Thermochemistry IB questions
1. Ammonium nitrate, NH4NO3, dissolves readily in water according to the equation:
NH4NO3(s)  NH4+(aq) + NO3-(aq)
H = 28kJ mol-1
Which of the following contribute(s) to the occurrence of this process?
I.
II.
a.
b.
c.
d.
The system moves to lower enthalpy.
The system becomes more disordered.
I only
II only
Both I and II
Neither I nor II
2. Which substance has the largest lattice energy?
a.
b.
c.
d.
NaF
KCl
MgO
CaS
3. A certain reaction is spontaneous below 100oC but is non-spontaneous at higher
temperatures. Based on this information, what are the signs of H and S?
a.
b.
c.
d.
H
+
+
S
+
+
-
4. Which ionisation requires the most energy?
a.
b.
c.
d.
Na(g)  Na+(g) + eNa+(g)  Na2+(g) + eMg(g)  Mg+(g) + eMg+(g)  Mg2+(g) + e-
5. The bond enthalpies in kJ mol-1 for several bonds are given below:
H – H 436
O – O 196
O = O 496
H – O 463
What is the enthalpy change, H, in kJ for the reaction below?
2 H2(g) + O2(g)  2 H2O(g)
a.
b.
c.
d.
442
92
- 484
- 834
6. At 0oC, the mixture formed when the following reaction reaches equilibrium consists
mostly of N2O4(g)
2 NO2(g)  N2O4(g)
What are the signs G, H, S at this temperature?
a.
b.
c.
d.
G
+
+
H
+
+
+
S
+
+
-
7. Excess thionyl chloride, SOCl2, can be removed from a reaction mixture by reacting
it with water according to the equation:
SOCl2(l) + H2O(l)  2 HCl(g) + SO2(g)
Use the following data to calculate the Ho for this reaction.
Hof
a.
b.
c.
d.
-1
(kJ mol )
SOCl2(l)
-245.6
H2O(l)
-285.8
HCl(g)
-92.3
SO2(g)
-296.8
–142.3
–50.0
+50.0
+142.3
8. 200 J of energy were given to a 10 g sample of copper. If the temperature of the
copper is increased by 50oC, what is the specific heat capacity of the copper?
a.
b.
c.
d.
0.25 J g-1 oC-1
0.40 J g-1 oC-1
2.5 J g-1 oC-1
4.0 J g-1 oC-1
9. Which of the changes below occurs with the greatest increase in entropy?
a. Na2O(s) + H2O(l)  2 Na+(aq) + 2 OH-(aq)
b. NH3(aq) + HCl(g)  NH4Cl(s)
c. H2(g) + I2(g)  2HI(g)
d. C(s) + CO2(g)  s CO(g)
10. For the reaction:
6 HCCH(g)  C6H6(g)
Ho = -597.3 kJ and So = -0.33 kJ K-1. This reaction
a.
b.
c.
d.
is spontaneous at 300 K and becomes non-spontaneous at higher temperatures
is spontaneous at 300 K and becomes non-spontaneous at lower temperatures
is non-spontaneous at 300 K and becomes spontaneous at higher temperatures
is non-spontaneous at 300 K and becomes spontaneous at lower temperatures
11. What types of hybridization of the carbon atoms in the compound
1 2
3
H2ClC–CH2–COOH
a.
b.
c.
d.
1
sp2
sp3
sp3
sp3
2
sp2
sp2
sp3
sp3
3
sp2
sp
sp2
sp
12. In which of the following pairs does the second substance have the lower boiling
point?
a.
b.
c.
d.
F2, Cl2
H2O, H2S
C2H6, C3H8
CH3OCH3, CH3CH2OH
13. Which substance exhibits only ionic bonding?
a.
b.
c.
d.
NaNO3
H2SO4
NH4Cl
MgBr2
14. Which molecule or ion does NOT have a tetrahedral shape?
a.
b.
c.
d.
XeF4
SiCl4
BF4
NH4+
15. When the substances below are arranged in order of increasing carbon-carbon bond
length (shortest bond first), what is the correct order?
I.
II.
III.
a.
b.
c.
d.
H2CCH2
H3CCH3
Benzene
I < II < III
I < III < II
II < I < III
III < II < I
16. What type(s) of intermolecular forces is/are present in CH3OCH3?
a.
b.
c.
d.
dipole-dipole, hydrogen bonds and van der Waals’
dipole-dipole and van der Waals’ only
hydrogen bonds and van der Waals’ only
van der Waals’ only
17. In which of the following is there at least one double bond?
I.
II.
III.
a.
b.
c.
d.
O2
CO2
C2H4
I only
III only
II and III only
I, II and III
18. According to VSEPR theory, which molecule would be expected to have the smallest
bond angle?
a.
b.
c.
d.
