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Author Topic: HSC Chemistry Question Thread  (Read 1040649 times)  Share 

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MysteryMarker

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Re: Chemistry Question Thread
« Reply #615 on: August 13, 2016, 06:27:30 pm »
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Kinda stuck on this question,

CH3NH2(aq) + H2O <----> OH- + CH3NH3+\

If the methyl amine (CH3NH2(aq)) reacts with HCl to produce the salt methyl ammonium chloride. Write the equation for this neutralisation reaction. Explain, with the aid of an equation, why this salt does no produce a neutral solution when dissolved in water.

Cheers.

RuiAce

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Re: Chemistry Question Thread
« Reply #616 on: August 13, 2016, 08:21:55 pm »
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Kinda stuck on this question,

CH3NH2(aq) + H2O <----> OH- + CH3NH3+\

If the methyl amine (CH3NH2(aq)) reacts with HCl to produce the salt methyl ammonium chloride. Write the equation for this neutralisation reaction. Explain, with the aid of an equation, why this salt does no produce a neutral solution when dissolved in water.

Cheers.
Given that the equation is reversible due to the presence of ⇌, we know that methyl amine is weakly basic and its conjugate; the methyl ammonium ion is weakly acidic.

Suppose we react with hydrochloric acid. Because we have a strong acid, the equation loses its nature as reversible. So it just becomes this.
CH3NH2(aq) + HCl(aq) -> CH3NH3Cl(aq)

If we dissolve this salt in water, it will dissociate.
CH3NH3Cl(s) ->  CH3NH3+ + Cl-

The chloride ion is neutral. Because HCl is a strong acid, Cl- is so basic to the point it basically doesn't react.
However, the weakly acidic methyl ammonium ion will just do the original equation's thing in reverse (i.e. donate a proton):

CH3NH3+ + H2O(l) -> H3O+ + CH3NH2(aq)

So in fact, the salt formed is slightly acidic. Not neutral.

MysteryMarker

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Re: Chemistry Question Thread
« Reply #617 on: August 13, 2016, 09:46:22 pm »
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Given that the equation is reversible due to the presence of ⇌, we know that methyl amine is weakly basic and its conjugate; the methyl ammonium ion is weakly acidic.

Suppose we react with hydrochloric acid. Because we have a strong acid, the equation loses its nature as reversible. So it just becomes this.
CH3NH2(aq) + HCl(aq) -> CH3NH3Cl(aq)

If we dissolve this salt in water, it will dissociate.
CH3NH3Cl(s) ->  CH3NH3+ + Cl-

The chloride ion is neutral. Because HCl is a strong acid, Cl- is so basic to the point it basically doesn't react.
However, the weakly acidic methyl ammonium ion will just do the original equation's thing in reverse (i.e. donate a proton):

CH3NH3+ + H2O(l) -> H3O+ + CH3NH2(aq)

So in fact, the salt formed is slightly acidic. Not neutral.

For the reaction between methyl amine and hydrochloric acid, shouldn't this be a neutralisation reaction as we established that methyl amine is a weak base and we know that hydrochloric acid is a strong acid? So shouldn't water also be a product in this reaction?



RuiAce

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Re: Chemistry Question Thread
« Reply #618 on: August 13, 2016, 09:51:03 pm »
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For the reaction between methyl amine and hydrochloric acid, shouldn't this be a neutralisation reaction as we established that methyl amine is a weak base and we know that hydrochloric acid is a strong acid? So shouldn't water also be a product in this reaction?
I didn't know you always had to produce water in neutralisation:

NH3 + HCl -> NH4Cl

MysteryMarker

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Re: Chemistry Question Thread
« Reply #619 on: August 14, 2016, 10:27:21 am »
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I didn't know you always had to produce water in neutralisation:

NH3 + HCl -> NH4Cl

OHHHHHHH rekt. Thanks man, i understand it now :P

Got another question:

The correct statement about biopolymers if:

A) biopolymers are made by living things but are not biodegradeable.
B)All biopolymers can be manufactured synthetically in the laboratory by condensation reactions.
C)Synthetic biopolymers are being produced from living organisms and are replacing polymers made from petrochemicals
D)All natural biopolymers are made by condensation reactions involving glucose monomers.

Cheers.
« Last Edit: August 14, 2016, 11:04:21 am by MysteryMarker »

jakesilove

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Re: Chemistry Question Thread
« Reply #620 on: August 14, 2016, 11:45:30 am »
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OHHHHHHH rekt. Thanks man, i understand it now :P

Got another question:

The correct statement about biopolymers if:

A) biopolymers are made by living things but are not biodegradeable.
B)All biopolymers can be manufactured synthetically in the laboratory by condensation reactions.
C)Synthetic biopolymers are being produced from living organisms and are replacing polymers made from petrochemicals
D)All natural biopolymers are made by condensation reactions involving glucose monomers.

