HSC Chemistry Question Thread

[anotherworld2b]

oh okay thank you for your help
I also had a question about the effect of water on equilibrium.
I am quite confused about it because sometimes it has a effect and sometimes it doesn't. How do you know whether or not water has an effect on equilibrium?

Hey! So we have



How did I know that? I googled it. There's no way you need to be able to 'guess' this reaction!

[scienceislife]

Any advice for making the most of chemistry lessons when you're barely learning anything from the teacher?

[Piccolo]

Hey fellas,

I've recently been revising my enthalpy of combustion work, and I had a question regarding why kerosene's enthalpy is so high. The answer is that it forms low energy bonds in the final products, and the lower the energy of the bond the more heat released. Why is that? Are low energy bonds strong or weak bonds, and why is more heat released in the formation of a low energy bond?

Thanks!

[jakesilove]

Any advice for making the most of chemistry lessons when you're barely learning anything from the teacher?

Hey! I had a not-so-incredible teacher as well; check out an article I wrote here: https://atarnotes.com/how-to-top-chemistry/. I reckon it's got all the info you need

[jakesilove]

Hey fellas,

I've recently been revising my enthalpy of combustion work, and I had a question regarding why kerosene's enthalpy is so high. The answer is that it forms low energy bonds in the final products, and the lower the energy of the bond the more heat released. Why is that? Are low energy bonds strong or weak bonds, and why is more heat released in the formation of a low energy bond?

Thanks!

Hey! This is definitely way beyond the syllabus, so I'll just briefly give you a conceptual understanding of the Chemistry behind this.

Essentially, Kerosene goes from a reactant containing high-energy bonds, to a state of low-energy products. So, you're question is; why is so much heat released in combustion? How does bond energy relate to enthalpy?

Essentially, the answer is simple; there is a lot of energy in the bonds of the reactants, and not much energy is the bonds of the products. Where did that energy go? Heat. Because we know that the bond energy is low, we also know that energy must have been released! Thus, the heat of enthalpy is high.

Does that kind of make sense? Again, this is definitely not useful in the HSC syllabus

[bsdfjnlkasn]

Hey there,

I was just wondering if someone could help me with the second part of this question - is it because CO is toxic? Causing respiratory problems?

Sulfur acid can dehydrate formic acid (methanoic acid, HCOOH) to carbon monoxide. Write an equation for the reaction. Why is this reaction too dangerous to perform in a school laboratory?

[jakesilove]

Hey there,

I was just wondering if someone could help me with the second part of this question - is it because CO is toxic? Causing respiratory problems?

Sulfur acid can dehydrate formic acid (methanoic acid, HCOOH) to carbon monoxide. Write an equation for the reaction. Why is this reaction too dangerous to perform in a school laboratory?

Yep, you're absolutely right; just spend some time discussing the dangers of CO, and you'll get the marks

[seventeenboi]

HI (2003 HSC q15)
the answer to this is apparently A, but I don't understand why wouldn't the pH drop sharply as indicated by C in the introduction of a strong acid ??

Thanks!

[RuiAce]

HI (2003 HSC q15)
the answer to this is apparently A, but I don't understand why wouldn't the pH drop sharply as indicated by C in the introduction of a strong acid ??

Thanks!

Because we're adding it to a buffer.

A buffer is made by mixing certain concentrations of an acid with its conjugate base. However the purpose of doing so is that a system is created whereby small changes in amounts of acids and bases added do NOT produce a strong effect on the system's pH. This is essentially how the pH in our blood is regulated should extra acidic/basic contents be added; the buffer will withstand the acid/base for a fair while before it starts breaking down.

If, on the other hand, we added it to something like water, then C would be valid.

[J.B]

Hi,
I was just wondering with the definition of a free radical: species with one or more lone or unpaired electrons.
I'm a bit confused as to what "lone or unpaired electrons" actually refers too, does this refer to elements that don't have a full outer shell? Like oxygen etc.
Thank you

[jakesilove]

Hi,
I was just wondering with the definition of a free radical: species with one or more lone or unpaired electrons.
I'm a bit confused as to what "lone or unpaired electrons" actually refers too, does this refer to elements that don't have a full outer shell? Like oxygen etc.
Thank you

Hey! This goes to a point in Chemistry that you have not actually been taught. Basically, even ignoring the idea of a 'full' outer shell, electrons like to exist in pairs. So, an empty shell with two electrons is more stable than an empty shell with only one electron. You don't really need to know/understand much about this!

[J.B]

Hey! This goes to a point in Chemistry that you have not actually been taught. Basically, even ignoring the idea of a 'full' outer shell, electrons like to exist in pairs. So, an empty shell with two electrons is more stable than an empty shell with only one electron. You don't really need to know/understand much about this!

Ok so for the HSC is it something along the lines of this that I need to understand:
Like are some single oxygen elements not free-radicals if all 6 electrons are in pairs, and then some single oxygen elements are free-radicals if it has two pairs of electrons, and then two single unpaired electrons?

[jakesilove]

Ok so for the HSC is it something along the lines of this that I need to understand:
Like are some single oxygen elements not free-radicals if all 6 electrons are in pairs, and then some single oxygen elements are free-radicals if it has two pairs of electrons, and then two single unpaired electrons?

Yep, that's right! Alternatively, Oxygen could lose an electron, leaving it with 5 valence electrons. Obviously, one of them would be unpaired! But, most commonly, one of the electrons jumps into a higher shell (for some complicated reason), leaving two sets of paired electrons, and two non-paired electrons

[J.B]

Yep, that's right! Alternatively, Oxygen could lose an electron, leaving it with 5 valence electrons. Obviously, one of them would be unpaired! But, most commonly, one of the electrons jumps into a higher shell (for some complicated reason), leaving two sets of paired electrons, and two non-paired electrons

Ok thank you

[J.B]

In this haloalkane when naming it. Is it 2,3-dibromo-4-chloropentane or 3,4-dibromo-2-chloropentane?

As the first one has a lower sum for the bromine, but my teacher gave us a rule to give the most electronegative halogen  (Cl>Br) the lowest numbers which would be the second one?