Question:
why do you suppose small-ring cycloalkenes such as
cyclohexenedon't exist as cis-trans isomers?

Cyclohexene doesn't exhibit this type of Isomerism. Cyclohexene is a tiny ring to exhibit the trans isomer. The Structural formula and component bonding characteristic within it limits it to show this phenomenon. Trying to force it to be trans would place a massive amount of ring strain on the molecule.

This characterises major rings within Alkene group up to cyclooctene.

Explanation:

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

the electron configuration that you gave me has 16 electrons ergo it has 16 protons ergo it is the element sulfur.