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Intermolecular Force. Content. Why ice float on water??? Importance of Hydrogen Bonds Water and Ice Hydrogen Bonding in Proteins Hydrogen Bonding in DNA. Basic Concept of intermolecular force Van der Waals’ force
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Content • Why ice float on water??? • Importance of Hydrogen • Bonds Water and Ice • Hydrogen Bonding in • Proteins • Hydrogen Bonding in DNA • Basic Concept of intermolecular force • Van der Waals’ force ~Permanent Dipole~Instantaneous dipole~Induced Dipole • Permanent dipole - permanent dipole interaction • Instantaneous dipole - induced dipole interaction • Summary of intermolecular forces • Hydrogen bonding • The evidence for hydrogen bonding • The origin of hydrogen bonding • Hydrogen Bonding in Water and Ice
Basic Concept of intermolecular force • Broadly speaking, there are two types of intermolecular forces,Van der Waals’ forcesandhydrogen bonding.
Van der Waals’ force • Van der Waals’ force is electrostatic attraction between dipole, i.e. the attraction between the positive end of one molecule and the negative end of another molecule. • There are the types of dipoles: permanent dipole,instantaneous dipoleandinduced dipole.
Permanent Dipole • A permanent dipole exists in all polar molecules. PermanentDipole δ+ δ-
Instantaneous dipole • An instantaneous dipole is atemporarydipole that exists as a result of fluctuation in the electron cloud. The temporary dipole induces a dipole in a neighbouring molecule. This results in a weak and temporary force of attraction between the two atoms.
Induced Dipole • An induced dipole is a temporary dipole that is created due to the influence of a neighbouring dipole (which may be a permanent or an instantaneous dipole) Induced Dipole
Van der Waals’ forces consists of three types of intermolecular attractions. They include the permanent dipole - permanent dipole interaction,permanent dipole -induced dipole interaction and instantaneous dipole -induced dipole interaction .
Permanent dipole - permanent dipole interaction • Polar molecules such as HCl have permanent moments. They tends to orient one another, as a result there are attractive forces between molecules. The attraction between the δ+ and δ- of the permanent dipoles of neighbouring molecules is permanent dipole - permanent dipole interaction. • Dipole-dipole attraction requires the presence of polar bonds and a unsymmetric molecule.
Sincethere are an attractive forces between molecules, • ∴it need more energy to break the attraction.
Permanent dipole -induced dipole interaction • When a non-polar molecule approaches a polar molecule (with a permanent dipole), a dipole will be induced in the non-polar molecule. The dipole induced will be in opposite orientation to that of the polar molecule. • The dipole – induced dipole interactions are generally weaker than dipole – dipole interactions
Instantaneous dipole -induced dipole interaction • Take argon as an example. The electron cloud distribution is generally symmetrical around the argon nucleus. However, due to the instant mobility of the electron cloud ,its position fluctuates all the time. • At any particular instant, it is likely to be concentrated on one side of the atom than the other. Thus, the atom possesses an electric dipole moment at that particular instant. It is known as the Instantaneous dipolemoment.
The instantaneous dipole will induce a dipole moment (known as the induced dipole moment) in the neighbouring atom by attracting opposite charges. If the positive end of the dipole is pointing towards a neighbouring atom, the induced dipole will then have its negative end pointing towards the positive pole of that dipole. That makes the instantaneous dipole attract the induced dipole. It is the type of weak attraction that exists between atoms of noble gases and between non – polar molecules. • Instantaneous dipole-induced dipole attraction also called dispersion force or London force. • They are much weaker than dipole – dipole interaction,
The evidence for hydrogen bonding • Many elements form compounds with hydrogen - referred to as "hydrides". If you plot the boiling points of the hydrides of the Group 4 elements, you find that the boiling points increase as you go down the group.
The increase in boiling point happens because the molecules are getting larger with more electrons, and so van der Waals dispersion forces become greater.
If you repeat this exercise with the hydrides of elements in Groups 5, 6 and 7, something odd happens.
