Origin of intermolecular forces • Intermolecular forces are the forces that hold the molecules together. • All intermolecular forces come from same origin ----- the polarity of the molecules.
Classification of intermolecular forces • Two types of intermolecular forces are hydrogen bond and Van der Waals' forces • Hydrogen bond only exist in the molecules that containing hydrogen atom bond to a highly electronegative atom (N,O,F) • Van der Waals' forces exits in All molecules, because ideal gas does not exist
What is Dipole moment • Ionic and polar covalent bonds have an unequal sharing of electrons between the two atoms. In these cases one end of the bond is more negative and the other more positive. If the molecule is a diatomic species then we call the molecule “polar” • These “bond dipoles” can be added up in more complicated molecules. Frequently the sum of the bond dipoles gives rise to an overall dipole moment in a polyatomic molecule. However, sometimes, due to symmetry or accident, the bond dipoles can cancel to give rise to a non-polar molecule (even though the individual bonds are polar).
Three types of interactions of Van der Waals' forces 1. permanent dipole-permanent dipole interactions Permanent dipoles: These occur when 2 atoms in a molecule have substantially different electronegativity — one atom attracts electrons more than another becoming more negative, while the other atom becomes more positive.
dipole-induced dipole interactions Induced dipoles: These occur when one molecule with a permanent dipole repels another molecule's electrons, "inducing" a dipole moment in that molecule. • instantaneous dipole-induced dipole interactions (Dispersion) Instantaneous dipoles: These occur due to chance when electrons happen to be more concentrated in one place than another in a molecule, creating a temporary dipole.
Relativemagnitude of different type of interaction Dipole-dipole > dipole-induced dipole > instantaneous dipole-induced dipole
Factors affecting strength of Van der Walls’ Force • Size of Electron cloud • The polarizability increases with the no. of electron in the molecule (i.e. size of electron cloud) • There is more displacement in the electron cloud and unequal distribution of charges • Strength of Van der Walls’ Force increase • Surface area of molecule • As the surface area of a molecule increase, the area contact to other molecules increase • Larger dipole can be induced • Strength of Van der Walls’ Force increase
Hydrogen bonds • A H-bond arises from an unusually strong dipole-dipole force. When H is bonded to a very electronegative element (F, O, N) the bond is polar covalent. H is unusual because with only one electron, it leaves a partially exposed nucleus (H has no other core electrons to shield the nucleus). • The bond can be thought of as forming between the hydrogen atom and the lone pairs of the F, N, or O.
Only F, O, and N? • The H-bond occurs between H-atom and a lone pair. Can other elements with lone pairs form H-bonds? e.g. Cl? • F, O and N are all 2nd row elements, which conveys certain properties: • They are the most electronegative elements • They are small (only 2s, 2p in outer shell) • They have lone pairs • Consequently, although, e.g. Cl lone pairs DO contribute to the dipole-dipole force, this force is not unusual in size (examine the previous plot). So we consider H-bonds ONLY to form between bonds involving FH, OH and NH
Drawing H-bonds • We indicate a H-bond using a dotted line. The H-bond is strongest when the bond angle is 180º.
Intramolecular H-bonds • All the H-bonds we’ve seen thus far have been between two molecules (intermolecular H-bonds). H-bonds can also occur within the same molecule (intra-molecular H-bond).
How does theory of intermolecular forces help to explain physical properties of substances? • Van der Waals' forces are much weaker than covalent bond, ionic bond & metallic bond • simple molecular substances have lower melting point & boiling point than giant covalent networks, ionic compounds & metals • Note that we cannot consider intermolecular forces as the ONLY FACTORS affecting melting point because the SYMMETRY of the molecule which affects the ESAE OF PACKING INTO A SOLID is also an important factor. • Although van der Waals' forces are usually consider as a type of weak forces, macromolecules with very large molecular sizes have very strong van der Waals' forces, e.g. polyvinyl chloride (high b.p.)
How does theory of intermolecular forces help to explain physical properties of substances? • Since Van der Waals' forces are much weaker than covalent bond, ionic bond and metallic bond, only small amount of energy is needed to break the intermolecular forces of molecular substances. • molecular crystals / liquids are volatile • molecular crystals are soft • Molecular crystals are non-conductors since there is no delocalized electrons.
How does theory of intermolecular forces help to explain chemical properties of substances? • DISSOLVING is a relationship between SOLUTE and SOLVENT.A solute is dissolved in a solvent providing that • The attractive force between the solute -solvent is greater than that between the solute - solute and between the solvent - solvent (attraction).
