Sample HKDSE Chemistry Questions (20240206)

1. Determine whether the following processes are physical changes, chemical changes or both.

(a) Radioactive decay

(b) Sublimation

(c) Respiration

(d) Nuclear fusion

2. Suggest how to detect the existence of mercury in qualitative analysis.

2024重DSE?

佢始終都係GCE O Level/GCSE程度加大碼

當然，相比GCE A Level/IBDP, DSE小巫見大巫

Solution:

(a) New substances (Matter that have mass) are formed, EXCEPT for gamma decay. A beta decay can be regarded as first-order chemical reaction.

Also, since the processes involve changes in entire atomic structure, a beta decay can be regarded as physical change, i.e. nuclear reaction.

(b) Change of state does not create new matter, so it is a physical change.

Note that the electronic configuration of the matter remains unchanged, so sublimation is not a chemical changes.

(c) During respiration, the nuclear structure of both reactants and products are unaltered, so it is not a physical change.

Rather, the electronic configuration of reactants are changed in order to form products, so respiration is chemical reaction.

(d) Same as (a)

so sublimation is not a chemical change*.

Solution:

Some cheap luncheons involve cleansing swine intestines using mercury fluid, so if we add Sulphur powder on them, scarlet HgS precipitates will be formed on the surface.

BTW, 部份安宮牛黃丸成份包括朱砂（HgS）

想死快啲就即管食多啲

就算你食嗰隻無朱砂，

由於蠍子亦是其中一個主要成份，

想花錢長生不老結果快咗去見毛主席就碌柒啦

3. Arrange the boiling points of the following 3 molecules in ascending order based on their empirical masses:

AlCl3, SiCl4, PCl5

4. Explain why H2O always undergoes heterolytic fissure while H2O2 homolytic fissure.

Hint: Which one of them often forms free radicals?

5. Given an excess amount of Benzene liquid which is carcinogenic, suggest a common method to neutralize it such that it is safe to pour the reactants to chemical waste bottles.

Solution:

AlCl3 is actually Al2Cl6. Since its molecular sizes are the greatest among the 3 chemicals, the intermolecular forces are great enough to push up its boiling point. So intuitively:

SiCl4 < PCl5 < AlCl3

H2O has the Lewis structure below:

It will form an adduct when reacting with SO3, so a pair electrons will transfer from one atom to another, which must be heterolytic.

For H2O2, the Lewis structure is as below:

The O-O single bond is quite weak due to lone pair electrons repulsion. With enough activation energy, homolytic fissure will occur to form hydroxyl free radicals ‧OH which can, with sufficient protons supplied, turn flesh into gases.

Solution:

Benzene has 6 carbon atoms only. With excess hot acidified KMnO4, it can break the carbon-carbon bonds of bond order 1.5 into an aliphatic carboxylic acid with acyl groups on its head and tail.

Note that Mn is a heavy metal and its solution should never be poured directly to the sink.