Chemistry 1050
LIQUIDS AND SOLIDS
Fall 2010
Text: Petrucci Herring Madura Bissonnette 10th Edition (Chapter 12)
Suggested text problems
Exercises: 1, 3, 5, 7, 15, 17, 21, 25, 39, 41, 43, 47, 51, 65
Text: Petrucci Harwood Herring Madura 9 th Edition (Chapter 12)
Exercises: 1, 3, 7, 11, 13, 15, 19, 21, 23, 33, 35, 37, 39, 43, 47, 49, 51, 53, 55, 57, 59, 61,
65
1.
Define or explain vapourization, condensation, vapour pressure, non-volatile liquid and
volatile liquid, viscosity and surface tension.
2.
Define boiling, normal boiling point and explain the effect of external pressure on
boiling point.
3.
The enthalpy of vaporization of liquid methanol is 38.0 kJ mol!1. The vapour pressure
of methanol at 5 °C is 40.0 mmHg. Estimate its normal boiling point.
4.
Methane has a normal boiling point of !164 °C and a vapour pressure of 42.8 atm at
!100 °C. Calculate the enthalpy of vaporization of liquid methane.
5.
Define or explain triple point, critical point, critical temperature, critical pressure,
vaporization curve, sublimation curve, fusion curve and supercritical fluid.
6.
The following questions refer to the phase diagram below.
(a)
Identify the regions (? on graph) where one stable phase exists e.g. solid, liquid
or gas (vapor).
(b)
Identify the sublimation, fusion, and vaporization curves.
(c)
Identify P c and Tc and the triple point.
(d)
Identify the changes that occur along the isotherm AB as one goes from
A to B.
(e)
Identify the changes that occur along the isobar CD as one goes from
C to D.
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Chemistry 1050
LIQUIDS AND SOLIDS
Fall 2010
(f)
Identify the changes that occur along the isobar EF as one goes from E to F.
(g)
Under what conditions of temperature can pressure be used to cause the vapor
phase to condense to a liquid? Under what temperature conditions is this
impossible?
(h)
What phase exists at point X?
(i)
Identify the normal melting and boiling points.
7.
Define the term hydrogen bond. Distinguish between the covalent bonding involving
hydrogen in water and hydrogen bonding involving water.
8.
The electronegativity of chlorine is slightly greater than that of nitrogen. Why is chlorine
not usually considered to form hydrogen bonds while nitrogen is known to form strong
hydrogen bonds?
9.
Describe the difference between a dipole-dipole interaction and a dispersion force.
Which interaction is present in every substance?
10.
Distinguish between polarizability and polarity. Describe how each influences the
intermolecular interactions between a pair of molecules.
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Chemistry 1050
LIQUIDS AND SOLIDS
Fall 2010
Determine the lattice energy for MgF2(s) from the following data: )H°f MgF 2 (s) =
11.
!1123 kJ mol!1, enthalpy of sublimation of Mg(s) = 148 kJ mol!1, bond dissociation
energy of F 2(g) = 159 kJ mol!1, I1 for Mg(g) = 738 kJ mol!1, I2 for Mg(g) = 1450
kJ mol!1 EA for F(g) = !328 kJ mol!1
12.
Pick the compound with the higher boiling point and give your reasoning:
(a) CO or N2 (b) NH3 or PH3 (c) CH3CH2OH or CH3OCH3 (d) CCl4 or CBr4 (e)
CH3CH2CH2CH3 or C 4H8 (the 4 carbons form a ring) (f) NaBr or HBr
12.
Consider the group VIIA hydrides,HF, HCl, HBr, HI
(a)
(b)
(c)
formula
Boiling point/°C
HF
19.5
HCl
!84.9
HBr
!67
HI
!35.4
Discuss the hydrides in terms of bond polarity and polarizability.
Explain why the boiling points increase from HCl to HI.
Explain why the boiling point of HF is much greater than the other hydrides
contrary to the trend of increasing boiling points from HCl to HI.
14.
Which substance do you expect to have the higher enthalpy of vaporization,
CH3COCH3 or CH3CH2CH2CH3? Explain briefly.
15.
Predict which type of solid (ionic, molecular, metallic or network covalent) is each of
the following:
16.
formula
T(fusion)
T(boiling)
other properties
PrI3
733°C
1377°C
liquid conducts electricity
OsF6
32°C
46°C
liquid does not conduct electricity
BN
not measured
not measured
sublimes at 3000°C
Hydrazine, NH2!NH2, is a liquid at room temperature while ethyl methyl ether,
CH3CH2!O!CH3, is a gas. Explain in terms of the relevant intermolecular forces.
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Chemistry 1050
LIQUIDS AND SOLIDS
Fall 2010
Predict which of the following ionic compounds will have the higher melting point and
17.
the greater solubility in water. Explain briefly.
