GS-2 Physical Sciences Question 43

[aref.qu6209]

I thought C-H bonds were relatively nonpolar? How can Sulfur and carbon have the same electronegativity, they are in different groups and periods.

[mcat_premed3832]

You are right! It is the mantra especially in organic chem to say that C-H bonds are relatively non-polar. But the key is the word "relatively"! C-H is usually compared to C-O or C-Br or some other halide, etc etc.

But you are given a table in this problem. So there is no reason to get this problem wrong unless you are so used to relying on memory rather than "going with the flow". I know there are some companies out there (we will not name!) that have exams based mostly on memory. But the new MCAT (the real one!) is mostly based on thinking, evaluating, analyzing, etc.

There is ALOT that can come up on the real MCAT that you have never heard of before. That's not important. What's important is that you have the basics down packed and that when you are presented new information, you go with it.

By the way, the data in the table (showing that C and S have electronegativities almost identical but C-H has a small but larger difference) is consistent with the table on Wikipedia: http://en.wikipedia.org/wiki/Electronegativity

[hua8986059]

omg... didn't even look at that table.. I was looking at the period table... and background knowledge....


Tricky, tricky.... indeed.

[mohsinalam5969]

You are right! It is the mantra especially in organic chem to say that C-H bonds are relatively non-polar. But the key is the word "relatively"! C-H is usually compared to C-O or C-Br or some other halide, etc etc.
In general, if the difference between the electronegativities of two atoms is greater than or equal to 2.0, then a bond between them will be ionic. If the difference in electronegativity is less than 2.0, then the bond will be covalent. If in a covalent bond the difference is close to zero, the bond will be nonpolar covalent, and between about 0.4 and 2.0 it will be polar covalent. How close does the difference have to be to zero before the bond is nonpolar covalent? Carbon and sulfur are both generally listed as having electronegativities of 2.5 while hydrogen has an electronegativity of 2.1 (see chart below). Consequently, the difference in electronegativity between carbon and hydrogen (0.4) is the same as the difference between sulfur and hydrogen (0.4). These electronegativity differences are right on the borderline between polar and nonpolar covalent. In fact, the bond between carbon and hydrogen is considered to be nonpolar while the one between sulfur and hydrogen is considered to be polar.
Hope it will help you.THANKS