Cheers.

Hey,

The answer here is definitely C. Just do this by elimination, in terms of what you have actually studied. Biopolymers may be biodegradable, they may not be, however you aren't supposed to know EVERY biopolymer out there! So, the answer can't be A. Again, you would need to know the reaction of EVERY biopolymer, some of which may be condensation reactions, most of which would be addition reactions, for the answer to be B. So, it isn't. Same goes for D. However, the REASON you learn about biopolymers is that we need to find alternatives to petrochemicals (the focus of the whole of Production of Materials). Plus, BIOpolymers are obviously produced by living organisms. So, the answer is C.

Jake
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RuiAce

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Re: Chemistry Question Thread
« Reply #621 on: August 14, 2016, 11:58:35 am »
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Hey Jake, please finish off the question that I only half completed at the bottom of the last page. I still can't show that sodium sulfate (prediction) would be the correct one

jakesilove

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Re: Chemistry Question Thread
« Reply #622 on: August 14, 2016, 12:10:01 pm »
+1
Hey Jake, please finish off the question that I only half completed at the bottom of the last page. I still can't show that sodium sulfate (prediction) would be the correct one

On it!
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jakesilove

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Re: Chemistry Question Thread
« Reply #623 on: August 14, 2016, 12:17:40 pm »
+1
This answer is incomplete. I'll ask that someone else finish it off because I'm not sure how to incorporate the acidification. I've started it off in the meantime

Since we know that all nitrates are soluble, there is no way that the precipitate formed could've involved the nitrate ion.

Hence, we consider substances involving the lead ion Pb2+ which are insoluble. Problem is that both PbSO4 AND PbCl2 are insoluble!

Begin a process of elimination.
D) is the first one to go. This is because AgCl is also insoluble. Since silver chloride is insoluble, it cannot have been the aqueous solution.
A) is the second one to go. The presence of the Cu2+ ion means that the solid would've been blue, when clearly it is white.

The last bit is the hard one.

So, for reasons unknown to me, and irrelevant to the HSC curriculum, Lead (II) Chloride is slightly soluble in Acidic conditions. It is also slightly soluble in water (but only a tiny bit!). Either way, the BETTER answer is going to be C, because Lead Sulfate is not soluble at all. This isn't something that you're supposed to know. Typical James Ruse going wayyy beyond the curriculum.

Jake
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MysteryMarker

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Re: Chemistry Question Thread
« Reply #624 on: August 14, 2016, 12:57:38 pm »
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Hey,

The answer here is definitely C. Just do this by elimination, in terms of what you have actually studied. Biopolymers may be biodegradable, they may not be, however you aren't supposed to know EVERY biopolymer out there! So, the answer can't be A. Again, you would need to know the reaction of EVERY biopolymer, some of which may be condensation reactions, most of which would be addition reactions, for the answer to be B. So, it isn't. Same goes for D. However, the REASON you learn about biopolymers is that we need to find alternatives to petrochemicals (the focus of the whole of Production of Materials). Plus, BIOpolymers are obviously produced by living organisms. So, the answer is C.

Jake

Thanks man, totally understand it now. Just got a few more questions if thats alright.

1. Sodium hydrogen carbonate is a common laboratory chemical. Explain why arrhenius acid/base definition is unable to account for the acid/base properties of this species whereas the BL theory can. Include chemical equations. (5marks)

For this question would i just state Arrhenius's definition + BL definition, and then show that HCO3- is amphiprotic. By its reaction with water, showing it accepting a hydrogen ion and donating a hydrogen ion, and as it can do both Arhennius would be unable to define this as a base/acid?

2. What is meant by the Eo of an electrode in an electrochemical cell. (2marks)

3. For the complete combustion of a fuel, is the water produced in the reaction gaseous or liquid?

Cheers.

RuiAce

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Re: Chemistry Question Thread
« Reply #625 on: August 14, 2016, 01:07:38 pm »
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Thanks man, totally understand it now. Just got a few more questions if thats alright.

1. Sodium hydrogen carbonate is a common laboratory chemical. Explain why arrhenius acid/base definition is unable to account for the acid/base properties of this species whereas the BL theory can. Include chemical equations. (5marks)

For this question would i just state Arrhenius's definition + BL definition, and then show that HCO3- is amphiprotic. By its reaction with water, showing it accepting a hydrogen ion and donating a hydrogen ion, and as it can do both Arrhenius would be unable to define this as a base/acid?

Also, note that Arrhenius thought bases had to ionise to produce OH-. You can't do that with NaHCO3 either.

