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worksheet 7 key - CH301 Fall 2009 Worksheet 7 Answer Key 1...

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CH301 Fall 2009 Worksheet 7 Answer Key 1. How can you decide whether a bond is polar or not? If a molecule has polar bonds, does that make the molecule polar? A polar bond is between two atoms with different electronegativities. One atom with hog the electrons, giving it a slightly negative charge, denoted δ-. The other atom w ill have a slightly positive charge, δ+. A molecule with polar bonds may not be polar overall. Symmetry of the molecule can cancel out the dipoles created by the polar bonds. For example, BF3 has three polar bonds but all these polar bonds are in the same plane and cancel each other out. If you replace one of the F with another atom then there would be polarity in the molecule because the electronegativities would not cancel out. 2. Fill in the chart and then rank the molecules in order of increasing polarity. NH3 O2 NF3 C2H5OH Draw structure Calculate ΔEN of all bonds ΔEN = 3-2.2 = .8 ΔEN=2.5-2.5= 0 ΔEN = 4 - 3 = 1 ΔEN C-H = 2.5-2.2 = .3 ΔEN C-O = 3.5-2.5 = 1 ΔEN O-H = 3.5-2.2 =1.3 Polar or nonpolar molecule? Polar Nonpolar Polar Polar and nonpolar ends O2 < C2H5OH < NH3 < NF3 Homonuclear diatomic molecules are never polar. 3. Rank the following in order of increasing polarity:
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D = B = A < C Molecule C is the only one where the dipoles do not cancel out. 4. Explain how carbon, with only 2 unpaired electrons, can form 4 bonds to fulfill its octet. Account for any energy changes (if you're putting in energy, where is it then released). What kind of hybrid orbitals will carbon then form? One of the electrons in the 2s orbital is promoted to the 2p orbital, giving carbon the electronic configuration of [He]2s12p3. Now there are 4 unpaired electrons that can bond. Promoting an electron takes energy, but the amount of energy released when bonds form compensates this. Also, only a small amount of energy is needed to initially promote the electron to the 2p orbital. If the electrons stay in the 2s and 2p orbitals then we would see 2 different types of bonds, but we know that we only see one type. In order to form 4 bonds these electrons will then hybridize their orbitals to form four sp3 hybrid orbitals.
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