How does theory of intermolecular forces help to explain chemical properties of substances? • non-polar solutes dissolve in non-polar solventsParaffin wax (C30H62) is a non-polar solute that will dissolve in non-polar solvents like oil, hexane (C6H14) or carbon tetrachloride (CCl4).Paraffin wax will NOT dissolve in polar solvents such as water (H2O) or ethanol (ethyl alcohol, C2H5OH).
How does theory of intermolecular forces help to explain chemical properties of substances? • polar solutes dissolve in polar solventspolar solutes like glucose (C6H12O6) will dissolve in polar solvents such as water (H2O) or ethanol ( alcohol, C2H5OH) • As the partially positively charged atom of the solute molecule is attracted to the partially negatively charged atom of the solvent molecule, and the partially negatively charged atom of the solute molecule is attracted to the partially positively charged atom of the solvent molecule. • And that's why glucose will NOT dissolve in non-polar solvents such as oil, hexane (C6H14) or carbon tetrachloride (CCl4).
How does theory of intermolecular forces help to explain chemical properties of substances? • Ionic solutes dissolve inpolar solventssodium chloride (NaCl) will generally dissolve in polar solvents such as water (H2O)but not in non-polar solvents. • since the positive ion of the soluteis attracted the partially negatively charged atom in the polar solvent molecule, and the negative ion of the solute is attracted to the partially positively charged atom on the solvent molecule
Summary of IM Forces • Dispersion forces exist between ALL molecules. The force increase in strength with molecular mass. • Forces associated with permanent dipoles are found only in substances with overall dipole moments (polar molecules). Their existence adds to the dispersion forces • When comparing substances of widely different masses, dispersion forces are usually more significant than dipolar forces. • When comparing substances of similar molecular mass, dipole forces can produce significant differences in molecular properties (e.g. boiling point).
Examples to show the existence of intermolecular forces • Example 1: Hot pressing hair • Example2: Protein • Example3: DNA • Example4: Soap and detergent • Example5: Water and ice • Example6: LCD
Example 1: Hot pressing hair • HOW TO MAKE ? • First the hair is washed and partially dried with a towel. A small amount of pressing oil is combed through the hair. This oil is for lubrication to allow the comb to pass through the hair more easily and also to act as a conductor of heat from the comb to the hair. • A metal pressing comb is heated to between 300 and 500 degrees Fahrenheit andis passed quickly through the hair.
Principle of hot pressing hair • The high temperature breaks the biochemical disulfide bonds between and within the keratin protein sand allows the hair to be straightened through the tension applied to the hair during the combing procedure. • After the comb has passed through the hair the temperature drops rapidly and this allows the broken biochemical bonds in the hair to reconnect and fix their new position. • This reformation of the bonds holds the hair in its new, straightened shape.
Principle of hot pressing hair • In the helical protein of hair, hydrogen bonds within individual helices of keratin, and disulfide bridges between adjacent helices, impart strength and elasticity to individual hairs. • Water can disrupt the hydrogen bonds, making the hair limp. When the hair dries, new hydrogen bonding allows it to take on the shape of a curler. Permanent wave solutions induce new disulfide bridges between the helices . • Genetically determined, natural curly hair also has a different arrangement of disulfide bridges compared with straight hair.
Example2: Protein • The primary structure of a protein is a pole peptide which is a polymer of amino acids .Polypeptide chains form a helical structure owing to the hydrogen bonds formed between the N-H and C=O groups. • This creates the secondary structure of proteins .In many proteins, including those in hair, wool and nauls , hydrogen bonding causes the polypeptide chains to become twisted into tightly coiled helices.
Polypeptides (proteins) • polypeptides are another biopolymer, and we shall examine their structure further…
Example2: Protein • ˙In the pleated sheet arrangementprotein chains run parallel or in alternating directions. Thechains are held together by hydrogen bonds that join the H of an NH group to the O in a CO group on the neighboring chain.
Example2: Protein • ˙In the a-helical arrangementprotein chain is coiled into a helix. Each NH group is hydrogen bonded to a CO group one helical turn (3.6 amino acid units) away in the same chain, giving a fairly rigid cylindrical structure with side chains on the outside.
Example3: DNA • DNA is present in the nuclei of living cells and carries genetic information. The DNA molecule consists of two helical nucleic acid chains which is very stable. • Each nucleic acid is made up of 3 components : a sugar , a phosphoric acid unit and a nitrogen-containing heterocyclic base : adenine , cytosine, guanine or thymine .The two nucleic acid chains are held together by hydrogen bonds. • These hydrogen bonds are formed between specific pairs of bases on the chains. The two strands coil tightly tound each other .
Example3: DNA • structure of DNA
Example4: Soap and detergent • Molecules liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. • the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surfaceto formsurface tension.Surface tension is a type of intermolecular forces.
