Intermolecular forces SAROJ MANOJ RAMAN, OCC Bonding Intermolecular forces SAROJ MANOJ RAMAN, OCC

Kinds of molecules Covalent bonding leads to two kinds of molecules They are bonded to each other by strong covalent bonds which form a network that is huge and so molecule has extremely high MP and BP E.g. diamond, sand Giant structures They have a few covalent bonds per molecule. Their MP and BP is very low and they exist as gases or low boiling liquids. E.g. methane, hydrogen chloride, Simple molecular structures

Proof for existence of intermolecular forces ON COOLING AND COMPRESSING Gases have large distances between their particles, so short range forces between molecules are insignificant. ON further COOLING AND COMPRESSING So gases liquefy. So (GL) i.e. Liquefy These weak forces become significant as the distances are reduced and their magnitude can now make a difference. So (LS ) i.e. solidifies Forces become more significant and hence the liquid solidifies. The distances are further reduced.

Types of Intermolecular forces Van der waals’ forces Arise due to random movement of electrons leading to the formation of instantaneous dipole and hence induced dipoles in molecules Their strength depends upon the molar mass of the molecule. These forces are effective over a short range. They are dependent on the surface area of the molecule Dipole-dipole forces These occur due to electrostatic attraction between molecules with permanent dipoles. They are significantly stronger than van der waals’ forces in molecules of a similar size. Molecule will have not just VVF but in addition to them the DPDP forces also. Hydrogen bonding This occurs in molecules that contain H bonded to N/O/F The non bonding e- pair on these N/O/F atoms interacts with the H atom that carries a high δ+ character coz its bonded to another of these small very electro- atoms. It’s the strongest of all the intermolecular forces.

Examples to explain Van der waals’ forces Boiling points of noble gases He – 4K Xe – 165 K More atomic mass hence more no. of electrons Boiling points of Alkanes Methane111 K Hexane 341 K As molar mass increases the VVF also increase Boiling points of Halogens As molar mass of halogens increases from F2 to I2 the boiling points also increase Boiling points of straight chain & branched Alkanes of same molar mass n pentane BP 309 K neo pentane BP 283 K Molar mass and very high surface area They have very high Plastics and polymers

Examples to explain dipole-dipole forces Hδ+— Clδ- ||||||||||||Hδ+— Clδ- electrostatic attraction The HCl molecule is polar and has slight negative and slight positive centres on it. This is a permanent DIPOLE and it interacts with other dipoles also and this electrostatic attraction will be DP-DP forces. Comparing the boiling points of non polar noble gases and polar hydrogen halides of same molar masses Boiling points of Non-polar Ar, Kr and Xe will be lower than those of Polar hydrogen halides like HCl, HBr and HI Because these are permanent dipoles they will have DP-DP forces along with VV forces

Hydrogen bonding Hδ+— Fδ- ---------------Hδ+— Fδ- hydrogen bond Considerably stronger than other intermolecular forces. Affects the physical properties of the compounds in which it exists.

Examples of H-Bonding Propane C3H8 only VVF BP is 231 K Comparing BP of Hydrogen peroxide, Fluorine and Hydrogen Chloride (Mr ~ 34-36) Hydrogen peroxide 431 K (polar, hydrogen bonded) Hydrogen Chloride 188 K (polar) Fluorine 85 K (non polar) Comparing BP of Propane, Ethanal and Ethanol (Mr~ 44-46) Propane C3H8 only VVF BP is 231 K Ethanal CH3CO-H VVF, polar BP is 294 K Ethanol CH3CH2OH VVF, polar, Intermolecular H bonding—352 K

Hydrogen Bonding in diff. compounds

Examples of H-Bonding contd…. Comparing the boiling points of hydrides of group 4,5,6,7 gp 4 compounds lowest boiling points as they are Tetrahedral, non polar, only VVF Methane (CH4) period 2 has lowest BP for the group and as we move to periods 3,4,5 the boiling point also increases. Why? Molar mass, Van der waals forces, so BP 

Examples of H-Bonding contd…. For Periods 3, 4, 5 as molar mass increases the bp also increases. What is the trend for Boiling points in periods 3,4,5 for different groups5,6,7? This is because they have in addition to VVF,H Bonding also . But for the 1st member in period 2 for these groups the BP is exceptionally high[ NH3<HF<H2O] . Why? There are 2 H atoms and 2 non bonding e pairs in 1 molecule. So is capable of 2 hydrogen bonds per molecule, extent of H bonding is the greatest, hence BP is highest. The deviation in trend is very high for water. Why?

Trends in Boiling points of Groups 4,5,6 and 7

Examples of H-Bonding contd…. [ NH3<HF<H2O] Explain this trend in boiling point of period 2 members of groups 5,6,7--- There are 2 H atoms and 2 non bonding e pairs in 1 molecule. So is capable of 2 hydrogen bonds per molecule, extent of H bonding is the greatest, BP highest. HF has 1 H atom and 3 non bonding e pairs per molecule. So extent of H bonding lesser than water so BP lesser than water. NH3 has 3H atoms and 1 non bonding e pair per molecule so the extent of hydrogen bonding lesser

Anomalous behavior of Water H Bonds In water liquid - 2/molecule In ice - 4/molecule Density Density of ice is less than that of water When ice melts from 0°C to 4°C then density of water increases unlike other liquids Molar enthalpy of fusion very high Molar enthalpy of vaporization

Structure of ICE Density of ice is less than water Each water molecule can form 4 H bonds to 4 other water molecules in a tetrahedral symmetry So the structure so produced resembles that of diamond but here there are H bonds holding the tetrahedron and not covalent bonds Open structure like a cage with large empty spaces in between so air is trapped in them, density is less and it floats on water.

Hydrogen Bonds in Water, Ice

Compare H2O and H2S Properties Water Hydrogen sulphide Molar enthalpy of fusion 6.0 kJmol-1 2.4 kJmol-1 Molar enthalpy of vaporization 41.0 kJmol-1 19.0 kJmol-1 State at room temperature & Pressure Liquid Gas Electro negativity Very high Less Size O atom is small S is larger than O Hydrogen Bonding 2 H bonds per molecule of water No H Bonding

Effect of H bonding on Organic compounds- case 1 Ethanoic acid (Mr =60) BP 391 K Liquid at rtp Due to H Bonds it can exist as dimer and its Mr seems ~ 120 Can bond with water molecules and is miscible Butane (Mr =58) BP 273 K Gas at rtp Only covalent bonds Immiscible in water

Dimer of Ethanoic acid

Effect of H bonding on Organic compounds- case 2 Propanone is miscible in water but it has no H bonding between propanone molecules This is because its O atoms can bond to water molecules.

Effect of H bonding on Organic compounds- case 3 p- nitro phenol or 4- nitro phenol is having intermolecular H bonds , stronger forces of interaction between molecules and so its BP is 279°C o- nitro phenol or 2- nitro phenol has intramolecular H bonds, weaker forces of interaction between molecules and so has lower BP of 216°C

Biological importance of H bonding DNA pairing occurs due to H bonds. Secondary structure of proteins (α helix and β pleated proteins) α helix-is an example of INTRA MOLECULAR H Bonding

Hydrogen bonds in DNA Base Pairs 2 H bonds between Adenine and Thymine 3 H bonds between Cytosine and Guanine