HSC Chemistry Question Thread

[bsdfjnlkasn]

Hey there,
 
Was just wondering which trial papers are the best to be doing now?
My exam is next week

Thank you!

[MisterNeo]

Hello! Would appreciate help for the following questions:
8. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20136376_1271772089614978_682044855_n.png?oh=ee5739c2bbd68d91e9daaa41e7fd0420&oe=5970AF0E
10. https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/20067722_1271778759614311_792814971_n.png?oh=c0f901e116ad084b5f9724edd3508723&oe=5971A02A
11. https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/20107688_1271783416280512_709372620_o.png?oh=b7b0ad8fcad5d15843f1ea220447f1be&oe=5971C7FF
TIA!

Hi! According to the syllabus, an acidic oxide reacts water to form an acidic solution and reacts with a base to form a salt. Calcium oxide is a base so I'd say the answer is both 2 and 3.
For the second one, 1 and 3 are immediately eliminated because conjugate pairs can only be one proton in difference. So the answer would be 2 and 4 since oxide ions can accept protons to form hydroxide. (This is why metal oxides are very basic in water.) The NH2^- can accept a proton to become ammonia.
And for the last one, you need to find the excess reagent (after balancing the equation) and put those moles in a 100mL solution. Then plug them into the pH equation.

Moles of OH^-=2(0.102x0.02)=0.00408mol
Moles of H⁺=(0.15x0.04)=0.06mol
Excess H⁺=0.00192mol
Concentration=0.0192mol/L
pH=1.71
Hope this helps

[bigsweetpotato2000]

Hey there,
 
Was just wondering which trial papers are the best to be doing now?
My exam is next week

Thank you!

For your trials? Definitely your school ones!
To maximise your score for trials, get familiar with your school's personal layout - even though they structure it like the HSC they tend to get... creative.
After trials though, head straight for HSC past papers!

Bigsweetpotato Farm

[statues]

Quickie:
In the hydration of Ethylene should I write the catalyst to be conc. phosphoric acid or dil. sulphuric acid?
I've got sources saying different things.

Thanks

[kiwiberry]

Quickie:
In the hydration of Ethylene should I write the catalyst to be conc. phosphoric acid or dil. sulphuric acid?
I've got sources saying different things.

Thanks

Both dilute phosphoric acid or sulfuric acid can be used!

[bsdfjnlkasn]

Hey there!

I'm not getting anything close to the answers given so would appreciate some guidance - thank you very much!

[Shadowxo]

Bit rusty so sorry for any mistakes
Energy released from 1.635g:
E=mc∆T
m=200mL, c=4.18, ∆T = 47 - 22 = 25 ºC
E = 200*4.18*25 = 20,900 J = 20.900 kJ
Energy released per gram = 20.900 /1.635 = 12.783 kJ
Energy released from 1.50g = 12.783 * 1.50 = 19.174 kJ
*remember sig figs, I ignored them but you shouldn't*
 Hope that helps

[seventeenboi]

hello
I know the answer to this question is a since all the other options contain substances which do not form a coordinate covalent bond. however, i was wondering why water forms a coordinate covalent bond? is it perhaps referring to the hydronium ion  

[MisterNeo]

hello
I know the answer to this question is a since all the other options contain substances which do not form a coordinate covalent bond. however, i was wondering why water forms a coordinate covalent bond? is it perhaps referring to the hydronium ion

Yes, you are correct! The answer would be A because oxygen, water, and ammonia can all form coordinate covalent bonds (ozone, hydronium ion, ammonium ion).

[bsdfjnlkasn]

Hey there,

Could someone please explain the following solution to me?
I got that it was B but still (I know it's annoying) the terminology is confusing me

Any help would be super appreciated!

Thanks

[MisterNeo]

Hey there,

Could someone please explain the following solution to me?
I got that it was B but still (I know it's annoying) the terminology is confusing me

Any help would be super appreciated!

Thanks

Hey! So an oxidising agent (also known as oxidant) causes oxidation and undergoes reduction itself (gains electrons).
On the reduction potential list that you get in exams, the strongest oxidising agents are the the very top with the largest positive potential, whereas the strongest reducing agents (reductants) are at the very bottom with negative potentials.

So with this question, the most powerful oxidant is the one with the most positive potential as it would appear at the very top of the potentials list, hence A⁺ is the answer.

[Mathew587]

Hey guys,
Can someone explain to me how to calculate E* for electrolytic cells? I'm confused as to which undergoes oxidation, reduction and when water is involved.
Ty

[beau77bro]

with the ionisation of concentrated sulfuric acid, is the reaction highly exothermic, or is it just that there is so much ionisation that the energy produced is enormous. I really can't tell from conquering, but I'm assuming it is a combination of both that produces a large amount of energy.

is it sufficient to say that there is very little water in the 98% concentrate, as well as the fact that most of the water forms hydrates resulting in very few ions being present. this then results in far greater ionisation when water is added and the corresponding heat produced from the strong ionisation, both of which contribute to the boiling and spititng of acid.

[MisterNeo]

with the ionisation of concentrated sulfuric acid, is the reaction highly exothermic, or is it just that there is so much ionisation that the energy produced is enormous. I really can't tell from conquering, but I'm assuming it is a combination of both that produces a large amount of energy.

is it sufficient to say that there is very little water in the 98% concentrate, as well as the fact that most of the water forms hydrates resulting in very few ions being present. this then results in far greater ionisation when water is added and the corresponding heat produced from the strong ionisation, both of which contribute to the boiling and spititng of acid.

It's a combination of both the ionisation of acid is highly exothermic, and that the acid is in its pure liquid form since there are a lot of molecules being ionised.
The reaction is so exothermic that the acid is added into water, not water into acid or else it will boil/splash everywhere.

[MisterNeo]

Hey guys,
Can someone explain to me how to calculate E* for electrolytic cells? I'm confused as to which undergoes oxidation, reduction and when water is involved.
Ty

Hey!
So for electrolytic cells, it is different to galvanic cells because the anode is the positive terminal and the cathode is negative.

For the NaCl example of an electrolytic cell:
A battery power source is used to pull electrons from the anode to the cathode (inert platinum rods)), and the anions (Cl-) in the solution migrate towards the anode because it is positively charged from its electrons being taken (negative anode means that negative ions go towards it).
The anions (Cl-) then get oxidised (electrons lost) at the anode to form chlorine gas with the electrons being taken to the cathode by the battery.
The cations (Na+) get reduced (electrons gained) at the cathode to form sodium metal deposit on the cathode rod, and the electrons come from the chlorine.

Electrolytic cells are non-spontaneous, thus the E* will be negative.
To calculate it, you just get the value of chlorine's oxidation (-1.36), and add it to sodium's reduction (-2.71).
(-1.36) + (-2.71) = -4.07V
If the concentration of NaCl is too low (<2M), hydrogen ions will be reduced instead of sodium ions, and oxygen ions will be oxidised instead of chlorine ions. So hydrogen gas will evolve at the cathode and oxygen gas will evolve at the anode.
Hope this helps