Polarity and Water Chapter 9.1 and 13

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-

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

Electronegativity Values of Chemical Elements

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

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

Electronegativity Trends in the Periodic Table

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.

Polar bonds EN: H Cl  EN = 0.9 2.1 3.0

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.

Dipole of water molecules  -  +

Intermolecular Attraction (Van der Waal’s Forces)

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  -

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)

Surface tension Capillarity Mercury, however …

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

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

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

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

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

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

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

Book assignment p. 451 - 454 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? When finished show us your work and you will receive your homework sheet