Project on Intermolecular Forces

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Project on Intermolecular Forces

• Members Liu Wing Chiu (19)
• Siu Nga Chiu (24)

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Intermolecular Forces

• The origin of intermolecular forces
• The classification of intermolecular forces
• Van der Waals force
• Hydrogen bonding
• Explore an example in depth to show the
  significance of existence of intermolecular
  forces.

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The Origin of Intermolecular Forces

• It is weak electrostatic force of attraction that
  exist an area of negative charge on one molecule
  and an area of positive charge on a second
  molecule.
• What causes intermolecular forces?
• Molecules are made up of charged particles
  nuclei and electrons. When one molecule
  approaches another, there is a multitude of
  interactions between the particles in the two
  molecules. Each electron in one molecule is
  subject to forces from all the electrons and the
  nuclei in the other molecule.

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• Intermolecular force is weak compared to covalent
  bond. It is relatively weak interactions that
  occur between molecules.
• There are 2 types of intermolecular forces (both
  of them are electrostatic attraction between
  dipoles formed by uncharged molecules.)
• 1. Van der Waals' force
• 2. Hydrogen bonding
• Van der waals force is formed by dipoles. There
  are 3 types of dipoles
• 1. Permanent dipoles
• 2. Instantaneous dipoles
• 3. Induced dipoles

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Permanent Dipole

• These molecules have a permanent separation of
  positive and negative charge.
• A simple example is HCl
• 
  
  
• ?
  ?-
• The pair of electrons in the covalent bond
  between hydroge and chlorine is
• unequally shared due to the difference in
  electronegativity between hydrogen and
• chlorine. Chlorine has a greater
  electronegativity compared to hydrogen and hence
• Chlorine tends to attract the bonded electron
  pair to itself. chlorine becomes slightly
• negatively charged (?-), hydroge atom has a
  partial positive charged (?) .The
• unsymmetrical distributed charge on the HCl
  molecule produces a permanent

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Instantaneous Dipole

• Instantaneous dipole is due to the fluctuation of
  electron clouds on non-polar molecules, positive
  and negative charges exist temporarily.

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Induced Dipole

• Induced dipole exist when a permanent dipole or
  instantaneous dipole comes close to a non-polar
  molecule, the non-polar molecule will be induced
  to form a dipole temporarily.

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The Classification of Intermolecular Force

• There are 2 major types of intermolecular force
• Van der Waals force
• It can be divided into three categories
• 1. Dipole-dipole Interactions
• 2. Instantaneous dipole-induced dipole
  Interactions
• 3. Dipole-induced dipole Interactions
• Hydrogen bond

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Classification diagram of intermolecular force
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Van der Waals Forces

• It is an attractive force which exist between all
  molecules.
• It is the weakest of intermolecular force.
• The force can be divided into three categories
• 1 Dipole-dipole Interactions
• 2 Dipole-induced dipole Interactions
• 3 Instantaneous dipole-induced dipole
  Interactions

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Dipole-Dipole Interactions

• Dipole-dipole interactions exist between
  molecules which are permanent dipole. They tend
  to orientate themselves that the attractive
  forces between molecules are maximized while
  repulsive forces are minimized.
• In the illustration
• the H end of HCl is permanently slightly
  positive charge. The Cl end of HCl has a
  permanent slight negative charge, the "H" in one
  molecule is attracted to the "Cl" in a neighbor.

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Instantaneous Dipole-Induced Dipole Interactions

• Also known as London forces or Dispersion Forces
• Instantaneous dipole-induced dipole Interactions
  exist in non-polar molecules. These forces
  result from temporary charge imbalances. The
  temporary charges exist because the electrons in
  a molecule or ion move randomly in the structure.
  The nucleus of one atom attracts electrons form
  the neighboring atom. At the same time, the
  electrons in one particle repel the electrons in
  the neighbor and create a short lived charge
  imbalance.
• These temporary charges in one molecule or atom
  attract opposite charges in nearby molecules or
  atoms. A local slight positive charge ? in one
  molecule will be attracted to a temporary slight
  ?- negative charge in a neighboring molecule.
• Note dispersion forces operate in all molecules
  whether they are polar or non-polar.