H2O
H2CO
CH4
NH3
19. Which of the following can exist in both polar and non-polar forms?
a.
b.
c.
d.
CH2Cl2
C2HCl
C2H2Cl2
C2H3Cl
20. What are the states of hybridization for the carbon atoms in NCCH2COOH
a.
b.
c.
d.
CN
sp
sp
sp2
sp2
CH2
sp3
sp2
sp2
sp3
COOH
sp2
sp3
sp3
sp2
Long Answer
21.
a. In hydrogen chloride, the hydrogen is joined to the chlorine by a single covalent
bond. Describe this bond in terms of electrons involved, the energy state, and the
polarity of the bond (4 marks)
b. The electronic structure of the carbon atom is 1s22s22p2. With the aid of this
information, answer the following (7 marks)
i.
Carbon typically forms 4 bonds. Explain this in terms of bonding and
energy.
ii.
Why is there only on CH3Cl even though s and p electrons from the
carbon are used in the bonding?
iii.
CH3Cl which contains on C-Cl bond is polar, but CCl4 which contains 4
such bonds is non-polar. Explain.
c. Describe the bonding in ethane and ethene. Use this description I discussing the
relative reactivities of the two compounds. (8 marks)
d. The shapes of the molecules are related to the bonding in the molecules. Starting
with an electron dot (Lewis) structure, deduce the shape of the POCl3 molecule.
Suggest why the Cl-P-Cl bond angle is approximately 103oC. (6 marks)
22.
a. Draw the Lewis structure of carbon dioxide. (1 mark)
b. Specify the hybridization of the carbon atom in carbon dioxide. (1 mark)
c. Describe and explain the shape of a molecule of sulfur dioxide. (2 marks)
d. Sulfur dioxide has a boiling point of 263 K whereas carbon dioxide boils at 195
K. Give two reasons for this difference. (2 marks)
e. Carbon dioxide, the ethanoate ion, CH3COO-, and the ethoxide ion, C2H5O-, all
contain at least one bond between carbon and oxygen. In which structure will the
carbon to oxygen bond length be in between those found in the other two
structures? Explain your answer. (3 marks)
Answers
1.
2.
3.
4.
5.
b
c
a
b
d
6. a
7. c
8. b
9. d
10. a
11. c
12. b
13. d
14. a
15. b
16. b
17. d
18. a
19. c
20. a
21.
I. Covalent bond, therefore an e- pair (sharing)
II. 1s e- for H and 3p e- from Cl
III. more stable as bond (than as separate atoms)
IV. polar as Cl is more EN than H
a.
1 mark
1 mark
1 mark
1 mark
Can receive marks for I and II by drawing Lewis dot structure
I. 2s22p2  2s12p3 or 1 e- promoted  2s12p3
1 mark
II. 4 unpaired e-, 4 bonds
1 mark
III. extra stability form 2 extra bonds
1 mark
Or less energy required to promote an e- compared to energy from the formation
of 2 bonds
b. i.
ii.
iii.
c.
sp3 hybridization- one s and three p to form 4
equivalent sp3 hybrid orbitals
2 marks
I. CCl4 has equal e- distribution
II. CH3Cl has unequal e- distribution
1 mark
1 mark
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
X.
ethane is sp3
C-C single bond
Overlap of orbitals
Considerable energy to break, unreactive
C2H4 sp2
1, 1 
 bond overlap of p orbitals
 weaker than ,  more reactive
high e- density I bond, electrophilic addition reaction
any other relevant point
1 mark
1 mark
1 mark
1 mark
1 mark
1 mark
1 mark
1 mark
1 mark
1 mark
any 8 of the above 10 for max 8 marks
d.
I.
II
III.
IV.
V.
O=P-(Cl)3
2 marks,
1 mark if lone pairs missing
4 negative charges/e- centers/e- densities
1 mark
tetrahedral
1 mark
P=O greater repulsion
1 mark
<109 as repulsion
1 mark
22.
a.
b.
c.
d.
e.
O=C=O
1 mark
sp
1 mark
bent or V shaped
1 mark
contains 3 centers of e- charge around the
central atom (two bonding, one non-bonding)
1 mark
Mr SO2 = 32+32=64; Mr CO2 = 44
1 mark
(or SO2 is a larger molecule)
SO2 is polar whereas CO2 is non-polar
1 mark
(both factors will increase strength of intermolecular
bonding and therefore boiling point)
CH3COO- contains C-O bond of intermediate bond
length (1 ½ bond order)
1 mark
CH3CH2O contains single bond (bond order of 1) 1 mark
Shown resonance structures of CH3COO1 mark