Today's Chemistry paper was not too difficult for the 13,000 candidates sitting it as part of their HKDSEs today. But, it did throw up a few puzzles.
“Overall, this year’s questions are definitely easier than those in the past,” said Beacon College chemistry tutor Kelvin Lau Chi-yin, who had just taken this year’s exam.
However, Lau said there was a long question in Paper 1 which some students might have found tricky. It was about the calculation of the equilibrium constant of the compound Fe(SCN)2+. Lau pointed out that students had to pay attention to the dilution effect of the reactant.
“They had to use the diluted reactant’s concentration in the calculation. If they neglected the dilution factor, the entire calculation would be wrong,” he warned.
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Li Kwing-hei, 17, of Diocesan Boys’ School, also mentioned the same long question, saying that's where he got stuck.
“I couldn’t figure out why the colour of the solution became paler, and why the equilibrium shifted to the left,” he said. “In the end, I just guessed it was because there were precipitates. I didn’t remember learning anything like this in class before.”
Lau explained that there were indeed precipitates between SO32- and Fe3+ which decreased the forward rate. This explains why the equilibrium position shifted to the left. Therefore, the concentration of Fe(SCN)2+ was reduced, as was the blood red colour intensity.
Kwing-hei also found one of the long essay questions, which was marked with an asterisk (meaning there was one point for organisation), tricky and time-consuming. The question was about the procedures of purifying zinc sulphate. “The question provided different chemicals to choose for the experiment. It just took me a while to figure out what reagents to use and what not to use,” he said.
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Lau explained students were doomed to fail this if they used chemicals that were unnecessary. “The key was to use excess zinc metal. First, dissolve the sample using distilled water. Then, add the excess zinc metal to the solution so the displacement reaction can take place and copper (II) ions can be precipitated out as copper.”
Lau emphasised that the zinc had to be "excess”, since if zinc metal wasn’t in excess, some copper (II) ions would still be left in the zinc sulphate solution and continue to contaminate it.
After the displacement reaction, Lau said the experiment was quite straightforward: Filter out the copper metal and unreacted zinc metal. And then use the typical crystallisation technique to isolate the crystal by single filtration, wash it with a small amount of cold distilled water and dry it overnight.
Candidates would have been wrong to choose iron, he said. "Because iron would become Fe2+, dissolve in the water and contaminate the Zn2+. So instead, you would end up getting a mixture of iron (II) sulphate and zinc sulphate. Therefore, students should have chosen to only use zinc metal and distilled water, thus both the iron and sodium hydroxide were irrelevant.”
Multiple choice questions
As for the multiple choice questions, Lau pointed out that a lot of students might not have answered question 16 correctly. It asked candidates to use a reagent to differentiate between concentrated hydrochloric acid and concentrated nitric acid.
“Students may think that sodium carbonate can reduce concentrated nitric acid to NO2, but that’s wrong. Sodium carbonate is not a reducing agent, thus it can’t be further oxidised. So even though concentrated nitric acid is a very strong oxidising agent, it won’t work.”
He added that when sodium carbonate is added to nitric acid, only sodium nitrate, carbon dioxide and water would be produced - rather than the brown gas nitrogen dioxide. Therefore, the correct answer was D.
Other topics worth paying attention to include the government’s scheme on sustainable development, Lantau Tomorrow Vision, Smart City Blueprint, and the authorities’ response to pressing social issues.
“A good way to prepare for this, if you have extra time, is to make notes of the recent Hong Kong budget highlights, and how they can be applicable to the various modules,” says Liu when asked about supplementary reading materials.
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Another star tutor, Kenneth Kwong Si-san (known as K Kwong), from Modern Education, said most of the multiple choice questions were quite straightforward and had appeared in past papers, including question 7 about cracking and question 2 about migration of ions.
Kwong also said that even though there was a new chemistry diagram in question 18, students should not have been intimidated by it. “It wasn’t directly linked to the question. If they looked carefully at what the question was asking, they would have known that only statement 2 was true. You can still spot the general features of the diagram, even if you don’t know anything about it,” said Kwong.
Industrial Chemistry
In the Industrial Chemistry section of Paper 2, Kwong said questions 1a and 1b had been quite common in past papers. However, question 1c might have worried students at first sight because the diagram was new.
Question 1c(i) also involved students’ knowledge of the core syllabus, rather than the elective syllabus, which was what Paper 2 supposedly was about.
“Questions 1c(ii) and 1c(iii) tested their common sense, and some students might not have expected that. And for 1c (iv), candidates would require an equation to calculate the answer.” Kwong added that if they hadn’t memorised it, they would probably lose all 3 marks.
Analytical Chemistry
Kwong said there were no surprise questions in Analytical Chemistry. He pointed out some students might have learned about paper chromatography but didn’t know that it’s the same as thin layer chromatography (TLC). It might therefore have caused some confusion.
“I always find Paper 2 harder," said student Kwing-hei. "When I did past papers, there were always a few questions I could not solve. Today, there’re about two questions that I am not entirely sure I got right … I found Industrial Chemistry harder than Analytical Chemistry.”
Regarding time control, Lau said, “I only had 15 minutes left after I finished all questions, so if candidates were not familiar with the syllabus, they might find it quite difficult to finish everything in time.”
Kwing-hei said he had managed to finish every question. “But when I left the examination centre, I heard some students saying that they had not.”