11.1 Intermolecular Forces Keeping Matter Together Nature’s Forces

Phases of Matter: Terminology Energy is required for phase change to occur. Solid-Liquid-Gas Triangle

Heating Cooling Curve From Steam to Ice and Vice-versa 4.184 J g ° 0.50 cal 1.84 J g ° 0.43 cal 4.184 J g ° 1 cal g o 6.01 kJ mol 80 cal g 40.67 kJ mol 540 cal g How much energy of 1g H2O 100°C to 0°C ? 540+100+80=720cal

Intermolecular Forces At the molecular level: Molecules or matter is held together by attractive force “glue” called intermolecular forces Energy added (K.E. increase)

Keeping Matter together Intramolecular Forces - Force which keeps integrity of molecule together, i.e., bonds or electrostatic bonding. Intermolecular Forces - Attractive force between molecules. Responsible for keeping matter in solid or liquid phase

The Forces be with You 2 Basic types of Intramolecular Force Ion - ion - Electrostatic attraction Covalent Bonds - Mutual sharing of electrons 4 Basic types of Intermolecular Force* 1. Ion - dipole : Ion is attracted to polar molecule 2. dipole - dipole: Polar molecules attracted to each other. 3. dipole - induce dipole: Polar molecules attracted to nonpolar molecules. 4. induce dipole -induce dipole (Van der Waal’s forces – also called London dispersion forces) nonpolar molecules attraction for each other due to electron cloud distortion. * plus one

Relative Strength Interaction Example Energy ion- ion Na+ Cl- 400 -4000 kJ/mol Covalent Bonds H - H 150-1100 kJ/mol ion-dipole (I-D) Na+ H2O 40-600 kJ/mol dipole - dipole (D-D) ICl ICl 5-25 kJ/mol dipole - induce dipole (D-ID) HCl O2 2-10 kJ/mol Van der Waal N2 N2 0.05 - 40 kJ/mol • H-Bond (10 - 40 kJ/mol

Ion - Ion Covalent Bonds Ion - Ion: Electrostatic attraction between ions Covalent Bonds: Bond between atoms as a result of electrons sharing. Bond Energy: = 926 kJ/mol Bond Energy: = 159 kJ/mol

Ion - Dipole Ion - Dipole: Charge and size dependent. Most important for larger charge and small ionic radius. Cation Ion Radius DHHyd (kJ/mol) Li+ 90 -515 Na+ 116 -405 K+ 152 -321 RB+ 166 -296 Cs+ 181 -268

Dipole - Dipole Dipole - Dipole: A permanent attractive intermolecular force resulting from the interaction of the positive end of one molecule with the negative end of another. Occurs between identical or different polar molecules. NonPolar Polar M(g/mol) bp (°C) M (g/mol) bp(°C) N2 28 -196 CO 28 -192 SiH4 32 -112 PH3 34 -88 GeH4 77 -90 AsH3 78 -62 Br2 160 59 ICl 162 97

Induce dipole - induced dipole: Vander Waal’s forces Van der Waal (Induced dipole-Induced dipole): Intermolecular force responsible for keeping nonpolar molecules (species) together. Polarisability - The ease of which an e- cloud can be distorted. Larger the atomic size, the greater the number of electrons, the greater the polarizability. Boiling Point of the Halogens and Noble Gases Halogen B.pt (K) Noble Gas B.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 Xe 166.1

Boiling point versus polarisability Graphs for noble gases and for series of nonpolar molecules; both show a family smooth increase of boiling point with atomic weight (larger degree of polarisability) due to increasing Van der Waal forces How about H2Te, (-20°C) H2Se, H2S, H2O Molar Mass Kr He SiH4 GeH4 SnH4 CH4 -250°C -100°C 0°C Temperature -200°C Ar Ne Xe

Boiling point Hydrogen compounds Graphs for family of hydrogen containing compounds and their boiling point. In general there is an increase in the boiling point except for H2O, HF and NH3. Why? There must be some other force that operate on these compounds which increases their intermolecular forces. Molar Mass (Period) H2Te H2Se H2O SiH4 SbH3 GeG4 CH4 -100°C 0°C 100°C Temperature HI SnH4 HF NH3 PH3 AsH3 HCl HBr H2S

A Special Type of Bonding H-Bonding H-Bonding: A special glue above and beyond dipole-dipole intermolecular forces. H-bonding is a strong type of intermolecular force (bond) between hydrogen and very electronegative elements ( 4 - 30 kJ/mol). N-H O-H F-H sometimes (Cl-H) Bichemical structural Integrity. Water possesses H-bond: Responsible for water’s unique properties.

Example: H-bonding a) C2H6 d) H3CCOOH No Yes b) CH3OH e) H3CCH2OH Which of the following substances exhibits H-bonding? Draw the H bonds between two molecules of the substances where appropriate. a) C2H6 d) H3CCOOH No Yes b) CH3OH e) H3CCH2OH Yes Yes c) H3CCONH2 f) H3CCOCH3 Yes No

Biological Integrity H-bonding is responsible for the structural integrity of Biological molecules. • Protein structures • DNA and RNA

H2O: Nature of Water Water is a liquid at room temperature as a direct consequence of hydrogen bonding between adjacent water molecules. (Most other molecules with comparable Molar mass is a gas at room temperature) Pure water is a liquid between 0°C and 100°C.

Example Identify the dominant intermolecular forces for each of the following substances, and select the substance with the higher boiling point in each pair; a) MgCl2 or PCl3 b) H3CNH2 or CH3F ion-dipole dipole-dipole H-bond dipole-dipole VdW VdW dipole-dipole VdW Higher Bpt VdW Higher Bpt b) CH3OH or CH3CH2OH e) Hexane or cyclohexane H-bond H-bond VdW VdW dipole-dipole dipole-dipole Higher Bpt VdW VdW More surface area Higher MWt.

Overview: Recognizing Intermolecular Forces Flowchart for recognizing the major types of intermolecular forces. Van der Waal’s forces occur in all instances. The strength of other forces generally increases proceeding from left to right

Summary of Nature’s Forces Bonding forces are relatively strong because they involve larger charges that are closer together. Ionic (400-4000 kJ/mol) Metallic (75-1000 kJ/mol) Intermolecular forces are relatively weak because they typically involve smaller charges that are farther apart. H-bond (10-40 kJ/mol) LDF (0.05 - 40 kJ/mol)