intermolecular forces and liquids and solids 1 why do some solids dissolve in water but others do...
TRANSCRIPT
• Why do some solids dissolve in water but others do not?
• Why are some substances are gaseous at room temperature, but others are liquid or solid?
• What gives metals the ability to conduct electricity, what makes non-metals brittle?
• The answers have to do with …
Intermolecular forcesIntermolecular forces
Questions
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Why mixtures mixWhy mixtures mix• Consider a glass of wine. Why do alcohol, water,
& pigment mix together?• There must be attractive forces.
Intramolecular forces occur between atoms
Intermolecular forces occur between molecules
• The factors that determine solubility are the strength of IMFs and speed of molecules.
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Intermolecular Forces
11.2
Intermolecular forces are attractive forces between molecules.
Intramolecular forces hold atoms together in a molecule.
Intermolecular vs Intramolecular
• 41 kJ to vaporize 1 mole of water (inter)
• 930 kJ to break all O-H bonds in 1 mole of water (intra)
Generally, intermolecular forces are much weaker than intramolecular forces.
“Measure” of intermolecular force
boiling point
melting point
Hvap
Hfus
Hsub
4
The strengths of intermolecular forces are generally weaker than either ionic or covalent bonds.
16 kJ/mol (to separate molecules)
431 kJ/mol (to break bond)
++-
-
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Types of Intermolecular Forces
3. Dipole-Dipole Forces
Attractive forces between polar molecules
Orientation of Polar Molecules in a Solid
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Types of intermolecular forces (between neutral molecules):
Dipole-dipole forces: (polar molecules)
SO O.. ::
....
:
+
--
SO O.. ::
....
:
+
--
dipole-dipole attraction
What effect does this attraction have on the boiling point?13
Polar molecules have dipole-dipole attractions for
one another.
+HCl----- +HCl-
dipole-dipole attraction
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Types of Intermolecular Forces4. Dispersion Forces – van der Walls forces/London forces (weakest)
Attractive forces that arise as a result of temporary dipoles induced in atoms or molecules
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Dispersion ForcesOccur between every compound and arise from the net attractive forcesamount molecules which is produced from induced charge imbalances
The larger the molecule the greater it’s Dispersion Forces are.
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London forcesLondon forces• Non-polar molecules do not have dipoles like
polar molecules. How, then, can non-polar compounds form solids or liquids?
• London forces are named after Fritz London (also called van der Waal forces)
• London forces are due to small dipoles that exist in non-polar molecules
• Because electrons are moving around in atoms there will be instants when the charge around an atom is not symmetrical
• The resulting tiny dipoles cause attractions between atoms/molecules
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London Dispersion Forces
• Non - polar molecules also exert forces on each other.
• Otherwise, no solids or liquids.• Electrons are not evenly distributed at
every instant in time.• Have an instantaneous dipole.• Induces a dipole in the atom next to it.• Induced dipole- induced dipole
interaction.
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“electrons are shifted to overload one side of an atom or molecule”.
London dispersion forces: (instantaneous dipole moment)( also referred to as van der Waal’s forces)
+ +- -
attraction
London Dispersion Forces
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London forcesLondon forces
Instantaneous dipole: Induced dipole:
Eventually electrons are situated so that tiny dipoles form
A dipole forms in one atom or molecule, inducing a
dipole in the other20
polarizability: the ease with which an atom or molecule can be distorted to have an instantaneous dipole.
In general big moleculesare more easily polarized
than little ones.
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Halogen Boiling Pt (K)
Noble Gas Boiling Pt (K)
F2 85.1 He 4.6
Cl2 238.6 Ne 27.3
Br2 332.0 Ar 87.5
I2 457.6 Kr 120.9
Which one(s) of the above are most polarizable?Hint: look at the relative sizes.
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Intermolecular Forces4. Dispersion Forces Continued
Polarizability is the ease with which the electron distribution in the atom or molecule can be distorted.
Polarizability increases with:
• greater number of electrons
• more diffuse electron cloud
Dispersion forces usually increase with molar mass.
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SO
O
What type(s) of intermolecular forces exist between each of the following molecules?
HBrHBr is a polar molecule: dipole-dipole forces. There are also dispersion forces between HBr molecules.
CH4
CH4 is nonpolar: dispersion forces.
SO2
SO2 is a polar molecule: dipole-dipole forces. There are also dispersion forces between SO2 molecules.
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Solubility and Intermolecular Forces
• Like dissolves like– Polar solutes dissolve in polar solvents
– Nonpolar solutes dissolve in nonpolar solvents
• Molecules with similar intermolecular forces will mix freely
• Molecules that can form H bondings will have greater solubility in water.
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Ionic Solute with Polar Solvent
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The hydration enthalpy is a measure of the strength of the interaction of the water molecule w/ the ion.
• Hydration enthalpy is the energy change when one mole of gaseous ions is converted to one mole of hydrated ions.
• It increases w/ the charge on the ion and decreases w/ the size of the ions. Therefore, it’s greater for cations than the anions since they’re smaller in size and the water molecules can give a better packing because of its bent geometry.e.g hydration enthalpy is great for Al3+ .
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Example:• Compare the solubilities of
a)ethanoic acid, ethanol and propane.
b)Propanone and HI d) ethanoic acid and pentanoic acid
c)
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4-nitrophenolb.p=279°C
Forms H bond mainly w/ other 4-nitrophenol
molecules
2-nitrophenolb.p=216°C
Forms H bond mainly intra-molecularly
H bonding
A crystalline solid possesses rigid and long-range order. In a crystalline solid, atoms, molecules or ions occupy specific (predictable) positions.
