Topic 5 Test Energetics IB Chemistry 3/6/17
1.
[30 marks]
Which equation represents the standard enthalpy of formation of liquid methanol?
A.
C(g) + 2H2 (g) + 12 O2 (g) → CH3 OH(l)
B.
C(g) + 4H(g) + O(g)→ CH3 OH(l)
C.
C(s) + 4H(g) + O(g) → CH3 OH(l)
D.
C(s) + 2H2 (g) + 12 O2 (g) → CH3 OH(l)
[1 mark]
Markscheme
D
Examiners report
This was a fair question with half the students answering it correctly. It was a definition question on the standard
enthalpy of formation of liquid methanol but almost 40% of the candidates selected choice A where carbon is given in
the gaseous state, not its solid under standard conditions.
2.
A simple calorimeter was set up to determine the enthalpy change occurring when one mole of ethanol is
combusted. The experimental value was found to be
K and 1.01 × 105
−867 kJ mol
−1
. The Data Booklet value is −1367
Pa).
During the experiment some black soot formed.
Which statements are correct?
I.
The percentage error for the experiment can be calculated as follows:
(1367 − 867) × 100%
II.
The difference between the two values may be due to heat loss to the surroundings.
III.
The black soot suggests that incomplete combustion occurred.
A.
I and II only
B.
I and III only
C.
II and III only
D.
I, II and III
Markscheme
C
Examiners report
[N/A]
kJ mol
[1 mark]
−1
(at 298
3.
Consider the following two equations.
2Ca(s) + O2 (g) → 2CaO(s)
ΔH Θ
[1 mark]
= +x kJ
ΔH Θ = +y kJ
Ca(s) + 0.5O2 (g) + CO2 (g) → CaCO3 (s)
What is ΔH Θ , in kJ, for the following reaction?
CaO(s) + CO2 (g) → CaCO3 (s)
A.
y − 0.5x
B.
y−x
C.
0.5 − y
D.
x−y
Markscheme
A
Examiners report
There was concern about the use of algebraic notation rather than actual numerical data. This has been used since
November 2010 so candidates should be familiar with this type of question. (In fact, some G2s in the past have
suggested it would be better to use algebraic notation!) In the event it was the sixth easiest question; over 81% of
candidates gave the correct answer and 9% gave B.
4.
Enthalpy changes of reaction are provided for the following reactions.
2C(s) + 2H2 (g) → C2 H4 (g)
ΔH Θ = +52 kJ mol−1
2C(s) + 3H2 (g) → C2 H6 (g)
ΔH Θ = −85 kJ mol−1
What is the enthalpy change, in kJ mol
−1
, for the reaction between ethene and hydrogen?
C2 H4 (g) + H2 (g) → C2 H6 (g)
A.
–137
B.
–33
C.
+33
D.
+137
Markscheme
A
Examiners report
[N/A]
[1 mark]
The Born-Haber cycle for MgO under standard conditions is shown below.
The values are shown in the table below.
5a. Identify the processes represented by A, B and D in the cycle.
[3 marks]
Markscheme
A: sublimation/atomization;
B: atomization/half dissociation enthalpy;
D: (sum of 1st and 2nd) electron affinity;
Do not accept vaporization for A and B.
Accept ΔHAT ⋅ or
ΔHEA .
Examiners report
This was the second most popular question and in general candidates demonstrated a good understanding of the Born
Haber cycle. Some candidates identified the process A as vaporization instead of atomization.
5b.
Define the enthalpy change, F.
[2 marks]
Markscheme
enthalpy change when one mole of the compound is formed from its elements (in their standard states);
under standard conditions / 25 °C/298 K and 1 atm/101.3
kPa/1.01 × 105 Pa;
Examiners report
Most candidates correctly stated the definition of enthalpy change of formation although some omitted to specify the
standard conditions.
5c.
Determine the value of the enthalpy change, E.
[2 marks]
Markscheme
−602 = 150 + 248 + 2186 + 702 + E;
−3888 (kJ mol−1 );
Do not allow 3889 (given in data booklet).
Allow 3888 (i.e no minus sign).
Award [2] for the correct final answer.
Examiners report
The majority of candidates correctly calculated the lattice enthalpy value.
5d.
Define the enthalpy change C for the first value. Explain why the second value is significantly larger than the first.[4 marks]
Markscheme
energy required to remove one electron;
from an atom in its gaseous state;
electron removed from a positive ion;
decrease in electron-electron repulsion / increase in nucleus-electron attraction;
Examiners report
The definition of the first ionization energy was stated correctly by most candidates but in a few cases the term
gaseous state was missing.
