HKDSE IS 2019 PANEL 3

Paper 1 Q1

Ans.:

Process: diffusion (1)

Example:

gas exchange at the air sac OR

oxygen diffuses out from the air sac / into the red blood cell OR

carbon dioxide diffuses into the air sac / out from the blood (1)

[Accept other correct alternatives]

ER:

■ The performance was poor.

■ Many candidates did not realise that the process being demonstrated was diffusion.

Quite a number of them mistook it for being dissolution or osmosis, and as such

they were unable to give a correct example of the movement process in human cells.

[a curve showing a drop in concentration to not lower than 0.5%]

ANS.:

Explanation: Water moves from compartment B into A (through membrane X) to

dilute the KMnO4 solution (1)

ER:

Satisfactory.

■ Most candidates were able to show in their sketch that the concentration of KMnO4(aq) decreases with time.

■ Candidates should have noted the following while drawing their sketches:

– the initial concentration and final concentration of KMnO4(aq) should be 1% and 0.5% respectively, and

– the curve should have a concave shape rather than a linear or a convex shape.

ANS.:

■ The higher mineral concentration in the cytoplasm ensures that the water potential

of the root cells is lower than that of soil water. (1)

■ This enables plants to absorb water by osmosis. (1)

ER: Well answered.

Most candidates demonstrated that they had mastered the concepts of osmosis and were able to explain the

flow of soil water to the root cells based on their difference in water potential.

ANS.:

Dissolve the mixture of sugar and salt in water and put the solution into compartment C.

(1)

Allow time for the ions of NaCl to diffuse to compartment D. Replace the water in

compartment D with distilled water from time to time. (1)

Recover the sugar by boiling away the water / allowing the water to evaporate from the

solution in compartment C. (1)

ER: Satisfactory.

Nevertheless, not many candidates were able to give a correct and complete description of the

experimental procedure.

Quite a number of candidates did not read the question carefully and failed to suggest how solid sugar

could be recovered from the resulting solution, i.e. by crystallisation or by evaporation.

A few candidates suggested methods that were not based on osmosis and the differential permeability of

the membrane to the ions of NaCl and sugar molecules. Thus, they were unable to remove NaCl from

the solution.

The presentation skills of candidates in describing experiments had room for improvement in most

cases.

ANS.:

The bonding between the H atom and F atom is covalent and HF exists as simple

covalent molecules. (1)

It does not have mobile ions / mobile electrons / charged particles or carriers to

conduct electricity. (1)

ER:

Satisfactory.

The lack of electrical conductivity of HF(l) is due to the fact that it has a simple molecular

structure and that there are no mobile charge carriers in its structure.

The weaker candidates wrongly stated that HF(l) has an ionic structure. Some candidates

erroneously related the poor electrical conductivity of HF(l) to its weak intermolecular attraction.

In fact, the latter is related to the melting point and boiling point of hydrogen fluoride.

Ans.:

2HF(aq) + H2O(l) 2H2(g) + F2O(g)

ER:

Satisfactory.

van der Waals’ forces

Accept: dipole-dipole attractions

ER:

Well answered.

Candidates were required to show, in the electron diagram, that all O and F atoms have an octet outermost electronic structure. Some candidates only showed one of these atoms having an octet structure.

Weaker candidates even drew an ionic structure for F2O. The intermolecular attraction is dipole-dipole attraction or van der Waal’s forces.

Some candidates wrongly mentioned ‘covalent bond’ which is, in fact, the attraction between the bonded atoms.

ANS.:

KF is an ionic compound. (1)

The K+ ions and F- ions in KF(s) are held together by strong ionic bonds forming an ionic

crystal. / Large amount of energy is needed to free the ions from the ionic lattice in

melting. (1)

ER: Well answered.

ER:

The performance was fair

. Candidates, who were weak at electrolysis, were unable to identify T as being the

positive pole of the battery.

The ionic half equation was poorly written.

Candidates should know that in writing ionic half equations, the electric charges on the

two sides of the equation must be balanced.

+ / anode

2F– F2 + 2e–

ANS:

The net reaction is 2HF H2 + F2 / HF is continuously consumed and so the mass

percentage of HF will drop. (1)

When the mass percentage of HF has dropped to below 38%, the mixture will solidify (at

70 – 130 C ) (1)

ER:

The performance was very poor.

Most candidates failed to realise that HF is consumed during the electrolysis and thus the percentage by mass of HF

in the electrolytic mixture will drop. When the percentage by mass of HF in the mixture drops to below 38%, the

mixture will solidify.

Many candidates appeared not to understand the chemistry involved. They wrongly proposed the occurrence of some

side reactions or the formation of potassium metal.

