Polarity. Q

Transcription

1 Unit 6 Polarity

2 Polarity Reflect: Hydrogen has a very low affinity for electrons, while oxygen has a very high affinity. What do you think the bond between the two is like?

3 Polarity Q

4 Polarity Nonpolar covalent bonds Electrons are shared equally No electronegativity difference Between same atoms (diatomic molecules) Example: H H

5 Polarity Polar covalent bonds Electrons are shared unequally, have δ+ and δ- end Polar ends = creates a dipole Difference in electronegativity Between different atoms Example: H O

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8 Electronegativity Difference: Polarity Subtract the two elements electronegativity to determine polarity Any difference = polar/unequal sharing More difference, more polar * Arrow shows the direction the electron cloud is being pulled.

9 Hydrogen Oxygen Bond Electronegativity of O: 3.4 Electronegatity of H:2.2 Electronegativity difference: 1.2 Polar or nonpolar? Polar

10 Hydrogen Hydrogen Bond Electronegativity of H: 2.2 Electronegatity of H: 2.2 Electronegativity difference: 0 Polar or nonpolar? Nonpolar

11 Nitrogen Oxygen Bond Electronegativity of O: 3.4 Electronegatity of N:_3.0 Electronegativity difference: 0.4 Polar or nonpolar? Polar (but not as polar as O-H)

12 Polarity Practice Multiple Choice # 1 10

13 Reflect: Shapes of Molecules Most molecules are made of polar bonds. How, then, can the overall molecule be nonpolar if it is composed of polar bonds?

14 Shapes of Molecules The most important thing to remember when attempting to predict the shape of a molecule based on its chemical formula and the basic premises of the VSPER model is that the molecule will assume the shape that most minimizes electron pair repulsions.

15 Shapes of Molecules In attempting to minimize electron pair repulsions, two types of electron sets must be considered: electrons can exist in bonding pairs, which are involved in creating a single or multiple covalent bond, or nonbonding pairs, which are pairs of electrons that are not involved in a bond, but are localized to a single atom.

16 Shapes of Molecules

17 Shapes of Molecules

18 Shapes of Molecules

19 Shapes of Molecules

20 Shapes of Molecules

21 Polarity of Molecules Reflect: Look at the bonds below and decide if they are polar (P) or nonpolar (NP): H H C H C Cl H F B F C O O H N- H

22 Shapes of Molecules Molecular Polarity: An uneven distribution of electron pairs between 2 bonded atoms. If polar bonds are arranged in a symmetrical pattern, the overall molecule can be nonpolar despite having polar bonds.

23 Shapes of Molecules PI 3 Shape: Pyramidal Polarity: Polar

24 Polarity of Molecules

25 Polarity of Molecules

26 Polarity of Molecules Practice Multiple Choice # 1-12

27 Intermolecular Forces Reflect: What happens when you put the ends of two magnets near each other? How might that relate to polar molecules?

28 Intermolecular Forces

29 Water Big difference in electronegativity =Strong IMF Therefore, High mp/bp Cohesion Adhesion High Surface Tension High Viscosity Low Vapor Pressure

30 Intermolecular Forces Hydrogen Bonding Strongest IMF Attraction between polar molecules H-X bond creates a VERY polar bond, thus stronger attraction (bigger difference in electronegativity)

31 Intermolecular Forces Dipole-Dipole Attraction between polar molecules Opposite charges attract each other, likes repel

32 Intermolecular Forces Dispersion (Van der Waals) Weakest forces Temporary distortion of electron cloud creates temporary polar molecules. A second atom can be induced (briefly) to also be polar, and thus weakly attract first atom Weak and fleeting attraction Bigger electron cloud = stronger dispersion forces (still weak relatively)

33 Intermolecular Forces Viscosity: Resistance to flow Surface Tension: Imbalance of forces at the surface of a liquid

34 Intermolecular Forces Practice Multiple Choice Questions #1-10

35 Reflect: IMF and Table H Draw the interactions between some polar molecules:

36 IMF and Table H

37 IMF and Table H Which substance 1. Most easily escapes to gas phase? Propanone 2. Has a boiling point of 80 C at STP? Ethanol 3. Has the strongest intermolecular forces? Ethanoic acid 4. If the pressure is reduced to 50 kpa, boils at approximately 82 C? water 5. Is the most volatile? propanone Line is boiling point. Boiling point is when vapor pressure = atmospheric pressure

38 IMF and Table H Practice Multiple Choice # 1-12