2. What is meant by the Eo of an electrode in an electrochemical cell. (2marks)

3. For the complete combustion of a fuel, is the water produced in the reaction gaseous or liquid?

Cheers.
1. That does show how it's a B-L acid because according to B-L theory, acids are proton donors.

Svante Arrhenius' definition stated that acids ionised in solution (water) to produce H+. When you dissolve your sample of sodium hydrogen carbonate you don't produce that; you just produce the sodium ion and the hydrogen carbonate ion.

NaHCO3(s) -> Na+ + HCO3-

So it fails to be an Arrhenius acid.

2. Your table of standard reduction potentials lists several Eo values for individual half equations.

The EMF produced in an electrochemical cell is just the sum of the two relevant half equations. It is thus, a measure of the voltage produced, from you making that electrochemical cell. (Remember: In a Galvanic cell, chemical energy is converted into electrical energy.)

3. Gas. The temperatures in combustion are extremely high.

MysteryMarker

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Re: Chemistry Question Thread
« Reply #626 on: August 14, 2016, 04:25:59 pm »
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Classify the salt formed from HCO3- as acidic, basic or neutral and justify its classification. Include a chemical equation to illustrate your answer. (2marks)

Kinda confused for this question, cause i know that HCO3- is amphiprotic so wouldn't its acidity/basicity depend on the solvent? Or would the salt just be neutral because assuming the solvent is water, it will produce an equal amount of hydrogen and hydroxide ions?

Cheers.

RuiAce

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Re: Chemistry Question Thread
« Reply #627 on: August 14, 2016, 04:33:02 pm »
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Classify the salt formed from HCO3- as acidic, basic or neutral and justify its classification. Include a chemical equation to illustrate your answer. (2marks)

Kinda confused for this question, cause i know that HCO3- is amphiprotic so wouldn't its acidity/basicity depend on the solvent? Or would the salt just be neutral because assuming the solvent is water, it will produce an equal amount of hydrogen and hydroxide ions?

Cheers.
Actually, for the hydrogen carbonate ion it exhibits basic properties with water.

HCO3- + H2O(l) ⇌ H2CO3(aq) + OH-

I think this is because water is a stronger acid than hydrogen carbonate. Or alternatively hydrogen carbonate is a stronger base than water. It's also found in the human body as a buffer system in this way.

I'm not entirely sure how this works though.

MysteryMarker

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Re: Chemistry Question Thread
« Reply #628 on: August 14, 2016, 06:15:07 pm »
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Ahh, so HCO3- acts as a base. Would this be because it is the strong conjugate base of the weak acid H2CO3?

Another question i have is justify the use of AAS in the mining industry, and in safeguarding the environment.

Just curious as to what points i should bring up for 'mining industry'.

Cheers.

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #629 on: August 14, 2016, 08:18:54 pm »
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Ahh, so HCO3- acts as a base. Would this be because it is the strong conjugate base of the weak acid H2CO3?

Another question i have is justify the use of AAS in the mining industry, and in safeguarding the environment.

Just curious as to what points i should bring up for 'mining industry'.

Cheers.

Hey MysteryMarker!

To your enquiry about the conjugate base/acid pair, yes and no. HCO3- is a strong base since H2CO3. However, it is only strong compared to water, meaning that when reacting with water, it is able to accept a hydrogen proton from water. But compared to NaOH which is a very strong base, HCO3- is less strong as a base. In our blood stream, HCO3- and H2CO3 act as buffer pairs and if HCO3- is a strong base then it wouldnt have the buffer effect since strong bases result in irreversible reactions. So when you talk about HCO3-, say its a strong base RELATIVE TO WATER.

In regards to AAS, I will just jot down a few dotpoints here for you to consider:
- Before AAS there was no way to detect the existence of trace elements in waterways (trace elements = extremely small quantities)
- AAS has helped industries to monitor heavy metal pollution into nearby waterways or atmosphere
- For example, if you do industrial chemistry, you can talk about the mercury cell which leeches the neurotoxin mercury into waterways and this can bioaccumulate and affect the health of animals going up the food chain
- If we wanna talk about mining industry, we can say that AAS helps to detect lead or mercury vapours that may rise from underground deposit caverns and these can really threaten workers' health
- And almost if you want to see the purity of the metal ore that is dug up from the ground, AAS would be useful for detecting small amounts of impurities

I think its quite hard to talk about the use of AAS in mining industry without being a mining engineer but generally AAS would just be used for detecting heavy metal existences and purity of materials. In HSC, they would ask for applications of AAS, but very unlikely to ask about its relation with mining industry. More likely they would be asking about AAS' use in water quality examination or just its general uses.

Best Regards
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