Just like the question we have discussed in the previous blogpost, this one was sent to me by a H2 Chemistry student for discussion.
Apparently, this question was also mentioned in the A-Level Chemistry Challenging Drill Questions for H1/H2 that is available in major Singapore bookstores.
Let’s take a look at the question:
Question:
Which gas can be most easily liquefied by cooling and applying pressure?
A. Ar
B. H2
C. HF
D. CH4
Now, try it out by applying the concepts you have learned in the previous blogpost.
Check out the answer and suggested solutions below:
Answer:
C
Common mistakes by students & their arguments:
When i post this question to my JC1 H2 Chemistry Small-Group Class, many gave A (Argon) as the answer or B (H2).
They argued that for gas molecules to be able to come together to become liquid, they must be very small (in size) as well as have weak intermolecular forces between their molecules. This will allow the molecules to come closer together much more easily.
It seemed a valuable argument at first thought, but after careful considerations the answer should be C (HF) if we apply the essential concepts we learned in earlier post.
With such disagreement and knowing that a book with suggested answer is available, i went to purchase a copy of A-Level Chemistry Challenging Drill Questions for H1/H2 as well as its Solutions Book.
From the book, I see the following suggested answer for the above question:
“To be easily liquefied at low temperatures and high pressures, the gas molecules must not occupy significant volumes and have strong attractive forces forces between molecules. Therefore, Ar is most easily liquefied as the attractive forces between gas molecules are the weakest.”
The answer given by the book is exactly the way some of my students think. However, this is the wrong concept! BEWARE especially if you have a copy of it.
Suggested Answer & Thought Process:
In order to find out if i am correct, i posted the question to two of my friends which are H2 Chemistry Lecturers in two of the top Junior Colleges (JC) in Singapore.
They agreed with my answer which is C (HF) and gave the following explanations which i totally i agree on.
H2 Chemistry Lecturer A:
“Ya, I think so
the one with the strongest intermolecular forces of attraction will have the highest boiling point too. Thus easiest to liquefy. E.g. Let’s say gas A has bp of 100 deg celsius, gas B 80 deg celsius. At 150 deg celsius, both are gases. But as I lower the temp from 150 deg celsius, the gas with bp 100 deg celsius will liquefy first
&
H2 Chemistry Lecturer B:
“yea. most easily liquefied ==> highest boiling point ===> strongest intermoecularl forces. Ans is thus C cos of hydrogen bonding. E.g. most easily liquefied means that it is easier to change the substance from the gas to liquid state (ie the substance prefers more to stay in the liquid state as compared to the rest). As such, it will be harder to boil it (change the substance from liquid to gas), and will have a higher boiling point.
Alternatively, we can look at the particles. For the gas that is most easily liquefied, it would have its particles closer, and less randomly arranged as compared to the rest. This in turns mean that the gas should have stronger intermolecular forces. Hope it helps. =)”
Hope you have learn something valuable today. Feel free to share it with anyone that might benefit from it. =)
Whether you are a A-Level Chemistry student (or equivalent), teacher or tutor, i would love to hear your views on it. Feel free to drop me a comment below.
PS: Like to give credit to the 2 H2 Chemistry Lecturers for sharing with us their thoughts.
