2 (g)
C3H7COOH (l)
Standard enthalpy of formation,
Hf in kJ mol-1
0.0
-393.5
0.0
-285.9
0.0
Standard entropy, Sf in J
K-1 mol-1
5.7
213.6
130.6
69.9
205.0
226.3
The enthalpy change for the combustion of butanoic acid at 25C is –2183.5 kJ mol-1. The
combustion reaction is:
C3H7COOH (l) + 5 O2 (g)  4CO2 (g) + 4H2O (l)
(a) Write the balanced equation for the formation of butanoic acid from its elements.
[1]
(b) Using the data above, calculate the standard enthalpy of formation, Hf, for butanoic acid.
[3]
(c) Calculate the standard entropy change, Sf, for the formation of butanoic acid at 25C.
[3]
(d) Calculate the standard free energy of formation, Gf, for butanoic acid at 25C.
[2]
(e) Is the reaction spontaneous at 25C? Explain your answer.
Topic 15 past paper questions
[1]
5
Answers
1.
Topic 15 past paper questions
6
2.
.
Topic 15 past paper questions
7
3. (N00)
Topic 15 past paper questions
8
Topic 15 past paper questions
9
4. (M04)
5. (a) (i) Mg(NO3)2 .6H2O (s)  Mg(NO3)2 (s) + 6H2O (g)
(ii) + 370 kJ mol-1
(b)
H = + 508 kJ mol-1
(c ) N2 (g) + O2 (g)  2 NO (g)
Topic 15 past paper questions
H = + 90
x = 675 kJ mol-1
10
6. (a) (i) Li
(ii) Cl
(b) (i) I (or At)
(ii) CsI
(c) (i)
o energy needed to break
o 1 mole of lattice
o into gases ions
(ii) size and charge
(d) + 775 kJ mol-1
7. (a) CH4
CH2O
+
O2 (g)
+

CH2O +

O2 (g)
H2O (g)
CO2 +
H2O(lg
2H2O (g)

CH4 (g) +
2O2 (g)  2H2O (l) + CO2 (g)
H = - 284 kJ
H = - 518 kJ
H = -88 kJ
2H2O(l)
H = -890 kJ
(b) (i) O2 element in standard state
(ii) - 117.1 kJ mol-1

(c) S = negative ; from 3 moles of gas to 2 moles of gas and 1 mole of liquid
(d) G = Hsystem - TSsys
= - - (-)
= - + less spontaneous
8. (N04)
Topic 15 past paper questions
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9. (M99)
Topic 15 past paper questions
12