5e.
The inter-ionic distance between the ions in NaF is very similar to that between the ions in MgO. Suggest with a
reason, which compound has the higher lattice enthalpy value.
Markscheme
MgO;
double ionic charge / both ions carry +2 and –2 charge/greater charge compared to +1 and –1;
Examiners report
The compound with higher lattice enthalpy was correctly identified including the reason.
[2 marks]
5f.
The standard enthalpy change of three combustion reactions is given below in kJ.
2C2 H6 (g) + 7O2 (g) → 4CO2 (g) + 6H2 O(l)
ΔH Θ = −3120
2H2 (g) + O2 (g) → 2H2 O(l)
ΔH Θ = −572
C2 H4 (g) + 3O2 (g) → 2CO2 (g) + 2H2 O(l)
ΔH Θ = −1411
[4 marks]
Based on the above information, calculate the standard change in enthalpy, ΔH Θ , for the following reaction.
C2 H6 (g) → C2 H4 (g) + H2 (g)
Markscheme
(C2 H6 (g) + 3 12 O2 (g) → 2CO2 (g) + 3H2 O(l))
ΔH Θ = −1560;
(H2 O(l) → H2 (g) + 12 O2 (g))
ΔH Θ = +286;
(2CO2 (g) + 2H2 O(l) → C2 H4 (g) + 3O2 (g))
ΔH Θ = +1411;
(C2 H6 (g) → C2 H4 (g) + H2 (g))
ΔH Θ = +137 (kJ);
Allow other correct methods.
Award [2] for –137.
Allow ECF for the final marking point.
Examiners report
The majority of candidates manipulated the thermo-chemical equations and calculated the correct answer of +137 kJ
although some reversed the sign.
Θ
5g. Predict, stating a reason, whether the sign of ΔS for the above reaction would be positive or negative.
[2 marks]
Markscheme
positive;
increase in number of moles of gas;
Examiners report
[N/A]
5h. Using bond enthalpy values, calculate
ΔH Θ for the following reaction.
[3 marks]
C2 H6 (g) → C2 H4 (g) + H2 (g)
Markscheme
Bonds broken (1C–C, 6C–H, or 1C–C, 2C–H) = 2825/1173;
Bonds made (1C=C, 1H–H, 4C–H) = 2700/1048;
+125 (kJ);
Allow 125 but not –125 (kJ ) for the final mark.
Award [3] for the correct final answer.
Examiners report
The bond enthalpy calculation had the usual mistakes of using the wrong value from the data booklet, bond making
minus bond breaking and –125 kJ instead of +125 kJ.
5i.
Suggest with a reason, why the values obtained in parts (b) (i) and (b) (iv) are different.
[1 mark]
Markscheme
bond enthalpy values are average values;
Examiners report
[N/A]
6.
Which equation best represents the bond enthalpy of HCl?
+
[1 mark]
−
A.
HCl(g) → H (g) + Cl (g)
B.
HCl(g) → H(g) + Cl(g)
C.
HCl(g) → 12 H2 (g) + 12 Cl2 (g)
D.
2HCl(g) → H2 (g) + Cl2 (g)
Markscheme
B
Examiners report
[N/A]
7.
Use the average bond enthalpies below to calculate the enthalpy change, in kJ, for the following reaction.
H2 (g) + I2 (g) → 2HI(g)
A.
+290
B.
+10
C.
–10
D.
–290
Markscheme
C
Examiners report
One G2 comment stated that the question should have asked for the enthalpy change in the reaction in units of
kJ mol−1 instead of kJ. This aspect of units has been discussed in detail in several previous subject reports and
teachers are referred to these reports for further reference.
[1 mark]
8.
A student measured the temperature of a reaction mixture over time using a temperature probe. By considering [1 mark]
the graph, which of the following deductions can be made?
I.
The reaction is exothermic.
II.
The products are more stable than the reactants.
III.
The reactant bonds are stronger than the product bonds.
A.
I and II only
B.
I and III only
C.
II and III only
D.
I, II and III
Markscheme
A
Examiners report
There were two G2 comments on this question, both suggesting that the graph given was confusing to candidates. In
this question candidates had to use a combination of ideas to ascertain that the correct answer is A, namely I. and II.
From the graph shown, candidates need to realise that the reaction is exothermic, and therefore from this information,
the products are more stable than the reactants. 55% of candidates got the correct answer.
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