2013 hkdse exam briefing - chem and cs(chem) part 3

(i) Unable to give the correct reaction products / failed to balance the chemical equation. (ii) Wrongly drew a reaction profile or an energy cycle.

Some candidates gave answers which wereincomplete, such as only stating that copper isless reactive than aluminium.

(a) Some candidates were unable to give a correctly balanced chemical equation for the reaction.

Many candidates failed to draw the structure of CsCl(s) showing each Cs+

ion surrounding by 8 Cl–

ions, and vice versa.

(ii) In CsCl , ions are strongly held by ionic bond. Relative movement of the ions can bring ions of the same charge close to each other, and will result in repulsion. So CsCl(s) is brittle.

(c) Cs(s) is more reactive than Na(s). The reactivity of Group 1 metals increases down the group. / The electron in the outermost shell of Cs is weakly bounded by the nucleus as compared with that of Na.

some failed to mention that for the Group I elements,chemical reactivity increases as the position of theelement decreases down the group.

Brown colour is observed. Due to the high concentration of KI in the solution, I–

ions are preferentially discharged to give I2 which dissolves in KI(aq) and gives a brown colour.

Some candidates wrongly mentioned that a brown solid or a brown gas is formed.

H+ ions is discharged and reduced to H2 at electrode B. (1)The amount of OH– ions increases at electrode B as H+ is beingconsumed. So universal indicator turns blue under alkaline conditions. (1)

(ii) Accept both ‘yes’ and ‘no’ answers. ‘No’: B is the negative electrode. Copper will not lose electrons to give Cu2+

at the negative electrode / Copper cannot undergo reduction at the negative electrode.

‘Yes’: Copper and carbon have different electrical conductivity. Therefore the solution near electrode B turns blue more quickly. / The current in the external circuit changes.

(i) Accept both ‘agree’ and ‘disagree’ answers. Agree: The hydrogen gas can be obtained from renewable source

(with one proper example) Disagree: The hydrogen gas used is produced from fossil fuel such as

steam reforming of natural gas. / Electrical energy is consumed in the production of hydrogen from water.

Common Mistakes: • Hydrogen can be obtained from air by fractional distillation.

• Suggested electrolysis of water (merely stating the fact would receive no marks because the question requiredcomments from environmental aspects.)

• For example, if hydrogen is produced from electrolysis of water, the process needs electricity. However, the generation of electricity can also pose harm to ourenvironment.

The airbag has to be inflated instantly when a car crash occurs. Fine powder can greatly increase the reaction rate by providing a large surface area for a reaction involving solid reactants.

KNO3 is added to react with sodium which is highly reactive / corrosive / flammable / strongly reducing.

Many candidates failed to recognise that the KNO3(s) inairbags can, through reaction, remove the sodium metalproduced from the decomposition of NaN3(s), becausesodium is flammable and can cause severe skin burns.

Common mistake: To produce more N2.

Some candidates failed to recognise that the volume of the reaction container is 10.0 dm3

Common mistakes: • Unable to give the correct equilibrium constant

expression.• Unable to solve the quadratic equation correctly. • Incorrect unit.

Common mistake: Overlooked the fact that extra Cl2(g) had been introduced into the system, and thus the final [Cl2(g)] cannot be lower than the initial [Cl2(g)].

Nitrogen < Lithium < Beryllium < Carbon (graphite)

N2 has the lowest melting point as it has a simple molecular structure, only weak van der Waals’ forces / intermolecular forces need to be overcome in melting.

Both Li and Be have metallic structure, metallic bond in Li is weaker than that in Be. So Li < Be in melting points.

C has the highest melting point as it has a giant covalent structure, large amount of energy is needed to break millions of strong covalent bonds between atoms in melting.

Communication mark

Common mistake: Many candidates failed to use the correct chemicalterminology.

• lithium molecules are present in lithium metal; • the inter-particle attraction in elemental lithium or

beryllium is ionic bond; and • graphite has a simple molecular structure.

H2, Ni / Pd / Pt

Some candidates wrongly proposed using NaOH(aq) as reagent, possibly mistaking the reaction would involvethe hydrolysis of ester.

Some candidates only gave H2(g) but without statingthe catalyst needed.

3 NaOH + HC OCOC17H33

H2C OCOC17H33 heat HC OH

H2C OH

+ 3 C17H33COO-Na+

H2C OCOC17H33 H2C OH

Some candidates gave incorrect structures for theglycerol formed.

Again, candidates were weak in balancing chemicalequations.

G reacts with methanol to give an ester.

However, some candidates wrongly gave the structureof a fatty acid.

Some candidates failed to recognise that G is more volatile than F as G has a smaller relative molecular mass than F.

Although the question explicitly stated that F is a vegetable oil, some candidates still mentioned that F is a solid.

LiAlH4 / NaBH4

Some candidates wrongly drew the structures of thecis/trans-isomers. They gave chemical structures with a smaller/larger number of carbon atoms in comparing with the correct one. Some wrongly mentioned that the alkenes are positional isomers.

Some candidates wrongly gave CH3CH2CHClCH2CH3, which is not optically active, as answer.