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Author Topic: HSC Chemistry Question Thread  (Read 1040669 times)  Share 

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anotherworld2b

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Re: Chemistry Question Thread
« Reply #2640 on: August 13, 2017, 01:54:12 pm »
+1
I got everything correct except for the state
eg,
CH2=Ch2 (g) + Br2(g) -> CH3CH2CH2CH3 (?)
vs
CH2=CH2(g) + H2O(g) -> CH3CH2OH (?)

Can you give an example?

jakesilove

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Re: Chemistry Question Thread
« Reply #2641 on: August 14, 2017, 02:40:57 pm »
+2
I got everything correct except for the state
eg,
CH2=Ch2 (g) + Br2(g) -> CH3CH2CH2CH3 (?)
vs
CH2=CH2(g) + H2O(g) -> CH3CH2OH (?)


By my understanding, there is no easy way to predict the state. It's just sort of 'you know it' or 'you don't'. I can't imagine you'd need to be able to predict this.
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anotherworld2b

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Re: Chemistry Question Thread
« Reply #2642 on: August 14, 2017, 04:05:11 pm »
+1
I see now :D
We have a prac where we have to identify 3 unknown compounds ( could be alkanes, esters, alcohols and ect)
I was wondering what would be some key observable changes that help identify which compound it is.
- So far I know that if you use the bromine test
Rapid discoloration of the added bromine water (orange to clear) confirms the presence of a suspected double bond. (identifies a alkene)
Whereas if it takes longer to change (it identifies a alkane)
- How would you know if a primary alcohol only oxidised to an aldehyde purely through observation? What would be some key changes?
I was also wondering how you could tell if a primary alcohol oxidized to a carboxylic acid through observations
I was also wondering what compounds a successful reaction with sodium could identify? You can test for hydrogen using a pop test I think?

By my understanding, there is no easy way to predict the state. It's just sort of 'you know it' or 'you don't'. I can't imagine you'd need to be able to predict this.
« Last Edit: August 14, 2017, 04:16:16 pm by anotherworld2b »

hansolo9

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Re: Chemistry Question Thread
« Reply #2643 on: August 15, 2017, 07:41:39 am »
+1
Hey guys :)
what are the limitations of the acid-base theories for laviosier davy arrhenius and B/L?

Natasha.97

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Re: Chemistry Question Thread
« Reply #2644 on: August 15, 2017, 08:20:59 am »
+7
Hey guys :)
what are the limitations of the acid-base theories for laviosier davy arrhenius and B/L?

Hi! :)

Lavoisier (Acids contained O2)
• Metal oxides tended to be basic
• Some oxides do not show any acidic/basic character, eg CO
• Some acids do not contain oxygen such as HCl

Davy (Acids contained H2)
• Many compounds containing hydrogen are not acidic (eg methane)
• Sometimes nitrogen isn't produced when acids react w/ metal, e.g when HNO3 reacts w/ metals, NO2 is produced

Arrhenius (Acids ionised in water to produce H+ ions, Bases ionise in water to produce OH- ions)
• No dissociated hydroxide ions involved in  metallic oxides and carbonates
• His model required the formation of ions, yet solid carbonates can quite often neutralise acids
• He did not expand on the role of the solvent, instead focussing only on the role of the solute in acids and bases
• Why certain solutions of salts are basic or acidic. For example sodium chloride is neutral, zinc chloride is acidic

As far as I can tell, Brφnsted-Lowry has no limitations (current accepted theory)

Hope this helps!  ;D




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MisterNeo

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Re: Chemistry Question Thread
« Reply #2645 on: August 15, 2017, 02:19:24 pm »
+3
I just did my Chem Trials and need clarification on this question.

Does adding a NaOH solution shift the equilibrium to produce more hydronium (more NaOH needed to neutralise), or does the NaOH react with molecular acetic acid?
(I chose C, others chose A)

Thanks heaps :)


hansolo9

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Re: Chemistry Question Thread
« Reply #2646 on: August 15, 2017, 08:28:59 pm »
+1
I just did my Chem Trials and need clarification on this question.

Does adding a NaOH solution shift the equilibrium to produce more hydronium (more NaOH needed to neutralise), or does the NaOH react with molecular acetic acid?
(I chose C, others chose A)

Thanks heaps :)

i'm pretty sure it does shift the equilibrium, and NaOH can only react with ions since it disassociates into two ions itself. :)
someone correct me if i'm wrong

Shadowxo

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Re: Chemistry Question Thread
« Reply #2647 on: August 15, 2017, 08:51:54 pm »
+2
I just did my Chem Trials and need clarification on this question.

Does adding a NaOH solution shift the equilibrium to produce more hydronium (more NaOH needed to neutralise), or does the NaOH react with molecular acetic acid?
(I chose C, others chose A)

Thanks heaps :)
I believe NaOH can react with molecular acetic acid. A weak acid reacts with a base regardless of whether it fully reacts with water. So, it doesn't matter that acetic acid is a weak base.
I'm a bit rusty so someone else feel free to correct me
It's good that you're thinking about it critically though :) It's always good to think of other possibilities and why they could / couldn't work
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VanillaRice

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Re: Chemistry Question Thread
« Reply #2648 on: August 16, 2017, 10:08:21 am »
+2
I see now :D
We have a prac where we have to identify 3 unknown compounds ( could be alkanes, esters, alcohols and ect)
I was wondering what would be some key observable changes that help identify which compound it is.
- So far I know that if you use the bromine test
Rapid discoloration of the added bromine water (orange to clear) confirms the presence of a suspected double bond. (identifies a alkene)
Whereas if it takes longer to change (it identifies a alkane)
- How would you know if a primary alcohol only oxidised to an aldehyde purely through observation? What would be some key changes?
I was also wondering how you could tell if a primary alcohol oxidized to a carboxylic acid through observations
I was also wondering what compounds a successful reaction with sodium could identify? You can test for hydrogen using a pop test I think?