(a) NaF(s) or KF(s) (b) CaS(s) or KBr(s)
18.
Which of the following gases would you expect to behave more like an ideal gas, CH4
or NH3? Explain briefly.
Answers
1.
vapourization:
Passage of molecules from the liquid state to the
gaseous or vapour state.
condensation:
Conversion of a gas or vapour to a liquid.
vapour pressure:
The pressure exerted by molecules of a substance in the
gaseous state (its vapour) in dynamic equilibrium with its
liquid at a given temperature.
non-volatile liquid: Liquid with a very low vapour pressure at room temperature e.g.
mercury.
volatile liquid:
Liquid with a high vapour pressure at room temperature
e.g. acetone.
surface tension:
energy or work needed to increase the surface area of a
liquid.
viscosity:
a liquid’s resistance to flow.
2.
boiling:
normal boiling point:
State in which bubbles of vapour form throughout a liquid
sample and escape with a pressure equal to the pressure
over the surface of the liquid.
Temperature at which the vapour pressure of a liquid is
equal to a standard atmosphere of pressure.
Effect of external pressure on boiling point. As the external pressure over the
surface of a liquid increases, the vapour pressure of the liquid required for boiling
increases. The temperature of the liquid must increase to provide the higher vapour
pressure. Hence boiling point increases.
3.
Boiling point = 339.0 K(65.8°C)
4.
9.29 kJ mol!1
5.
Triple point:
a specific temperature and pressure where 3 phases are
in equilibrium (usually solid, liquid and vapor).
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Chemistry 1050
Critical point:
Critical temperature:
Critical pressure:
Sublimation curve:
Vaporization curve:
Fusion curve:
Supercritical fluid:
6.
(c)
(d)
(e)
(f)
(g)
(h)
LIQUIDS AND SOLIDS
Fall 2010
temperature and pressure at which a liquid and vapor
become indistinguishable. It is the highest point on the
vaporization curve.
the temperature at the critical point.
the pressure at the critical point.
line on a phase diagram representing equilibrium
between solid and vapor.
line on a phase diagram representing equilibrium
between liquid and vapor. It shows how boiling point
changes with external pressure.
line on a phase diagram representing equilibrium
between solid and liquid. It shows how melting point
changes with external pressure.
The state of a substance at or above its critical
temperature.
Pc = critical pressure
Tc = critical temperature
vapor → liquid → solid
solid → liquid
solid → vapor
Increasing the pressure causes the vapor phase to condense to a liquid only
at temperatures below the critical temperature. It is not possible to do this
above the critical temperature.
a supercritical fluid exists at point X.
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In hydrogen bonding, a hydrogen forms a bridge between two very electronegative
atoms. The X!H.....X bond angle is usually very close to 180°C. The H has a
significant partial positive charge as a result of being covalently bonded to a very
electronegative atom (N, O, or F) and is attracted to the concentrated negative charge
on the small volume lone pairs of N, O, or F.
The O forms two electron pair (polar covalent) bonds, one to each H atom (short
bonds). The oxygen can bridge through its two lone pairs to H’s on two other water
molecules ( long H-bond).
8.
Chlorine doesn’t form effective hydrogen bonds due to the large volume of its 3 rd shell
lone pairs vs the small volume of nitrogen’s 2nd shell lone pair which allows a close
approach to a second molecule containing a hydrogen with a big M+ charge.
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Chemistry 1050
LIQUIDS AND SOLIDS
Fall 2010
A dipole-dipole attraction is the attraction between the permanent dipoles in polar
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molecules. Dispersion forces are present in non-polar and polar molecules alike and
are the only forces present in non-polar species. They are instantaneous dipoleinduced dipole forces in which temporary dipoles in two non-polar species attract one
another. In a polar species, dispersion forces temporarily strengthen the permanent
dipoles leading to stronger interactions.
10.
Polarizability refers to the tendency for charge separation to occur in a molecule or
atom. The dispersion forces in a sample of molecules, both polar and non!polar,
increase in strength with the polarizability of the molecule’s electrons. Polarity refers
to the degree of charge separation in a polar bond or polar molecule. It is best
represented for a molecule by its dipole moment. The more polar the molecule, the
greater its dipole moment.
11.
)H (lattice) = !2962 kJ molG1 lattice energy for MgF2(s) = +2962 kJ mol!1
12.(a) TB CO > TB N2 Both have 14 electrons and 2 atoms and have similar polarizability and
dispersion forces but CO is polar and has additional dipole-dipole forces to overcome.
(b) TB NH3 > TB PH3. PH3 has more electrons than NH3 (18 vs 10) and hence has greater
polarizability and dispersion forces to overcome. However, NH3 is more polar and can
form hydrogen bonding (N:----H!N). When comparing molecules which are not too
different in size, hydrogen bonding is the dominant force to overcome and this will give
NH3 the significantly higher boiling point.
