Heavier doesn't mean stronger
- The idea
- Predict
- See it
- Why
- Check
The idea
“The heavier molecule should boil higher or hold on tighter, because it has more mass.”
Heavy things are harder to move, so heavy molecules seem like they should boil higher or grip tighter. Molecules stick by attraction, and charges can outweigh mass by a lot.
The short version
The mistake: The heavier molecule should boil higher or hold on tighter, because it has more mass.
What's actually true: If mass doesn't track the result, something else does, usually hydrogen bonding, polarity or charge.
- Boiling pulls molecules away from their neighbors. The harder they attract each other, the more heat that takes.
- In a hydrogen bond, a slightly positive H on O or N is drawn to a slightly negative O or N nearby. Dimethyl ether has no H on its oxygen, so it can't do this with itself.
- The same check works elsewhere. Size, charge and polarity decide what crosses a bilayer. An active site's shape and charges decide what an enzyme binds.
On the exam: If a lighter molecule beats a heavier one, mass is out. Find what changes with the result.