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Chem EK Exam 1, Passage 1, Question 4, page 258
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Chaitali_3347
#1 Posted : Thursday, June 11, 2020 2:09:59 AM
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Chem EK Exam 1, Passage 1, Question 4

1) why is CH4 not considered a hydrogen bond? i understand that hydrogen bonds occur between H and F, O and N but i'm just wondering about other molecules like CH4 and why we never classify them as hydrgoen bonds.

2)Knowing that CH4 is non polar, how can the correct answer be D?. I understand that the other answer choices are incorrect, but what is the source of the dipole induction in a non polar substance like CH4? is there another substance we assume such as H2O as the source of dipole induction?

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INSTR_Katerina_102
#2 Posted : Thursday, June 11, 2020 2:19:21 AM
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Hello,

1)Hydrogen bonding only really occurs when F-H, N-H and O-H bonds are present. This is because there is such a large electronegativity difference that the hydrogen is pretty much "naked" and has not very much electron density shielding its partial positive charge.

Because the electronegativity difference between C and H is very small (about .35), the sharing of electrons is pretty much equal and you don't get this polarization. As a result, CH4 cannot hydrogen bond.

2) Induced dipole - induced dipole is another name for london dispersion forces - as each molecule's electron density is a probability function, there can always be an instant where the electrons are unevenly distributed and the molecule can be partially positive at one end, and partially negative on the other end. This influences a molecule right beside it, and this propagation results in the attraction between particles (London dispersion forces). As you have also pointed out, you can get at this by elimination.

Please let me know if this is unclear.
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