March 1, 2015

Homework Help: Chemistry

Posted by Alekya on Thursday, August 2, 2007 at 5:08pm.

Hi I have two questions and I was hoping someone could help me with them.

1. Classify the following compounds as ionic or covalent
a. MgCl2
b. Na2S
c. H2O
d. H2S

2. Which compound in each pair exhibits the stronger intermolecular hydrogen bonding?
a. H2S vs. H2O
b. HCl vs. HF
c. HBr vs. HCl
d. NH3 vs. H20

Thanks for your help.

The secret to #1 is to look up the elecrronegativities of each element and subtract them. For example, H is about 2.1 and O is about 3.5. The difference is 1.4 We usually call compounds with a difference of 1.8 or so ionic. Less than that will be covalent. Technically, few molecules are all one or the other. On this scale MgCl2 will be ionic and H2O will be covalent. Now you do the other two.
The secret to #2 is to look at electronegativities, also. S is about 2.5 and O is about 3.5 so H2O is more likely to form H bonding. For NH3 and H2O, N is about 3.0 and O is 3.5 so H2O is more likely to form H bonding than NH3. The compound in which the non-hydrogen element has the higher EN have a tendency to more likely form H bonding. I will leave the others for you.

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