(c)
TB CH3CH2OH > TB CH3OCH3. Both have 26 electrons and 9 atoms and hence have
similar polarizability and dispersion forces. CH3CH2OH is more polar (O!H is more
polar than O!C) but more importantly CH3CH2OH can form hydrogen bonding between
pairs of molecules and this represents a significantly greater force to overcome in
order to boil.
(d)
TB CBr4 > TB CCl4. Since they are both class AX4 and non-polar, dispersion forces are
the only ones to consider. CBr4 has more electrons (146 vs 74) and is more
polarizable and has greater dispersion forces to overcome in order to boil.
(e)
TB CH3CH2CH2CH3 > TB C4H8. Both are non-polar with nearly the same #electrons (34
vs 32) but different shapes. CH3CH2CH2CH3 has an elongated shape and greater
polarizability than the more compact ring shape of C4 H8 . Hence it has greater
dispersion forces to overcome in order to boil.
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Chemistry 1050
LIQUIDS AND SOLIDS
Fall 2010
(f) TB NaBr > TB HBr. NaBr forms an ionic crystal which always have greater forces to
overcome (i.e. cation-anion forces) than the dispersion and dipole-dipole forces of
HBr.
13.(a) Bond polarity decreases from HF to HI (as )EN decreases) but polarizability
increases from HF to HI due to increasing numbers of electrons. (10<18<36<54).
(b)
The boiling points increase due to the increase in polarizability and dispersion forces.
(c)
HF has the highest boiling point because only HF can form strong hydrogen bonds.
(HF:---H!F). The valence lone pairs of HCl, HBr and HI are too big.
14.
)Hva p CH3COCH3 [(CH3)2C=O] > )Hvap CH3CH2CH2CH3. CH3CH2CH2CH3 has
slightly more electrons (34 vs 32) and more atoms (14 vs 10) and hence is more
polarizable and will have greater dispersion forces. However the difference in
dispersion forces should not be great. CH3COCH3 is polar due to the C=O and it’s
additional dipole-dipole forces should give it the greater total forces to overcome and
the greater enthalpy of vaporization.
15.
PrI3 ionic (cation/anion forces are relatively strong to account for relatively high melting
and boiling points. An ionic liquid contains mobile ions which can conduct a current.
OsF6 molecular (low melting and boiling point characteristic of a molecular compound
with weak intermolecular forces and a non-conductor.)
BN network covalent (very high sublimation temperature indicative of very strong
intermolecular forces which are characteristic of network covalent solids.)
16.
At a given temperature, the phase of a substance is indicative of the relative strength
of intermolecular forces vs the average kinetic energy of the molecules. A gas
indicates weaker intermolecular forces than a liquid. The particles of a gas have
enough kinetic energy to overcome intermolecular forces but the particles of a liquid
do not, they have just enough energy to move around but not enough to escape and
form a vapour.
CH3 CH2!O!CH3 is a somewhat polar molecule (C!O!C part is V-shaped) with
significant non-polar fragments ( CH3CH2 and CH3). It has more electrons than (34 vs
18) stretched over more atoms (12 vs 6) and hence is more polarizable and has
greater dispersion forces. However, NH2!NH2 is extensively hydrogen-bonded and
since the two molecules are not largely different in size, hydrogen bonding give
NH2!NH2 the greater total intermolecular forces. At room temperature, molecules of
both have the same average kinetic energy. However such energy is not enough for
molecules of NH2!NH2 to totally overcome their intermolecular forces and form a
vapour but it does represent sufficient energy for molecules of CH3CH2!O!CH3 to
overcome their weaker intermolecular forces and form a vapour.
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Chemistry 1050
LIQUIDS AND SOLIDS
Fall 2010
+
17.(a) NaF has higher melting point and should be less soluble in water. K is a bigger ion
than Na+ hence the distance between the centres of + and ! charge is larger reducing
the cation!anion attraction and the lattice energy and melting point of KF vs NaF. The
lower lattice energy of KF makes it easier for water molecules to pull ions out of the
crystal to dissolve them increasing solubility.
(b) CaS has higher melting point and should be less soluble in water. K+ is a bigger ion
than Ca2+ and Br! is bigger than S2! hence the distance between the centres of + and
! charge is larger. The charges on the ions of KBr (+1 and !1) are lower than those
of CaS (+2 and !2). Both factors of internuclear distance and ion charges reduce the
cation!anion attraction, the lattice energy and melting point of KBr vs CaS. The lower
lattice energy of KBr makes it easier for water molecules to pull ions out of the crystal
to dissolve them increasing solubility.
16.
CH4 is the more ideal gas because it has weaker IMF’s. CH4 has only dispersion
forces but NH3 is about the same size and would have comparable dispersion forces.
It is also polar and has dipole!dipole forces and can form hydrogen bonding.
Developed by Dr. Chris Flinn
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