Intermolecular Forces

1. Intermolecular Forces

2. The atom has a centrally located nucleus with a positive charge. surrounded by a cloud of electrons (with a negative charge). +

3. Cl Cl Cl Cl Cl Cl Cl Cl Cl Na Na Na Na Na Na Na Na Na Na Na Ionic substances (electrolytes) bond by a process of transferring electrons to become stable. An electron is transferred from sodium (Na) to chlorine. . forming two oppositely charged ions. . Na Na which attract each other forming an ionic bond. Chloride is surrounded by sodium ions, sodium in turn is surrounded by chloride. which is surrounded by sodium and so on.

4. Cl H Molecular substances bond by sharing electrons to become stable. Covalent bond Covalently bonded substances form molecules. Ionic bonded substances do not form molecules.

5. This type of interaction, covalentbonding, would be considered to be intramolecular(within the molecule). What we’ll be seeing is the interactionbetween molecules, intermolecular, ‘bonding’ between molecules.

6. Here’s an example of two elements forming a covalent bond. Hydrogen comes in pairs. It is already covalently bonded. So is oxygen. water H2 O2 Covalently bonded substances form independent molecules. water H2

7. Ionically bonded substances form crystal lattices. NaCl

8. When atoms covalently bond, the electrons might not be shared equally between the two atoms. . . and away from the hydrogen nucleus. Electrons are pulled towards the chlorine side. . . and this side to have a slightly positive charge. H Cl d+ d- 2.1 3.5 Causing this side to have a slightly negative charge. . . Chlorine has a greater electronegativity….. a stronger attraction for the shared electrons.

9. This is a polar molecule. A molecule that has a dipole (2 poles). HCl d- d+ Vectors are used to indicate the direction of electron pull.

10. HCl HCl HCl HCl d+ d+ d+ d+ d- d- d- d- Molecules that are polar attract each other.

11. Cl Of course, two atoms that share their electrons equally would be nonpolar. Cl Cl 3.5 3.5 Equal sharing of electrons = nonpolar bond

12. Which would boil or vaporize first? Nonpolar substances are usually gases or have low boiling points. Polar molecules Nonpolar molecules

13. Hydrogen bondingis an extreme case of polar bonding. HFHN-HO- Three of the highest electronegative elements. Hydrogen with just one electron to share

14. Water is an example of hydrogen bonding between molecules of water. Between both H-O bonds, oxygen has the greater pull on the shared electrons. d- 3.5 H O d+ 2.1 H 2.1 d+ The result is that the electrons in both H-O bonds are pulled towards the oxygen atom.

15. Water is an example of hydrogen bonding between molecules of water. d- H O d+ H d+

16. d- d+ d+ d+ d+ d+ d- d+ d- This is hydrogen bonding. Hydrogen bonding is the attraction between molecules when hydrogen is bonded to oxygen, fluorine or nitrogen.

17. d- d- d+ d+ d+ d+ d- d+ d+ HF HF NH3 NH3 H bonding between three different molecules. H2O H2O

18. H H H H F F F F d+ d+ d+ d- d- d- d+ d- Hydrogen bonding – when H is bonded to F, O, or N it is attracted to F, O, or N on a neighboring molecule.

19. H H H H O O O O H H H H d+ d+ d-

20. H H H H H H H H H O O O C C C H H H CH3OH

21. d- d- d+ d+ d+ d+ d- d+ d+ HF HF NH3 NH3 H bonding between three different molecules. H2O H2O

22. H H N H H F O H H H bonding between three different molecules.

23. Even nonpolar molecules have some weak attraction between molecules. Cl Cl Cl2 Consider the equal sharing of electrons in a Cl2 molecule.

24. This type of attraction between nonpolar molecules is called London dispersion forces. As the molecules near each other the electron clouds are repelled from each other. d- d+ d+ d- Cl2 Cl2 Causing the molecules to temporarily become polar.

25. d- d+ d- d+ d- d+ d+ d+ d+ d- Review dipole-dipole forces H2O hydrogen bonding The charged side on a molecule is attracted to the oppositely charged side on another molecule. HCl HCl London dispersion forces When heat is removed, the molecules lose their energy of motion, slow down and the intermolecular forces will hold the molecules solidly in place. In this case forming ice. The hydrogen proton on one molecule is attracted to the negative side of another molecule when H is bonded to F, O, or N. Removing even more heat is needed to solidify this substance. d- d+ d+ d- Temporary dipoles are induced when two molecules near each other. H2O

26. It’s these intermolecular forces that cause molecules to become solids. For example, it’s the hydrogen bonding between water molecules that cause water to change to ice. It explains why substances are solids, liquids and gases at room temperature. It explains why substances freeze and boil at different temperatures.

27. strong ionic bonding between charged ions Ionic substances solid (crystals) weak attraction between slightly charged molecules Polar covalent liquid, gas or solid weaker attraction between non-charged molecules Nonpolar covalent gas or liquid

28. Types of Intermolecular Forces Types Examples Ion-Ion forces (ionic bonding) NaCl, NH4NO3 Ion-dipole forces NaCl in H2O Hydrogen bonding H2O, NH3, HF Dipole-dipole H2S, HCl, CH3Cl London forces Ar, O2, N2

29. Sample Exercise 11.3 List the substances BaCl2, H2, CO, HF and Ne in order of increasing boiling points. First, identify the type of intermolecular force, then arrange them from weakest to strongest. BaCl2 ion-ion London forces H2 dipole-dipole CO hydrogen bonding HF London forces Ne H2 < Ne < CO < HF < BaCl2