HSC Chemistry Question Thread

[RuiAce]

Hi, I'm actually from Victoria so might be slightly different content
First of all [H3O+] = [H+] ( the H+ ions react with water to form [H3O+], they're used interchangeably)
So high [H3O+] means highly acidic, and high [H+] also means highly acidic, as they're basically the same thing
If something's basic, it has a low [H+] (or high [OH-])
And with the K_a, not sure if it's in HSC course but likely is. Do you know K? If so, Ka is just the acidity constant as [H2O] is constant, so it's like K*[H2O]
Basically for each weak acid, there's an equilibrium. We use the term Ka to represent the acidity constant, there is a defined Ka for each weak acid. If the acid is HA and it reacts with H2O to form H3O+ and A- ions, the Ka = [H3O+][A-]/[HA], and the [H3O+] = [A-] as they react 1:1, so Ka = [H3O+]²/[HA]
So the Ka for ethanoic acid is Ka = [H3O+]²/[CH3COOH]. This Ka is always constant at equilibrium, regardless of the concentrations so if all the concentrations decrease then this expression (sometimes called Q) isn't equal to the Ka, so in order for it to be in equilibrium again, the concentrations need to change so it's equal to the Ka. If all concentrations decrease, since it's [H3O+]²/[CH3COOH] then [H3O+] must increase and [CH3COOH] must decrease to return to equilibrium

Hope this helps

The equilibrium constant stuff is only taught in the industrial chemistry option.

In the core topic "the acidic environment" they only deal with equilibrium principles from a qualitative perspective.

[Shadowxo]

The equilibrium constant stuff is only taught in the industrial chemistry option.

In the core topic "the acidic environment" they only deal with equilibrium principles from a qualitative perspective.

Ah, here it's a core topic and reasonably important

[bsdfjnlkasn]

Oh ok cool!

Either way i'll need to learn it as we study industrial chemistry for our elective 

[anotherworld2b]

Hi i was wondering if anyone knew any tricks/tips on tackling hydrocarbon structural diagrams. I have difficulty when it comes to naming it correctly especially when it is a cyclo structure

[shreya_ajoshi]

Hi i was wondering if anyone knew any tricks/tips on tackling hydrocarbon structural diagrams. I have difficulty when it comes to naming it correctly especially when it is a cyclo structure

This is a pretty helpful link: https://www.youtube.com/watch?v=Oz6z2HyNU-k
https://www.youtube.com/watch?v=LUOpbV__zU8 --> for the cyclo structures

[anotherworld2b]

thank you

This is a pretty helpful link: https://www.youtube.com/watch?v=Oz6z2HyNU-k
https://www.youtube.com/watch?v=LUOpbV__zU8 --> for the cyclo structures

[hanaacdr]

Hi
could i please get some help on this question,

explain in terms of bond type and strength
(i)- why addition polymers are generally hydrophobic?
(ii) why PVC loses structural integrity when placed in sunlight for long periods of time?

thank you

[RuiAce]

Hi
could i please get some help on this question,

explain in terms of bond type and strength
(i)- why addition polymers are generally hydrophobic?
(ii) why PVC loses structural integrity when placed in sunlight for long periods of time?

thank you

I think for ii you just need to know that it does, not the why part. Don't remember learning about the why, but it should have something to do with the chemical structure.

For i though, those strong carbon chains (the first part of the question) are what give it the hydrophobic nature. These strong carbon chains may form intermolecular forces with other carbon chains in the form of dispersion forces, however they don't really interact with water. As a rule of thumb, polar substances interact with other polar substances (such as water), and non polar with non polar (eg octane).

Yet again, forgot a lot of the intricacies but that's the basic idea. Someone else can input further.

[hanaacdr]

I think for ii you just need to know that it does, not the why part. Don't remember learning about the why, but it should have something to do with the chemical structure.

For i though, those strong carbon chains (the first part of the question) are what give it the hydrophobic nature. These strong carbon chains may form intermolecular forces with other carbon chains in the form of dispersion forces, however they don't really interact with water. As a rule of thumb, polar substances interact with other polar substances (such as water), and non polar with non polar (eg octane).

Yet again, forgot a lot of the intricacies but that's the basic idea. Someone else can input further.

thank you!