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Dipole-Induced Dipole Interactions

• Also known as induction force.
• When a polar molecule approaches a nonpolar
  molecule, the permanent dipole on the polar
  molecule can distort the electron cloud of the
  nonpolar molecule, forming an induced dipole.

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Van der Waals Radius VS Covalent Radius

• Van der Waals radius is one half of the distance
  between the nuclei of two atoms in adjacent
  molecules.
• Covalent radius is one half of the distance
  between two atoms in the same molecules.
• Van der Waals radius of a non-metal is always
  larger than the corresponding covalent radius
  because the covalent radius because covalent bond
  is much stronger than van der Waals forces.

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The Strength of the Van der Waals Forces

• The strength of the van der Waals' forces depends
  on size of electron cloud (how easily the
  electron cloud is distorted or polarized).
• For all molecules, the more number of electron
  (or weaker attraction force between nucleus and
  electrons), causing the higher in polarizability.
  The degree of uneven distribution of electron
  cloud is higher and the strengths induction force
  and dispersion force become stronger. Thus the
  stronger van der Waals forces.
• The strength of van der Waals' forces is also
  related to the surface area (or shape) of the
  molecule.
• For molecules with similar relative molecular
  masses or size, the higher contact surface area,
  the stronger van der Waals forces.

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Hydrogen Bonding

• Hydrogen bond is a electrostatic force of
  attraction existing between polar hydrogen(?)
  and electronegative atom(?-) of dipoles.
• The hydrogen bond is weaker than the covalent
  bond, but relatively strong compared to van der
  Waals force.
• Hydrogen bonding is a unique type of
  intermolecular molecular attraction. There are
  two requirements. 1. The first is a covalent
  bond between a H atom and either F, O, or N
  (These are the three most electronegative
  elements.) 2. The second is an interaction of
  the H atom in this kind of polar bond with a
  lone pair of electrons on a nearby atom of F, O,
  or N.

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The presence of hydrogen bonding has an important
effect on the properties of various substances

• The melting and boiling points of the period two
  hydrides NH3, H2O and HF are much higher than are
  expected if only  dipole-dipole forces were
  acting between the molecules.
• The solubility of molecular substances in water
  is greatly influenced by their ability to form
  H-bonds with water molecules.
• Water has several unusual properties which are
  related to H-bonding
• High melting and boiling points given
• High surface tension.
• Expansion on freezing due to the formation of a
  regular open-cage network of H-bonded water
  molecules.
• Liquids with hydrogen bonds between molecules
  usually have higher viscosity than comparable
  liquids that don't.
• H-bonding can influence acidity. H-bonded
  hydrogen atoms are often less likely to
  dissociate as H ions.
• H-bonding also plays important roles in
• The folding of proteins.
• The structure of DNA.
• The manner in which hydrated crystals cleave.

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Hydrogen Bonding in Alcohols

• An alcohol is an organic molecule containing an
  -O-H group.
• Any molecule which has a hydrogen atom attached
  directly to an oxygen or a nitrogen is capable of
  hydrogen bonding. Such molecules will always have
  higher boiling points than similarly sized
  molecules which don't have an -O-H or an -N-H
  group. The hydrogen bonding makes the molecules
  "stickier", and more heat is necessary to
  separate them.
• Ethanol, CH3CH2-O-H, and methoxymethane,
  CH3-O-CH3, both have the same molecular formula,
  C2H6O.

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Solubility

• If there are strong solute-solvent interactions,
  the solvent is soluble in the solute.
• Most ammonium, nitrate and sulphate salts are
  soluble in water since they form hydrogen bonds
  with water molecules.
• The high solubility of alkanols in water is cause
  by the formation of hydrogen bond.
• Carbohydrates have many OH groups which can form
  hydrogen bond with water. Therefore carbohydrates
  with low relative molecular mass are soluble in
  water.

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Hydrogen Bonding in an Ice Crystal

• Ice has a lower density than water as ice has an
  open structure. In ice, each molecule is
  tetrahedral bonded to other molecules by hydrogen
  bond.

