Download - Lecture 11- Intermolecular Forces
![Page 1: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/1.jpg)
Dr. Rabih O. Al-Kaysi
Ext: 47247Email: [email protected]
General Chemistry Course # 111, two creditsSecond Semester 2009
King Saud bin Abdulaziz University for Health Science
Textbook: Principles of Modern Chemistry by David W. Oxtoby, H. Pat Gillis, and Alan Campion (6 edition; 2007)
![Page 2: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/2.jpg)
Lecture 11Lecture 11 Intermolecular ForcesIntermolecular Forces
![Page 3: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/3.jpg)
• Physical properties of substances understood in terms of kinetic molecular theory:• Liquids are almost incompressible, assume the shape but not
the volume of container:– Liquids molecules are held closer together than gas molecules, but
not so rigidly that the molecules cannot slide past each other.• Solids are incompressible and have a definite shape and
volume:– Solid molecules are packed closely together. The molecules are
so rigidly packed that they cannot easily slide past each other.
A Molecular Comparison of Liquids and Solids
![Page 4: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/4.jpg)
Comparison of Liquids Comparison of Liquids and Solids Cont.and Solids Cont.
![Page 5: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/5.jpg)
•These are the forces holding solids and liquids together are called intermolecular forces.
Intermolecular ForcesIntermolecular Forces
![Page 6: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/6.jpg)
• The attraction between molecules is an intermolecular force.
• Intermolecular forces are much weaker than intramolecular forces (e.g. 16 kJ/mol vs. 431 kJ/mol for HCl).
• When a substance melts or boils the intermolecular forces are broken (not the covalent bonds).
• However, when a substance condenses, intermolecular forces are formed.
What are Intermolecular What are Intermolecular ForcesForces
![Page 7: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/7.jpg)
Vaporizing HCLVaporizing HCL
Vapor phase
Liquid phaseHeating Liquid Solutions
Formation intermolecular forces
intermolecular forces are broken
![Page 8: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/8.jpg)
Inter- Inter- vs. vs. Intramolecular ForcesIntramolecular Forces
The covalent bond holding a molecule together is an intramolecular force.
![Page 9: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/9.jpg)
Properties Reflecting Properties Reflecting Molecular Force Strengths Molecular Force Strengths
• Boiling and melting points reflect the strengths of intermolecular forces.
• High boiling points indicate strong attractive forces between molecules.
- For example, HCl boils at -85oC at room temperature due to its weak attractive forces.
• Melting points increase with increasing attractive forces (i.e., molecules become harder to separate).
![Page 10: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/10.jpg)
Types of Molecular Forces Types of Molecular Forces
• There are four types of molecular forces:• Ion-dipole Forces
• Dipole-dipole Forces
• London Dispersion Forces
• Hydrogen Bonding Forces
• The lateral three forces are general called van der Waals forces (developed by Johannes van der Waals) and exist between neutral molecules
• The ion-dipole forces exist between ions and polar molecules.
![Page 11: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/11.jpg)
Water (HWater (H22O)O)
![Page 12: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/12.jpg)
Molecular PolarityMolecular Polarity
![Page 13: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/13.jpg)
• Interaction between an ion and a dipole.• Dipole is a polar molecule (e.g. water).• Strongest of all intermolecular forces.
Ion-dipole ForcesIon-dipole Forces
![Page 14: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/14.jpg)
• Dipole-dipole forces exist between neutral polar molecules.
• Only effective when polar molecules are close together.• These forces are weaker than ion-dipole forces.• There is a mix of attractive and repulsive dipole-dipole
forces as the molecules tumble (free flow in liquids)• If two molecules have about the same mass and size, then
dipole-dipole forces increase with increasing polarity.
Dipole-dipole ForcesDipole-dipole Forces
![Page 15: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/15.jpg)
Dipole-dipole Forces Dipole-dipole Forces Schematic Schematic
![Page 16: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/16.jpg)
• Weakest of all intermolecular forces.• Primary property that cause nonpolar substances to
condense to liquids and to freeze into solids at low temperatures.
• Form when electrons occupy positions around the nucleus in two adjacent atoms causing a temporary dipole.
• The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom).
London Dispersion ForcesLondon Dispersion Forces
![Page 17: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/17.jpg)
• The nucleus of one molecule (or atom) attracts the electrons of the adjacent molecule (or atom).
• For an instant, the electron clouds become distorted.• In that instant a dipole is formed (called an
instantaneous dipole).
Formation of London Dispersion Formation of London Dispersion ForcesForces
![Page 18: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/18.jpg)
• Dispersion forces are present in all molecules whether polar or nonpolar
• The larger the molecule (the greater the number of electrons) the more polarizable.
• London dispersion forces increase as molecular weight increases.
• London dispersion forces depend on the shape of the molecule.
• Example: neopentane (gas at 25oC), n-pentane (liquid at 25oC)
Properties Effecting London Properties Effecting London Dispersion ForcesDispersion Forces
![Page 19: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/19.jpg)
Trends in London Dispersion Trends in London Dispersion ForcesForces
Notice that as the molecular weight increases the boiling points of the halogen increases, indicating greater London dispersion forces between atoms.
![Page 20: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/20.jpg)
• By experiments: boiling points of compounds with H-F, H-O, and H-N bonds are abnormally high.
• In the case of NH3, H2O, and HF, additional intermolecular forces must be present which increases the amount of heat energy needed to separate the atoms.
• These additional intermolecular forces are called hydrogen bonds.
Hydrogen BondingHydrogen Bonding
![Page 21: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/21.jpg)
• Notice that the hydrogen is attached to the most electronegative elements. Thus, causing the hydrogen to acquire a significant amount of positive charge.
The Origin of Hydrogen BondingThe Origin of Hydrogen Bonding
![Page 22: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/22.jpg)
Hydrogen Bonding SchematicHydrogen Bonding Schematic
![Page 23: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/23.jpg)
• Hydrogen bonds are responsible for:• Ice Floating
– Ice is ordered with an open structure to optimize H-bonding.
– Therefore, ice is less dense than water.
– In water the H-O bond length is 1.0 Å.
– The O…H hydrogen bond length is 1.8 Å.
– Each + H points towards a lone pair on O.
Hydrogen Bonding in HHydrogen Bonding in H22OO
![Page 24: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/24.jpg)
Comparing Intermolecular ForcesComparing Intermolecular Forces
• Dispersion forces are found in all substances.• Their strength depends on molecular shapes and weights.
• Dipole-dipole forces add to the effect of dispersion forces.• They are found only in polar substances.
• H-bonding is a special case of dipole-dipole interactions.• Strongest of the intermolecular forces involving neutral species.
• Most important for hydride compounds (NH3, H2O, etc.).
• Ion-dipole forces are interactions between ionic and polar molecules.• Ion-dipole are stronger than H-bonds.
• Covalent bonds are stronger than any of these reactions.
![Page 25: Lecture 11- Intermolecular Forces](https://reader033.vdocuments.mx/reader033/viewer/2022061119/546aee2bb4af9f8e2c8b49ba/html5/thumbnails/25.jpg)
Intermolecular Forces Intermolecular Forces ChartChart