HKDSE Chemistry Enrichment Questions (20241104)

Preamble:

話明係enrichment ， DSE當然唔考。此系列旨在提供預科至本科程度化學知識。如何你本身對化學無興趣，你大可以直接㩒上一頁繼續同郊登一眾無知無恥動漫狗公一齊癲。

Question 1:

Apart from addition polymerization and condensation polymerization, suggest one more mode of polymerization as common practices.

Solution:

Coordination polymerizations are used to obtain polymers with very high stereospecificity. For this purpose, very special catalysts may be used. For example, special Ziegler–Natta catalysts are used in the polymerization process to synthesize isotactic polypropylene (i-PP) and high density polyethylene (HDPE).

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Question 2:

In what circumstances will water be an acid or base?

Solution:

The acid anhydride for sulphuric acid (H2SO4) is SO3. It is a Lewis acid because it can accept lone pair electrons from a Lewis base. In this case, water (H2O) will donate a lone pair electrons to SO3 to regenerate sulphuric acid, and the process is called neutralization because the oxidation number of the sulphur atom remains +6.

When facing a dehydrated acid such as HCl in gaseous phase, water molecules will take away a proton from the acid such that a conjugate base, i.e. chloride ion will be formed. The formation of H3O+ ion indicates the water molecule is acidified/protonated.

Question 3:

Suggest a method to supply oxygen from common fruits and vegetables.

Solution:

Fruits and vegetables are abundant in nitrate(V) ions, say KNO3 (aq).

Since K+ and Na+ ions are very difficult to reduce in room conditions, the NO3- ions won't decompose to yield NO2(g). Instead, Oxygen gas is liberated when the salt is heated:

2 KNO3 (aq) --> 2 KNO2 (aq) + O2 (g)

Note that nitrate(III) ions as a product is a strong carcinogen.