Although for the most part the trend is exactly the same as in group 4 (for exactly the same reasons), the boiling point of the hydride of the first element in each group is abnormally high. • In the cases of NH3, H2O and HF there must be some additional intermolecular forces of attraction, requiring significantly more heat energy to break. These relatively powerful intermolecular forces are described as hydrogen bonds.
The origin of hydrogen bonding • The molecules which have this extra bonding are:
Notice that in each of these molecules: • The hydrogen is attached directly to one of the most electronegative elements, causing the hydrogen to acquire a significant amount of positive charge. • Each of the elements to which the hydrogen is attached is not only significantly negative, but also has at least one "active" lone pair.Lone pairs at the 2-level have the electrons contained in a relatively small volume of space which therefore has a high density of negative charge. Lone pairs at higher levels are more diffuse and not so attractive to positive things.
The δ + hydrogen is so strongly attracted to the lone pair that it is almost as if you were beginning to form a co-ordinate (dative covalent) bond. It doesn't go that far, but the attraction is significantly stronger than an ordinary dipole-dipole interaction. • Hydrogen bonds have about a tenth of the strength of an average covalent bond, and are being constantly broken and reformed in liquid water. If you liken the covalent bond between the oxygen and hydrogen to a stable marriage, the hydrogen bond has "just good friends" status. On the same scale, van der Waals attractions represent mere passing acquaintances!
Hydrogen Bonding in Water and Ice • Hydrogen bonding exists in both water and ice. • In water, the molecules are in constant motion. Hydrogen bonds are formed and broken continually. The arrangement of a minimum and molecules are in random. • In ice, molecular motion is of a minimum and molecules are oriented in such a way that the maximum number of hydrogen bonds are formed.
Why ice can float on water??? • Each water molecule is made of 2 hydrogen atoms and 1 oxygen atom. These are connected to one another by very strong chemical bonds called covalent bonds. Water molecules are connected to each other by much weaker chemical bonds called hydrogen bonds between the positively charged hydrogen atoms, and one negatively charged oxygen atom in a neighboring water molecule. • Ice floats because it is less dense than liquid water. Ice is about 9% less dense.
Ice has an open-cage structure with very inefficient packing of molecules.The hydrogen bonds not only help to held the water molecules rigidly as solid , but also hold the molecules apart & ice is less dense than water. As there are many space in open-cage structure, when the bond broke, some molecules can occupy the space,the volume of ice decrease.
At 0℃(ice melt),much of hydrogen bond structure is broken(the regular lattice breaks up), water molecules pack more closely together & density rise. • When the temperature is above 0℃, kinetic energy increase, and causing an increasing % of hydrogen bonds. • When the temperature is above 4℃, vibration increase, the volume increase, and cause the decreasing of density. • When the temperature is 100 ℃, all hydrogen bond will break.
Importance of Hydrogen Bonds Water and Ice • In water, the strong hydrogen bonding result in some ordered packing of water molecules over a short range. The fact that ice is less dense than water at 0 ℃ makes ponds and lakes freeze from the surface downwards. The layer of ice insulates the water below and prevents complete solidification. That allows fish, aquatic plants and other aquatic organisms to survive.
Hydrogen Bonding in Proteins • Hydrogen bonding plays a crucial role in the structures of many substances such as proteins and deoxyribonucleic acids (DNA). The primary structure of a protein is a polypeptide which is a polymer of amino acids.
Polypeptide chins 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 nails, hydrogen bonding causes the polypeptide chains to become twisted into tightly coiled helices.
Hydrogen Bonding in DNA • DNA is present in the nuclei of living cells and carries genetic information. It consists of two macromolecular strands spiralling around each other in the form of a double helix.
The DNA molecule consists of two helical nucleic acid chains. Each nucleic acids is made up of three 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. For example, two hydrogen bonds are formed between an adenine unit in one chain and a thymine unit in the other while three hydrogen bonds are formed between a guanine unit in one chain and a cytosine unit in the other.
When a cell divides, the double helix of the DNA molecule uncoils by breaking down the hydrogen bonds. A complementary chain is formed adjacent to each of the newly formed double helices is received by each of the two cells. This gives an explanation for the same DNA in every normal cell of an organism.
Group Member • CHEUNG YI TONG (6) • TSUI MAN KI (27)
