Polarity and Hydrogen Bonding Post-Lab

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Presentation transcript:

Polarity and Hydrogen Bonding Post-Lab water

http://ed.ted.com/lessons/how-polarity-makes-water-behave-strangely-christina-kleinberg

Why does the water bend??? The comb picks up electrons giving it a negative charge. The positive charges in the water are attracted to the negative charges from the comb, and the negative charges in the water are repelled. Overall, the water has a net charge difference, so it bends toward the comb.

Water is a polar molecule--the O pulls the electrons more strongly, so it has a slightly more negative charge. Oxygen is more electronegative than hydrogen.

Polar molecules have both: 1. Polar bonds (Determined by electronegativity) 2. Symmetry in all directions

Electronegativity: tendency of an atom to attract a bonding pair of electrons. Electronegativity values are found in Table S of the reference table More protons moving left to right = more “pull” on electrons Fewer electron shells bottom to top = more “pull” on electrons

Examples: CO → EN difference = 3.5 - 2.5 = 1.0 CO has a polar covalent bond because the EN difference is within the range 0.4 and 2.0. Cl2 → EN difference = 3.0 - 3.0 = 0 Cl2 has a nonpolar covalent bond because the EN different is within the range 0.0 and 0.4.

Electronegativity difference between TWO atoms gives us Polarity of the bond So now we need to know the SYMMETRY of the molecule to determine the overall polarity of the molecule...

For some symmetry practice... Are these shapes symmetrical or asymmetrical? No symmetry Symmetry

Now look at these molecules… Are they symmetrical or asymmetrical? No symmetry Symmetry Now look at these molecules… Are they symmetrical or asymmetrical?

Symmetrical → NONPOLAR No symmetry Symmetry Asymmetrical → POLAR Symmetrical → NONPOLAR

SUMMARY: Polarity and Bond Type Polar Covalent Molecules *asymmetrical with polar bonds *unequal distribution of electrons and charge *dipole= positive & negative side *EN difference between 0.4 and 2.0 Nonpolar Covalent Molecules *symmetrical or nonpolar bonds *equal distribution of electrons and charge *EN difference between 0.0 and 0.4

Hydrogen Bonding Hydrogen Bond = a weak bond between two molecules resulting from an electrostatic attraction between a proton in one molecule and an electronegative atom in the other

So… why can a fly walk on water? Hydrogen Bonding So… why can a fly walk on water? The water molecules are attracted to each other (cohesion) because water is asymmetrical and polar (unequal sharing of electrons), which causes hydrogen bonding between the water molecules. So, if you are careful not to break these hydrogen bonds, an object can be held up by the water molecules due to surface tension. But hydrogen bonds are weak, so it cannot hold much weight or handle a lot of force.