1. Polarity and Water
Chapter 9.1 and 13

3. Ionic or molecular; polar or nonpolar
If a nonmetal atom can pull electrons completely to itself, relatively to a metal atom, the bond will be
ionic.
Na Cl-
If two (nonmetal) atoms are identical, the electron inside their bond will be right in the middle and the bond will be
nonpolar covalent
Br---Br
Many bonds are somewhere inbetween: sharing electrons, but not symmetric, making the bond
polar covalent
+ H---O-

4. Electronegativity: An Attraction for Electrons
How each atom fares in a tug-of-war for shared electrons is determined by comparing the electronegativities of the two bonded atoms.
Electronegativity is the tendency of one atom to attract the electrons inside a chemical bond

5. Electronegativity Values of Chemical Elements

6. Electronegativity is the tendency of one atom to attract the electrons inside a chemical bond

7. Difference in Electronegativity EN makes bonds …
2.0 – 3.3
ionic
0.5 – 1.9
polar
0.0 – 0.4
nonpolar

8. Electronegativity Trends in the Periodic Table

9. Polar covalent bonds
are called polar because the unequal electron sharing creates two poles across the bond.
Electrons
Closer to
higher EN
more
negative
Just as a battery has separate positive and negative poles, so polar covalent bonds have poles.
The negative pole is centered on the more electronegative atom in the bond.

10. Polar bonds
EN: H Cl  EN = 0.9

11. Exceptions: For example CO2 has two polar bonds
Exceptions: For example CO2 has two polar bonds. However, the dipoles point in opposite directions and cancel out. As a molecule CO2 is nonpolar.
On the other hand, water is a bent molecule and both dipoles point upward. As a molecule H2O is polar.

13. Dipole of water molecules
 -
 +

14. Intermolecular Attraction (Van der Waal’s Forces)

15. Hydrogen bonds
are a connection between the hydrogen atoms of one molecules and a highly electronegative atom of another molecule, but not a full covalent bond.
 +
2  -

16. The two DNA strands in a double helix are held together by many hydrogen bonds;
2 between each thymine (T) and adenine (A) and
3 between each cytosine (C) and guanine (G)

17. Surface tension
Capillarity
Mercury, however …

18. less dense than cold liquid water
Density of water
Water has its
highest density at 4C
Therefore…
Ice Floats
Ice crystal
less dense than cold liquid water

19. Demos: dollar + paper cup
Water has a high specific heat
(lots of energy is absorbed when heating up water or during vaporization)
Water is earth’s thermostat (Moderating climate)
Cools down your body during perspiration, etc.
Demos: dollar + paper cup

20. Water the “Universal Solvent”
Dissolves polar covalent and ionic compounds
… thus rarely pure
Solution: Homogeneous mixture e.g saltwater
Solute: Dissolved particles in a solution salt
Solvent: The dissolving medium in a solution water
The concentration of a solution is a measure of the amount of solute that is dissolved in a given quantity of solvent
dilute solution
concentrated solution
Saturated solution

21. Is a measure of concentration in chemistry
Molarity
Is a measure of concentration in chemistry
Molarity (M) is the number of moles of a solute dissolved per liter of solution, mol/L.
Molarity (M) = moles of solute liters of solution
example
2 mol glucose = 0.4 mol/L = 0.4 M
5 L solution

22. Water dissolving ionic compounds such as NaCl
Polar water molecules surround ions facing each with its oppositely charged dipole

23. Why is sugar highly soluble in water?
Polar water molecules form multiple hydrogen bonds with each sucrose molecule
simulation 19

24. Most compounds dissolve better
at higher temperature
Think of sugar in hot tea
or ice tea
(but there are exceptions)

25. Book assignment p
1) Describe how water dissolves ionic compounds
2) Describe how water dissolves polar covalent compounds
3) Explain the expression “like dissolves like”. Which compounds does water dissolve and which compounds are typically not soluble in water?
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