11.1 Intermolecular Forces Keeping Matter Together

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1 11.1 Intermolecular Forces Keeping Matter Together Nature s Forces Dr. Fred Omega Garces Chemistry 201 Miramar College 1 Intermolecular Forces Keeping Matter Together

2 Phases of Matter: Terminology Energy is required for phase change to occur. Solid-Liquid-Gas Triangle 2 Intermolecular Forces Keeping Matter Together

3 Heating Cooling Curve From Ice to Steam and Vice-versa Stage1 Stage2 Stage3 Stage4 Stage J g 0.49 cal g 40.7 kj mol 80 cal g J g 6.01 kj mol 540 cal g 2.08 J g 0.43 cal g 1 cal g o Heat Addition What is the energy needed to take 1g H 2 O at 0 C to 100 C? =720cal 3 Intermolecular Forces Keeping Matter Together

4 Intermolecular Forces At the molecular level: Molecules or matter is held together by glue called intermolecular forces SOLID LIQUID GAS Energy added (K.E. increases) 4 Intermolecular Forces Keeping Matter Together

5 Keeping Matter together Intramolecular Forces - Force which keeps molecule together, i.e., bonds. Intermolecular Forces - Attractive force between molecules. Responsible for keeping matter in solid or liquid phase. 5 Intermolecular Forces Keeping Matter Together

6 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 (London dispersion forces, LDF) nonpolar molecules attraction for each other due to electron distortion. * plus one 6 Intermolecular Forces Keeping Matter Together

7 Relative Strength Interaction Example Energy ion- ion Na + Cl kj Covalent Bonds H - H kj ion-dipole (I-D) Na + HCl kj dipole - dipole (D-D) HCl HCl 5-25 kj dipole - induce dipole (D-ID) HCl O kj London Dispersion (LD) N 2 N kj H-Bond (10-40 kj/mol 7 Intermolecular Forces Keeping Matter Together

8 Ion - Ion Covalent Bonds Ion - Ion: Electrostatic attraction between ions Na Na+ 11 p 12 n 11 p 12 n 11 p 12 n 9 p 10 n F F- 9 p 10 n 9 p 10 n NaF Bond Energy: = 910 kj/mol Covalent Bonds: Bond between atoms as a result of electrons sharing. F F F 2 Bond Energy: = 155 kj/mol 9 p 10 n 9 p 10 n 9 p 10 n 9 p 10 n 8 Intermolecular Forces Keeping Matter Together

9 Ion - Dipole Ion - Dipole: Charge and size dependent. Most important for larger charge and small ionic radius. Cation Ion Enthalpy of Hydration (pm) (KJ/mol) δ - δ + Li Na K Rb Cs Distance between ion center and negative pole of dipole 9 Intermolecular Forces Keeping Matter Together

10 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) N CO SiH PH GeH AsH Br ICl Intermolecular Forces Keeping Matter Together

11 Induce dipole - induced dipole: London Dispersion Forces London Dispersion Force (Induce dipole-induce dipole): Intermolecular force responsible for keeping nonpolar molecules (species) together. Polarizability - 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) F He 4.6 Cl Ne 27.3 Br Ar 87.5 I Kr Xe Intermolecular Forces Keeping Matter Together

12 Boiling point versus polarizability Graphs for noble gases and for series of nonpolar molecules; both show a fairly smooth increase of boiling point with atomic weight (larger degree of polarizability) due to increasing dispersion forces 12 Intermolecular Forces Keeping Matter Together

13 Boiling point Hydrogen compounds 100 C H 2 O Graphs for family of hydrogen containing compounds and their boiling point. In general there is an increase in the boiling point except for H 2 O, HF and NH 3. Why? There must be some other force that operate on these compounds which increases their intermolecular forces. Temperature 0 C -100 C HF NH 3 CH 4 H 2 S HCl PH 3 SiH 4 H 2 Se AsH 3 HBr GeG 4 H 2 Te SbH 3 HI SnH 4 Molar Mass (Period) 13 Intermolecular Forces Keeping Matter Together

14 A Special Type of Bonding H-Bonding H-Bonding: A special glue above and beyond dipoledipole intermolecular forces. H-bonding is a strong type of intermolecular force (bond) between hydrogen and very electronegative elements (10-40 kj/mol). N-H O-H F-H sometimes (Cl-H) Biochemical structural Integrity. Water possesses H-bond: Responsible for water s unique properties. 14 Intermolecular Forces Keeping Matter Together

15 Example: H-bonding Which of the following substances exhibits H-bonding? Draw the H bonds between two molecules of the substances where appropriate. a) C 2 H 6 d) H 3 CCOOH b) CH 3 OH e) H 3 CCH 2 OH c) H 3 CCONH 2 f) H 3 CCOCH 3 15 Intermolecular Forces Keeping Matter Together

16 Biological Integrity H-bonding is responsible for the structural integrity of Biological molecules. Protein structures DNA and RNA 16 Intermolecular Forces Keeping Matter Together

17 H 2 O: 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 are gas at room temperature) Pure water is a liquid between 0 C and 100 C. Temperature 100 C 0 C -100 C H 2 O HF NH 3 CH 4 H 2 S HCl PH 3 SiH 4 H 2 Se AsH 3 HBr GeG 4 H 2 Te SbH 3 HI SnH 4 Molar Mass (Period) 17 Intermolecular Forces Keeping Matter Together

18 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) MgCl 2 or PCl 3 b) H 3 CNH 2 or CH 3 F ion-dipole dipole-dipole H-bond dipole-dipole LDF LDF dipole-dipole LDF Higher Bpt LDF Higher Bpt c) CH 3 OH or CH 3 CH 2 OH e) Hexane or cyclohexane H-bond H-bond LDF LDF dipole-dipole dipole-dipole Higher Bpt LDF LDF More surface area Higher Bpt Higher MWt. 18 Intermolecular Forces Keeping Matter Together

19 Overview: Recognizing Intermolecular Forces Flowchart for recognizing the major types of intermolecular forces. London dispersion forces occur in all instances. The strength of other forces generally increases proceeding from left to right 19 Intermolecular Forces Keeping Matter Together

20 Summary of Nature s Forces Bonding forces are relatively strong because they involve larger charges that are closer together. Ionic ( kj/mol) Covalent ( kj/mol) Intermolecular forces are relatively weak because they typically involve smaller charges that are farther apart. H-bond (10-40 kj/ mol) LDF ( kj/mol) 20 Intermolecular Forces Keeping Matter Together