The Role of Surfactants • Surfactants are molecules that act as a bridge between polar and • non-polar molecules, thereby considerably increasing solubility. • They achieve this by having a polar/ionic end, which interacts preferentially with polar molecules, and a non-polar end, which interacts preferentially with non-polar molecules.
Detergent action • A detergent is just one type of surfactant. Surfactant molecules occupy the spaces between water molecules at the surface, reducing the force of attraction between the water molecules, lowering the surface tension.
Example5: Water and ice • Hydrogen bonding in Water or ice • A water molecule is composed of two hydrogen atoms (H) and one oxygen atom (O). The atoms of hydrogen and oxygen are bound by sharing their electrons with one another. This bond is called a “covalent bond”. • However, since oxygen atoms pull electrons more strongly than hydrogen atoms, the oxygen atom in a water molecule has a slightly negative charge and the hydrogen atoms have a slightly positive charge. • So adjacent water molecules are attractedto one another through the slightly negatively charged oxygen atoms and the slightly positively charged hydrogen atoms. This interaction is called “hydrogen bonding”.
Water • Structure of water • A water molecule is formed when two atoms of hydrogen bondcovalently with an atom of oxygen. In a covalent bond electrons are shared between atoms. In water the sharing is not equal. The oxygen atom attracts the electrons more strongly than the hydrogen. This gives water an asymmetrical distribution of charge. • Molecules that have ends with partial negative and positive charges are known as polar molecules. It is this polar property that allows water to separate polar solute molecules and explains why water can dissolve so many substances.
The special case of water • Water is perhaps the most unusual liquid. Each water molecule is H-bonded to FOUR other water molecules (donating 2 H atoms and accepting two H-atoms to the lone pairs), forming a tetrahedral network.
ice • Structure of ice • In ice, each water forms four hydrogen bonds with O---O distances of 2.76 Angstroms to the nearest oxygen neighbor. The O-O-O angles are 109 degrees, typical of a tetrahedrally coordinated lattice structure. The density of ice Ih is 0.931 gm/cubic cm. • This compare with a density of 1.00 gm/cubic cm. for water.
ice • Structure of ice • There are lots of different ways that the water molecules can be arranged in ice. The one below is known as "cubic ice", or "ice Ic". It is based on the water molecules arranged in a diamond structure.
Why does ice float on water? • The arrangement of water molecules in ice creates an open structure . This accounts for the fact that ice is less dense than water at 273K . • When ice melts, the regular lattice breaks up .the water molecules can then pack more closely together ,so its liquid form has a higher density. • In water , the strong hydrogen bonding results in some ordered packing of water molecules over a short range.
Why does ice float on water? • The fact that ice is less dense than water at 273K make ponds and lakes freeze from the surface downwards. • The layer of ice insulates the water below and prevent complete solidification. That allows fish ,aquatic plants and other aquatic organisms to survive.
Example6: LCD • A liquid crystal display, or LCD, is a thin, lightweight display device with no moving parts. It consists of an electrically-controlled light-polarising liquid trapped in cells between two transparent polarising sheets. • The polarising axes of the two sheets are aligned perpendicular to each other. Each cell is supplied with electrical contacts that allow an electric field to be applied to the liquid inside
About liquid crystal • Liquid crystals are a class of molecules that, under some conditions, inhabit a phase in which they exhibit isotropic, fluid-like behavior – that is, with little long-range ordering – but which under other conditions inhabit one or more phases with significant anisotropicstructure and long-range ordering while still having an ability to flow.
Example6: LCD • Before an electric field is applied, the long, thin molecules in the liquid are in a relaxed state. Ridges in the top and bottom sheet encourage polarisation of the molecules parallel to the light polarisation direction of the sheets. • Between the sheets, the polarisation of the molecules twists naturally between the two perpendicular extremes. Light is polarised by one sheet, rotated through the smooth twisting of the crystalmolecules, then passes through the second sheet. • The whole assembly looks nearly transparent. A slight darkening will be evident because of light losses in the original polarising sheet.
Example6: LCD • Reflective twisted nematic liquid crystal display. • 1.Vertical filter film to polarize the light as it enters. • 2.Glass substrate with ITO electrodes. The shapes of these electrodes will determine the dark shapes that will appear when the LCD is turned on. Vertical ridges are etched on the surface so the liquid crystals are in line with the polarized light. • 3.Twisted nematic liquid crystals. • 4.Glass substrate with common electrode film (ITO) with horizontal ridges to line up with the horizontal filter. • 5.Horizontal filter film to block/allow through light. • 6.Reflective surface to send light back to viewer.