Physical chemistry - forces within and betweenmolecules?

Question

a) Which would you expect to have the higherboiling point, cis-dichloroethane or trans-dichloroethane?
Why?
b) Explain the trend in boiling points of the hydrogen halides:
i) HCl (-85 degree celsius)
ii) HBr (-67 degree celsius)
iii) HI (-35 degree celsius)

For part a), my answer is cis-dichloroethane because the bulkygroups are on the same side which requires more energy to breakdown the molecule. However, I'm not sure if my answer'scorrect.

For part b), is boiling point related to the molecular mass? Biggermolar mass => higher boiling point?
If so, why? a) Which would you expect to have the higherboiling point, cis-dichloroethane or trans-dichloroethane?
Why?
b) Explain the trend in boiling points of the hydrogen halides:
i) HCl (-85 degree celsius)
ii) HBr (-67 degree celsius)
iii) HI (-35 degree celsius)

For part a), my answer is cis-dichloroethane because the bulkygroups are on the same side which requires more energy to breakdown the molecule. However, I'm not sure if my answer'scorrect.

For part b), is boiling point related to the molecular mass? Biggermolar mass => higher boiling point?
If so, why?

Explanation / Answer

a) cis-dichloroethane b/c the bulky groups are on the sameside. b) All three are ionic compounds (salts), which boil at muchhigher temperatures than other materials b/c a lot of energy isneeded to separate the cations from the anions. Adding so muchenergy to ionic compounds requires a lot of heat, which explainsthe boiling point. Cl, Br, and I are all in Period 7 of theperiodic table (halogens). The electronegativity (andelectron affinity and ionization energy) all go up as you go up theperiod, which would explain the boiling point going up from HI toHBr to HCl.

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