HKDSE Chemistry Question Review (2021-01-07)

1. What is the type of the following reaction?

C2H6 --> C2H4 + H2

Solution:

Cracking.

It rarely occurs because the hydrocarbon chain is so short, and hydrogen free radicals are difficult to escape from methane unless the C-C bond has homolytic fissure.

H3C· --> H2-·C· + H·

2H· --> H2

2 H2-·C· --> C2H2

Methane will NOT be formed because otherwise the reaction would be reduction of ethane in excess.

2 H2-·C· --> C2H4*

2. Explain why hambeast has unpleasant odor when he/she sweat.

Solution:

Hydrolytic Rancidity occurs when excess fatty acids are diffused from bloodstream with water as sweat. Then a lot of ketones will result in and smells.

bad* smell

Oxidative Rancidity will occur when the hambeast is exposed to direct sunlight. This time fatty peroxides will result in another bad smell.

3. 女人自青春期會有體香，然而一踏入中年體香就消失。這是怎麼的一回事?

Solution:

We learn that ester will form in acidic condition when alkanol and carboxylic acid react together. Obviously bloodstream does have a lot of fatty acids going through human body. Since liver heats up the bloodstream and the pH of bloods is below 7, we can conclude that women bloodstream must have alkanol during adolescence.

A plausible explanation is that women liver will have sugar alcohol formed when glucose inside is metabolized. When such metabolism ceases to occur, we can say that her body starts to decay.