PAPER 2 Q3

ANS.:

There is oil / fat on the surface of our fingers. (1)

The oil / fat is converted to slippery soap. (1) [Accept: glycerol].

ER:

The performance was fair.

Quite a number of candidates failed to realise that there is oil/fat on the surface of our fingers. Some simply

regurgitated what they had learnt in junior science that alkalis can give a slippery feel. Some wrongly mentioned that

NaOH has emulsifying property. They seemingly did not realise that the structure of an emulsifier includes a

hydrophilic part and a hydrophobic part.

ANS.:

P3O105– ions bind with Mg2+ and Ca2+ ions to prevent them from forming scum /

insoluble compounds with soap ions .

ER:

The performance was fair. Many candidates did not realise that P3O105-reacts preferentially with Mg2+(aq) and

Ca2+(aq) ions in hard water.

ANS:

P3O105–, when discharged into rivers and seas, provides nutrients for algal growth. (1)

After the algae die, they are decomposed by the microorganisms in water. (1)

The process uses up dissolved oxygen in water and causes suffocation and death of

other aquatic lives.

ER

The performance was fair. Most candidates knew that P3O105-, when discharged into water

bodies, would provide nutrients for algal growth.

Many could not explain why algal bloom is harmful to aquatic lives, i.e. the decomposition

of dead algae by microorganisms in water and the respiration of algae (especially at night)

can use up the dissolved oxygen in water.

ANS:

■ A catalyst provides an alternative reaction pathway (1)

■ which has a lower activation energy for the formation of products (1)

ER:

Satisfactory. A catalyst works by providing an alternative pathway with a lower energy barrier for product

formation.

Some candidates mentioned only one of these points. Other candidates gave incorrect answers such as ‘a catalyst

makes the reactant particles collide more effectively’ and ‘a catalyst provides a shorter reaction pathway’.

ANS:

At a higher temperature, the reactant molecules would have higher kinetic energy (1)

There will be a higher collision rate between the reactant molecules. (1)

There is also a higher portion of reactant molecules having enough energy to overcome

the energy barrier. (1)

ER:

Satisfactory. Raising the operating temperature of the reaction process can help increase the kinetic energy of

the reactant particles. Its effect on the reaction rate is twofold, i.e. increasing the collision frequency of the

reactant particles, and increasing the proportion of particles with kinetic energy greater than the energy

barrier of the reaction. However, quite a number of candidates only mentioned one of these effects.

ANS:

the cost is higher because more fuel is burnt to maintain the reaction temperature/ the

risk is higher or more dangerous

ER:

Satisfactory. Some candidates gave vague answers such as ‘the reaction will become difficult to control’.

ANS:

Glycerol is a by-product in the saponification in the production of soap. This reduces the demand on

propene, which can have other uses.

[Accept: giving a correct disadvantage of using propene; e.g. it’s a petroleum product which is a finite

resource / oil drilling may cause environmental problems]

ER:

The performance was fair.

A merit of using glycerol over using propene is that glycerol is a renewable material. Some candidates gave answers

such as ‘glycerol is a renewable energy source’ and did not receive any marks.

A few candidates argued in terms of the biodegradability of glycerol. As glycerol is a by-product of saponification,

using it to manufacture acrylonitrile can help reduce waste. Thus, the biodegradability of glycerol is not regarded as a

merit in this respect.

ER:

■ The performance was fair. Common mistakes of candidates in writing the repeating

unit included:

– showing carbon atoms which form greater/smaller than four covalent bonds,

and

– incorporating N atoms into the backbone of PAN.

ANS:

Air contains oxygen gas. (1)

At a high temperature (1000 C), the carbon atoms in the polymer would react with

oxygen / PAN can burn to give carbon dioxide, water and nitrogen compounds. (1)

ER:

Poor performance.

Many candidates failed to realise that the carbon atoms in PAN would react with oxygen if it is heated in air.

Thus, a nitrogen atmosphere, which is chemically inert, has to be used.

ANS:

(12.0 × 3) (12.0 × 3 + 1.0 × 3 + 14.0 × 1) (1 M)

= 0.679 g (or 0.68g) (1 A)

[no marks for no / wrong unit]

ER:

The performance was fair. Many candidates failed to demonstrate a good grasp of the concepts regarding

reacting masses. Thus, they were unable to correctly carry out the calculation.

ANS:

Making the body/frame/structural parts of automobiles/bike/wheelchair (for

wheelchair racing/wheelchair basketball) (1+)

Carbon fibre has a greater strength than steel but it is much lighter. This reduces the

weight of the automobiles/bike/wheelchair. (1)

ER:

Satisfactory. Quite a number of applications were accepted. But some candidates

suggested inappropriate applications, such as making raincoats. These candidates

should know that no raincoats are made of steel.