HSC Chemistry Question Thread

[WLalex]

make sense?

Yes thank you to both you and Rui

[Albertenouttaten]

Hi everyone,

I got a few questions so bare with me;

1) When you are asked to write the redox reactions and say one equasion gives 2 electrons and other equation accept 1 electron. When writing the two redox reaction, do you write them balanced or do you just balance them when you are writing your final net ionic equation?

2) Would Ammonia be considered a raw material in the Solvay process? (Industrial Chemistry Option topic)

3)Is a weak acid weak base neutralization an exothermic reaction?

4) When trying to determine the ions present in a solution, typically you would add in Nitric acid first to bubble off the CO3- ions (carbonate ions), what would be the equation for this reaction?

5) Do we have to remember every step in the AAS process (Atomic Absorption Spectrometry)?

Thanks to everyone who answers and very sorry for the large amount of questions!

[Krijan]

Hii Im Krijan,
I was reading over my notes and it seems like i forgot how to name isomers and stuff, so like for example C2H2CL2F2, and there is a structure there, and ur job is to name it, in this case its 1,1-dichloro-2,2-difluroethane. The issue with me is that i cant remember how to name that, was that some sort of rule?
please help hahah
Thanks

[nimasha.w]

hey i don't know if this makes sense but when you have to calculate the hydrogen ion concentration of a diprotic or triprotic acid, do you have to find the hydrogen ion concentration and then divide by 2/3?

[RuiAce]

Hi everyone,

I got a few questions so bare with me;

1) When you are asked to write the redox reactions and say one equasion gives 2 electrons and other equation accept 1 electron. When writing the two redox reaction, do you write them balanced or do you just balance them when you are writing your final net ionic equation?

2) Would Ammonia be considered a raw material in the Solvay process? (Industrial Chemistry Option topic)

3)Is a weak acid weak base neutralization an exothermic reaction?

4) When trying to determine the ions present in a solution, typically you would add in Nitric acid first to bubble off the CO3- ions (carbonate ions), what would be the equation for this reaction?

5) Do we have to remember every step in the AAS process (Atomic Absorption Spectrometry)?

Thanks to everyone who answers and very sorry for the large amount of questions!

1. Given your half equations the way you put it, to combine them into the redox reaction equation you would have to multiply 2 to both the reactants and products, of the one with 1 electron. I don't fully understand what you're saying even though it looks like you probably know what you're doing

2. Yes but not sure if the term "raw materials" is necessary. The good thing is that it gets recycled.

3. All neutralisations are exothermic, including weak acid + weak base. It's always H⁺ + OH^- -> H2O which is an exothermic reaction.

4. CO₃^2- + 2 HNO_3(aq) -> CO_2(g) + H₂O(l) + 2 NO₃^-

5. I did but according to Jake you don't have to. Might want to double check with him.

[ProfLayton2000]

Hi everyone,

I got a few questions so bare with me;

1) When you are asked to write the redox reactions and say one equasion gives 2 electrons and other equation accept 1 electron. When writing the two redox reaction, do you write them balanced or do you just balance them when you are writing your final net ionic equation?

2) Would Ammonia be considered a raw material in the Solvay process? (Industrial Chemistry Option topic)

3)Is a weak acid weak base neutralization an exothermic reaction?

4) When trying to determine the ions present in a solution, typically you would add in Nitric acid first to bubble off the CO3- ions (carbonate ions), what would be the equation for this reaction?

5) Do we have to remember every step in the AAS process (Atomic Absorption Spectrometry)?

Thanks to everyone who answers and very sorry for the large amount of questions!