I don't think you can apply a general rule for observable changes when interconverting functional groups (e.g. a primary alcohol to an aldehyde), since the solution may, for example, remain colourless.

Are you allowed to perform further tests on the compounds?
For example, I've used the Benedict's reagent test for aldehydes, base-promoted hydrolysis phenolphthalein test for esters, and the dissolve + test pH test for carboxylic acids to test for different functional groups. You could test for both aldehydes and carboxylic acids to see which has formed from the oxidation of a primary alcohol (may be a combination).
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smshs2017

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Re: Chemistry Question Thread
« Reply #2649 on: August 18, 2017, 10:34:34 am »
+1
Hey guys, can someone please help me with all the equations, including word equations, for the cyclohexane/cyclohexene + bromine water prac that you need to know.
Thanks.
« Last Edit: August 18, 2017, 10:40:54 am by smshs2017 »

Natasha.97

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Re: Chemistry Question Thread
« Reply #2650 on: August 18, 2017, 12:00:51 pm »
+10
Hey guys, can someone please help me with all the equations, including word equations, for the cyclohexane/cyclohexene + bromine water prac that you need to know.
Thanks.

Hi :)

Cyclohexane + Bromine Water
- C6H12(aq) + Br2(aq) → C6H11Br(aq) + HBr(aq)
- Cyclohexane + Bromine → 1-bromohexane + Hydrogen bromide
- Observation: No change in colour (orange)
- Substitution reaction (one H is replaced with one Br)
- Requires UV light and time (slow reaction)

Cyclohexene + Bromine Water
- C6H10(aq) + Br2(aq) → C6H10Br2(aq)
- Cyclohexene + Bromine → 1,2-dibromohexane
- Observation: Orange to colourless
- Addition reaction (Br2 is added across the double bond)
- Occurs quickly under normal light

Hope this helps  ;D
« Last Edit: August 18, 2017, 03:27:11 pm by 13cheungjn1 »
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jakesilove

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Re: Chemistry Question Thread
« Reply #2651 on: August 19, 2017, 05:13:42 pm »
+2
Hi :)

Cyclohexane + Bromine Water
- C6H12(aq) + Br2(aq) → C6H11Br(aq) + HBr(aq)
- Cyclohexane + Bromine → 1-bromohexane + Hydrogen bromide
- Observation: No change in colour (orange)
- Substitution reaction (one H is replaced with one Br)
- Requires UV light and time (slow reaction)

Cyclohexene + Bromine Water
- C6H10(aq) + Br2(aq) → C6H10Br2(aq)
- Cyclohexene + Bromine → 1,2-dibromohexane
- Observation: Orange to colourless
- Addition reaction (Br2 is added across the double bond)
- Occurs quickly under normal light

Hope this helps  ;D


Such a great, succinct explanation. Love it!
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smshs2017

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Re: Chemistry Question Thread
« Reply #2652 on: August 19, 2017, 05:18:13 pm »
+2
Hi :)

Cyclohexane + Bromine Water
- C6H12(aq) + Br2(aq) → C6H11Br(aq) + HBr(aq)
- Cyclohexane + Bromine → 1-bromohexane + Hydrogen bromide
- Observation: No change in colour (orange)
- Substitution reaction (one H is replaced with one Br)
- Requires UV light and time (slow reaction)

Cyclohexene + Bromine Water
- C6H10(aq) + Br2(aq) → C6H10Br2(aq)
- Cyclohexene + Bromine → 1,2-dibromohexane
- Observation: Orange to colourless
- Addition reaction (Br2 is added across the double bond)
- Occurs quickly under normal light

Hope this helps  ;D

thank you so much!!!!!!!

Piza

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Re: Chemistry Question Thread
« Reply #2653 on: August 20, 2017, 06:27:14 pm »
+1
I just did my Chem Trials and need clarification on this question.

Does adding a NaOH solution shift the equilibrium to produce more hydronium (more NaOH needed to neutralise), or does the NaOH react with molecular acetic acid?
(I chose C, others chose A)

Thanks heaps :)



Hey! This is just my answer but feel free to correct me if I am wrong.
As we know, the ionisation of acetic acid is an equilibrium reaction. Therefore, the addition of NaOH will neutralise the hydronium ions, thereby causing equilibrium to shift to the right. This will continue to do so until all of the acetic acid has ionised, and so the answer is A (10mL).

BatBec

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Re: Chemistry Question Thread
« Reply #2654 on: August 20, 2017, 08:53:35 pm »
+1
Hey everyone,
I'm having a bit of trouble with understanding half-equations for compounds. The question below is from an assignment and I can't figure out how to do it.

Describe, using half equations, what happens in the electrolysis of aqueous solutions of sodium sulfate and magnesium nitrate.

Thanks in advance for your assistance  :)