Unit 6 Objectives 11, 12

1. Unit 6Objectives 11, 12 Polarity of Water, Hydrogen Bonding in Water

2. Bond Types (Review) • A nonpolar covalent bond results from even sharing of electrons • A polar covalent bond results from uneven sharing of electrons • An ionic bond results from a transfer of electrons

3. Bond Polarity • Electrons are not always shared equally in a covalent bond. Sometime one element has a stronger pull on the electrons.

4. Water • Draw the LDS for water: • Classify the bond type (nonpolar covalent, polar covalent, or ionic) for the O-H bond: • Look up the electronegativity values for each atom • 3.5 – 2.1 = 1.4, polar covalent

5. Polar Molecules • In a polar molecule, one end of the molecule is slightly negative (δ-) and the other end is slightly positive (δ+)

6. What makes a molecule polar or nonpolar? • Even though a molecule has polar bonds, it may not be a polar molecule • The shape (molecular geometry) of a molecule needs to be determined before the molecular polarity can be determined

7. Polarity of Molecule is Different! • Polarity of the molecule depends on 1. Magnitude of electron distribution (Diff, in EN) 2. Direction of the pull • If two atoms are pulling away from a third in opposite directions – then often the force cancels and electrons stay put

8. Examples of polar and nonpolar molecules • Example 1 – CO2: • CO2 has polar bonds (3.5 – 2.5 = 1.0) • The molecule is nonpolar because its molecular geometry is linear • Often molecules that do not have lone pairs on the central atom are nonpolar, if all of the external atoms are of the same element

9. Examples of polar and nonpolar molecules • Example 2 – H2O: • H2O has polar bonds (3.5 – 2.1 = 1.4) • The molecule is polar because its molecular geometry is bent • Often molecules that have lone pairs on the central atom are polar

10. Water is Special • The polar bonds and the lone pairs on the oxygen make water a POLAR molecule and a result it has special properties that relate to its ability to interact with other molecules.

11. Attractions Between Molecules • Intermolecular forces are WEAKER than ionic and covalent bonds • The strongest type of intermolecular force is hydrogen bonding • Its NOT actually a bond • It has 5% of the strength of an average covalent bond

12. Hydrogen Bonds • Hydrogen bonding is defined as attractive forces in which a hydrogen covalently bonded to a very electronegative atom (F, O, or N) is also weakly attracted to an unshared electron pair of another electronegative atom (F, O, or N) • This always involves a H atom • Hydrogen Bonding in Water Animation

13. Hydrogen Bonding in Water • The hydrogen atom in one water molecule is attracted to the lone pairs on the oxygen atom in another water molecule • Hydrogen bonds are represented as dashed lines

14. Hydrogen Bonds in Water • Hydrogen bonds are important in determining the properties of water • Surface tension • Capillary action • Density • Boiling Point

15. Solubility of Water • “Like dissolves like” explains the solubility of water • Because water is a polar molecule, it will dissolve all other polar molecules or ionic compounds (such as sugar, NaCl) • Water will not dissolve nonpolar molecules (such as oil)

16. Video on Like Dissolves Like Water and NaCl: both are Polar so they mix well Water and oil: Water is polar and oil is nonpolar – they do not mix

17. Surface Tension • Definition: How hard it is break the surface • Caused by the Cohesion of the water molecules: the ability of the molecules to stick together • Water molecules stick together because of the hydrogen bonds

18. COHESIONand Surface Tension • Water molecules experience attractions to neighboring water molecules in all directions • Water molecules at the surface experience the attractions to neighboring water molecules only sideways and downwards, which creates suface tension http://www.chem1.com/acad/sci/aboutwater.html

19. Examples of Surface tension of Water Ref:

20. Cohesion vs. Adhesion • Cohesion: How sticky the water molecules are to EACH OTHER • Adhesion: How sticky the water molecules are to ANOTHER SURFACE

21. Capillary Action: An example of Adhesion • Most plants have adapted to take advantage of water’s adhesion that helps move water from the roots to the leaves (capillary action) http://mvhs1.mbhs.edu/riverweb/jigsaw/PoW.pdf

22. Density • Water has a unique property because it expands as it freezes, making the density of ice (0.925 g/mL) less than that of liquid water (1.00 g/mL) mbari.org

23. Hollow structure reduces density • Hydrogen bonding gets stronger as the temperature gets lower causing the structure of ice to be completely hydrogen bonded, and these bonds force the crystalline structure to be very "open“ (which increases the volume)

24. Boiling Point • As liquid water boils (at 100 °C), the water molecules are able to break the forces of attraction i.e. the hydrogen bonds to each other and escape as the gas molecule http://www.elmhurst.edu/~chm/vchembook/163boilingpt.html Animated GIF "Boiling" - Courtesy of General Chemistry Help - Purdue University

25. Qualities of Water • Video on Water • Pay attention to Specific Heat and What it Means

26. References • http://www.infoplease.com/cig/biology/water.html • http://www.chem1.com/acad/sci/aboutwater.html • http://mvhs1.mbhs.edu/riverweb/jigsaw/PoW.pdf • http://www.iapws.org/faq1/freeze.htm • http://www.elmhurst.edu/~chm/vchembook/163boilingpt.html • All images taken from www.google.com/images