An amorphous solid does not possess a well-defined arrangement and long-range molecular order.
A unit cell is the basic repeating structural unit of a crystalline solid.
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Properties of Ionic Compounds
1. Crystalline solids - a regular repeating arrangement of ions in the solid:
– Ions are strongly bonded together.– Structure is rigid.
2. High melting points• Coordination number- number of ions of
opposite charge surrounding it
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- Page 198
Coordination Numbers:
Both the sodium and chlorine have 6
Both the cesium and chlorine have 8
Each titanium has 6, and each oxygen has 3
NaCl
CsCl
TiO2
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Do they Conduct Electricity?
• Conducting electricity means allowing charges to move.
• In a solid, the ions are locked in place.• Ionic solids are insulators.• When melted, the ions can move around.3. Melted ionic compounds conduct.
– NaCl: must get to about 800 ºC.– Dissolved in water, they also conduct
(free to move in aqueous solutions)
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- Page 198
The ions are free to move when they are molten (or in aqueous solution), and thus they are able to conduct the electric current.
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Ionic solids are brittle
+ - + -
+- +-+ - + -
+- +-
• Strong Repulsion breaks a crystal apart, due to similar ions being next to each other.
Force
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Types of CrystalsCovalent (Network or macromolecular) Crystals – e.g.
Silica(SiO2),SiC, BN , Si.Stronger than IM forces.• Held together by covalent bonds• Hard, high melting point• Poor conductor of heat and electricity
diamondgraphitecarbon
atoms
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This p bond overlap forms a huge p bonding network.
• Electrons are free to move through out these delocalized orbitals.
• The layers slide by each other.
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• Take a look at the tables on p. 129, 133, and 134.
• Allotropes are different forms of an element that exist in the same physical state. e.g. C( graphite, fullerene, graphite), oxygen (O2 and O3)
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The giant covalent structure of silicon dioxide
• There are three different crystal forms of silicon dioxide. The easiest one to remember and draw is based on the diamond structure.
• Crystalline silicon has the same structure as diamond. To turn it into silicon dioxide, all you need to do is to modify the silicon structure by including some oxygen atoms.
• Notice that each silicon atom is bridged to its neighbours by an oxygen atom. Don't forget that this is just a tiny part of a giant structure extending on all 3 dimensions.
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Types of Crystals
Molecular Crystals• Formed from molecules• Held together by intermolecular forces• Soft, low melting point• Poor conductor of heat and electricity
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Examples of Molecular solids
Iodine, I2• Iodine is a dark grey crystalline solid with a purple
vapour. M.Pt: 114°C. B.Pt: 184°C. It is very, very slightly soluble in water, but dissolves freely in organic solvents.
• Iodine is therefore a low melting point solid. The crystallinity suggests a regular packing of the molecules.
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Examples of Molecular solids
Ice
• Ice is a good example of a hydrogen bonded solid.• There are lots of different ways that the water molecules
can be arranged in ice. This is one of them, but NOT the common one - I can't draw that in any way that makes sense! The one below is known as "cubic ice", or "ice Ic". It is based on the water molecules arranged in a diamond structure
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Types of CrystalsMetallic Crystals – Typically weaker than covalent, but
can be in the low end of covalent• Lattice points occupied by metal atoms• Held together by metallic bonds• Soft to hard, low to high melting point• Good conductors of heat and electricity
Cross Section of a Metallic Crystal
nucleus &inner shell e-
mobile “sea”of e-
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Metallic Bonds are…• How metal atoms are held together in the
solid.• Metals hold on to their valence electrons
very weakly.• Think of them as positive ions (cations)
floating in a sea of electrons:
+ + + ++ + + +
+ + + +59
How does electricity flow in metal?
• In metals, free electrons carry the current.
• In metal wire, all the atoms share the electrons in their outermost electron shell and these electrons are free to move anywhere within the wire.
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Strength and Workability
• Malleability and ductility• Metals are described as malleable (can be
beaten into sheets) and ductile (can be pulled out into wires). This is because of the ability of the atoms to roll over each other into new positions without breaking the metallic bond.
• If a small stress is put onto the metal, the layers of atoms will start to roll over each other. If the stress is released again, they will fall back to their original positions. Under these circumstances, the metal is said to be elastic.
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1) Ductility 2) Malleability
Due to the mobility of the valence electrons, metals have:
and
Notice that the ionic crystal breaks due to ion repulsion!
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Malleable
+ + + +
+ + + ++ + + +
• Mobile electrons allow atoms to slide by, sort of like ball bearings in oil.
Force
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Strength and Workability
• If a larger stress is put on, the atoms roll over each other into a new position, and the metal is permanently changed.
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An amorphous solid does not possess a well-defined arrangement and long-range molecular order.
A glass is an optically transparent fusion product of inorganic materials that has cooled to a rigid state without crystallizing
Crystallinequartz (SiO2)
Non-crystallinequartz glass 67
Name type of solid Force(s) Melting Pt. (oC)
Boiling Pt. (oC)
Ne
molecular
-249
-246
H2S
molecular
-86
-61
H2O
molecular
0
100
Mercury
metallic
-39
357
W
metallic
3410
5660
CsCl
ionic
645
1290
MgO
ionic
2800
3600
Quartz (SiO2)
covalent network
1610
2230
Diamond (C)
covalent network
3550
4827
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