[Noorijaz]

hi

i havent made any notes for chemistry for the beginning of year 12.POM
Can i get away with not making notes and just use someone elses notes or should i go through the pain to make notes.
Furthermore is it better to tpye or hand write notes.

thanks heaps

[RuiAce]

hi

i havent made any notes for chemistry for the beginning of year 12.POM
Can i get away with not making notes and just use someone elses notes or should i go through the pain to make notes.
Furthermore is it better to tpye or hand write notes.

thanks heaps

Well, personally, I never made notes for chem. I just downloaded another set of notes and combined that with my textbook.

So "can you" get away with it? Yeah, I'd say so. Is it recommended? Probably not.


As I always say, typing v.s. handwriting is UP TO YOU. Typing usually makes it neater, but if you hand write them you're really activating your brain and have a better chance at storing it in your head. Everyone is different on what they choose to do.

[anotherworld2b]

Hi I just wanted to confirm whether that I wrote in my notes is correct
The periodic table is arranged horizontally in order of increasing atomic number in such a way the trends can be seen. Would atomic number be the same as atomic weight?

I also wanted to ask for relative atomic mass is it usually written as a percentage or amu? I saw two questions that are nearly identical in wording but one is in amu and the other in percentage 

[bsdfjnlkasn]

Hi I just wanted to confirm whether that I wrote in my notes is correct
The periodic table is arranged horizontally in order of increasing atomic number in such a way the trends can be seen. Would atomic number be the same as atomic weight?

I also wanted to ask for relative atomic mass is it usually written as a percentage or amu? I saw two questions that are nearly identical in wording but one is in amu and the other in percentage 

The horizontal arrangement note is definitely correct. Just look at a periodic table, what do you see? You have hydrogen in the top left corner which has an atomic number of 1, then you move along to Helium, atomic number 2. Because He is at the end of the first horizontal group (called a period) we move down to the next period and look to the far left where Lithium (3) is sitting etc etc. The main trends that you can observe across periods are electronegativity and first ionisation energy, so that's right also.

Atomic number and atomic weight are not the same thing. Atomic number is the big round number we were discussing earlier that identifies an element's position. Atomic weight on the other hand, is the (usually) smaller number that tells us what the element's weight is. It is measured in atomic mass units (amu) and is usually corrected to 2 decimal places. Relative atomic mass and atomic weight are the same thing. I haven't ever seen either given as a percentage. Maybe post the questions you were referring so that we can take a look and see if the question was actually talking about something else 

Finally for an example, let's look at Lithium - It's atomic number is 3 whereas it's atomic weight is 6.94 amu

Hopefully that helped with your queries and let me know if you need anything else cleared up 

[Shadowxo]

Hi I just wanted to confirm whether that I wrote in my notes is correct
The periodic table is arranged horizontally in order of increasing atomic number in such a way the trends can be seen. Would atomic number be the same as atomic weight?

I also wanted to ask for relative atomic mass is it usually written as a percentage or amu? I saw two questions that are nearly identical in wording but one is in amu and the other in percentage 

Just adding some extra information
Atomic number is the number of protons in the nucleus of that element. It is always the same for a specific element, and a whole number. Periodic table is arranged left to right with increasing atomic number (/ protons) and usually, as the number of protons increases, the number of neutrons increases too, to hold the element together. More protons -> more weight so usually left to right is also increasing weight.
Protons and neutrons are around 2000x heavier than electrons, so atomic weight is primarily made up of the mass of protons and neutrons
Atomic weight is the weight of the entire element, protons and neutrons (the average, as some elements have different isotopes due to different neutron numbers). Average number of neutrons can also be found by atomic weight - atomic number.

For relative atomic mass, I've usually seen it as amu, with the percentage abundance of different isotopes being given as information to work out the relative atomic mass

[QC]

Hi
could i please get some help on this question,

explain in terms of bond type and strength
(i)- why addition polymers are generally hydrophobic?
(ii) why PVC loses structural integrity when placed in sunlight for long periods of time?

thank you

Ok, depending on how many marks the Q is worth add more detail
(i) Addition polymers are generally hydrophobic as they are saturated (all electron bonds are used/single) which means they do not react with water. As well as this, they are non-polar and if you remember the water topic last year, that means that they do not form a solution as water doesn't create strong enough inter molecular bonds to break the existing bonds between the polymer molecule.
(ii) As for the PVC losing structural integrity, as Rui said you probably don't need to know this but if your interested/want to beef up your answer, in sunlight the PVC molecules are hit by UV radiation which emits a photon that is able to weaken the plastic (photodegredation). Weathering and other things can also cause PVC to lose its structural integrity. Side note: A lot of the time companies coat these plastics to enable them to withstand more conditions, you can talk about this in the properties and uses dot point.