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Hydrogen Bond in Water

• Many other unique properties of water are due to
  the hydrogen bonds. For example, ice floats
  because hydrogen bonds hold water molecules
  further apart in a solid than in a liquid, where
  there is one less hydrogen bond per molecule. The
  unique physical properties, including a high heat
  of vaporization, strong surface tension, high
  specific heat, and nearly universal solvent
  properties of water are also due to hydrogen
  bonding.

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Hydrogen Bonding in DNA

• Hydrogen bonds play an important role in the
  base-pairing duplication of DNA (A-T,C-G).
  Matching of the bases produces an accurate
  duplicate of the original DNA chain.

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Boiling Points of Some Hydrides

• The figure shows the normal boiling point
  temperatures for several related substances. This
  boiling point diagram tells us about the
  intermolecular forces between a homologous series
  of small hydrogen containing molecules. Although
  for the most part the trend is that the boiling
  points increase as going down the group. The
  boiling point other hydride of the first element
  in each group is abnormally high. In the cases of
  NH3, H2O and HF there are hydrogen bond
  attraction, requiring significantly more heat
  energy to break.

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Explore an example in depth to show the
significance of existence of intermolecular forces
The Hardness of Calcium Sulfate (CaSO4)
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Gypsum (Hydrated CaSO4 , CaSO42H2O )

• In the gypsum crystal structure, calciums are
  coordinated by six oxygens from sulphate, and by
  two oxygens from water (H2O). Two sheets of
  sulphates are bound together by calciums forming
  double sheet layers. At each side of these layers
  are water molecules, which form weak hydrogen
  bonds to the next layer in the structure.
• It is sectile and slightly flexible

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Anhydrite (Anhydrous CaSO4)

• Anhydrite has the same composition as Gypsum, but
  contains no water in its structure. There are
  only strong ionic bonds between ions.
• It is very hard and very difficult to cleave

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• Calcined gypsum has an unusual property when
  mixed with water at normal temperatures, it
  recombines with the water that was driven off
  during calcination, and sets to form a strong
  gypsum crystal lattice
• CaSO4½H2O 1½H2O ? CaSO42H2O
• This reaction is exothermic.
• The anhydrous form, called anhydrous calcium
  sulfate (sometimes anhydrite), is produced by
  further heating to above approximately 180C
  (356F) and has the chemical formula CaSO4.
  Anhydrite reacts slowly with water to return to
  the dihydrated state.

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Plaster of Paris (Calcium Sulfate Hemi-hydrate,
CaSO4, ½ H2O)

• Plaster of Paris, or simply plaster, is a type of
  building material based on calcium sulfate
  hemi-hydrate, nominally (CaSO4)2. ½ H2O. It is
  created by heating gypsum to about 150C.
• (CaSO4, 2 H2O) heat  (CaSO4, ½ H2O) 1.5 H2O
• When the dry plaster powder is mixed with water,
  it re-forms into gypsum, initially as a paste but
  eventually drying into a solid. The structure is
  made up of sheets of Ca2 and SO42- ions held
  together by hydrogen bonds in the water
  molecules. The grip between these sheets is
  easily broken, so plaster is fairly soft.
• Its major use is in building, statuary, ceramics,
  dental plates, fine metal parts for precision
  instruments, and surgical splints.

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Use of Plaster of Paris

• Plaster is used as a building material similar to
  mortar or cement. Like those materials plaster
  starts as a dry powder that is mixed with water
  to form a paste, which then dries into a hard
  surface. Unlike those materials plaster remains
  quite soft after drying, and can be easily
  manipulated with metal tools or even sandpaper.
  Plaster was a common building material for wall
  surfaces in a process known as lath and plaster,
  in which a series of wooden strips were covered
  with a semi-dry plaster and then hardened into a
  flat surface. Today this building method has been
  almost completely replaced with drywall.

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• Plaster expands while drying, then contracts
  slightly just before hardening completely. This
  makes plaster excellent for use in molds, and it
  is often used as an artistic material for
  casting. Plaster is also commonly spread over an
  armature (form), usually made of wire, mesh or
  other materials. In medicine, it is also widely
  used as a support for broken bones a bandage
  impregnated with plaster is moistened and then
  wrapped around the damaged limb, setting into a
  close-fitting yet easily removed tube.