1. When you write your half equations, you can keep them as you see on the data sheet (eg Zn->Zn2+ + 2e-, Ag+ + e- -> Ag), but when it comes to writing full redox reaction, you must balance it such that the number of electrons released by the oxidation reaction match the ones gained by the reduction reaction (eg. 2Ag+ + Zn -> Ag + Zn2+)

2. I do industrial Chem so Idk

3. Any acid base neutralisation is exothermic, but less so that strong acid/strong base neutralisations.

4. 2HNO3 + CO3 2- --> CO2 + H2O + 2NO3-  (CO2 is gas, water is liquid and rest is aq)

5. Fairly sure! There is a monochromatic light source, a flame with the sample vapourised, a prism and detector which measures % absorbance. (While you're at it make sure you know it's advantages and practical uses)

*Can someone check this for correctness plz*

[RuiAce]

Hii Im Krijan,
I was reading over my notes and it seems like i forgot how to name isomers and stuff, so like for example C2H2CL2F2, and there is a structure there, and ur job is to name it, in this case its 1,1-dichloro-2,2-difluroethane. The issue with me is that i cant remember how to name that, was that some sort of rule?
please help hahah
Thanks

There's heaps of "rules", but depending on what you get given you only apply the rules that are relevant.

Because you just typed the chemical formula we aren't really sure as to what you mean. Naming questions involve a structural formula

So you're better off providing an example and ask us to guide you through the rules you need. Feel free to provide 2 at a time though (unless they're like identical)

[RuiAce]

hey i don't know if this makes sense but when you have to calculate the hydrogen ion concentration of a diprotic or triprotic acid, do you have to find the hydrogen ion concentration and then divide by 2/3?

For the moles of the actual acid itself? Yes

[Krijan]

There's heaps of "rules", but depending on what you get given you only apply the rules that are relevant.

Because you just typed the chemical formula we aren't really sure as to what you mean. Naming questions involve a structural formula

So you're better off providing an example and ask us to guide you through the rules you need. Feel free to provide 2 at a time though (unless they're like identical)

I have attached, the structure image, But in simple words, just say in exam i was to get this structure and was asked to name it, can you please tell me how would i possibly name it?
Thank You

[RuiAce]

I have attached, the structure image, But in simple words, just say in exam i was to get this structure and was asked to name it, can you please tell me how would i possibly name it?
Thank You

So there are 3 fluorine atoms and 2 chlorine atoms.

First thing I think about when I see a CFC - alphabetical order.

Second thing I think about when I see a CFC - minimise the sum of the 'locants' (i.e. the numbers I give).

That's how I decide which carbon I'm supposed to start on.

So notice how, preserving alphabetical order, the two options are:
1,1-dichloro-2,2,2-trifluoroethane (counting left to right)
2,2-dichloro-1,1,1-trifluoroethane (counting right to left)

Sum up the locants in each case:
1+1+2+2+2=8
2+2+1+1+1=7

Clearly the second one has a smaller sum, so the answer is
2,2-dichloro-1,1,1-trifluoroethane

[jakesilove]

I have attached, the structure image, But in simple words, just say in exam i was to get this structure and was asked to name it, can you please tell me how would i possibly name it?
Thank You

First, note that the groups (chlorine, flourine etc.) need to be named in alphabetical order. So, Chlorine then Flourine.

Note that there are two carbons. So, ethane group.

If there are two, we put the prefix 'di'. If there are three, we put the prefix 'tri'.

We number EACH of the molecules. So, in this case, we get something like

2,2-dichloro-1,1,1-trifluroethane.

Note that the numbers should be the minimum possible total value. We could have named the compound

1,1-dichloro-2,2,2-trifluroethane

But that would add up to a total number that is more than our original name!

[:3]

Hiya,
For question B), why is the answer calculating the percentage of CO2 in the dry ice? Since it's asking for the purity, don't we find the mass of ice then find the percentage of that?

Thanks.

[JemexR]

For Q 18 of the 2010 paper, why is the answer C?

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf

For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?

And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf

[Krijan]

Thanks Jake and Rui, That really helped, i now remember the alphabetical rule thingi. hahah

[Furrrball]

Hiya,
For question B), why is the answer calculating the percentage of CO2 in the dry ice? Since it's asking for the purity, don't we find the mass of ice then find the percentage of that?
(Image removed from quote.)
Thanks.

It's talking about dry ice-that's solid carbon dioxide