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March 29, 2024, 08:02:26 pm

Author Topic: HSC Chemistry Question Thread  (Read 1040773 times)  Share 

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RuiAce

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Re: Chemistry Question Thread
« Reply #960 on: October 26, 2016, 03:01:05 pm »
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Hey another q , just asking to confirm. In q 2 in the multi choice the weak acid required the SAME volume of base to reach equivalence as the strong acid because the student titrated equal conc of 0.1M of acid? However in the graph , because of different concentrations a LARGER volume of base is needed to reach equivalence.

Is this right?
Thanks again
The answer to Q2 is that they are the same because you titrated with a strong base. So long as one of the acid or base is strong, the neutralisation goes to completion. Hence the degree of ionisation of the weak base is useless because the same volume (so this part you are right) of the same STRONG base was used

RuiAce

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Re: Chemistry Question Thread
« Reply #961 on: October 26, 2016, 03:11:51 pm »
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Whereas for the graph, whether or not the acid is strong or weak will matter.

In every instance we titrate with a strong base, so we need to consider what happens using the equivalence point.

Note that:
Strong acid + Strong base produces a neutral salt
Strong acid + Weak base produces an acidic salt
Weak acid + Strong base produces a basic salt.

The equivalence point tells us which is what.

For the first acid, the pH jumping from 3 to 10 hints that we have an equivalence point at around pH 7, I.e. Neutral. So working backwards that should be a strong acid strong base reaction.

For the second, it goes from 6 to 10, which hints an equivalence point of 8. This is suggestive of a strong base weak acid reaction.


The graphs are characteristic. Frequently appears in acids and isn't hard to memorise

onepunchboy

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Re: Chemistry Question Thread
« Reply #962 on: October 26, 2016, 03:41:16 pm »
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Whereas for the graph, whether or not the acid is strong or weak will matter.

In every instance we titrate with a strong base, so we need to consider what happens using the equivalence point.

Note that:
Strong acid + Strong base produces a neutral salt
Strong acid + Weak base produces an acidic salt
Weak acid + Strong base produces a basic salt.

The equivalence point tells us which is what.

For the first acid, the pH jumping from 3 to 10 hints that we have an equivalence point at around pH 7, I.e. Neutral. So working backwards that should be a strong acid strong base reaction.

For the second, it goes from 6 to 10, which hints an equivalence point of 8. This is suggestive of a strong base weak acid reaction.


The graphs are characteristic. Frequently appears in acids and isn't hard to memorise

wait so that means both acid 1 and 2 will require to same volume of base to reach equivalence because there is a strong base reacting?

RuiAce

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Re: Chemistry Question Thread
« Reply #963 on: October 26, 2016, 03:50:45 pm »
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I guess the deal with volume is correct in what you had at the start, but I always analyses it via the pH, never via the volume

Neutron

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Re: Chemistry Question Thread
« Reply #964 on: October 26, 2016, 10:37:38 pm »
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Hi! I think I'm just a wreck right now but if someone could explain this to me I would really appreciate it! Like I thought because strong acids ionise more H+ ions, that means they would need more base (i.e more OH ions) to neutralise but I guess not

massive

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Re: Chemistry Question Thread
« Reply #965 on: October 27, 2016, 07:56:46 am »
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is petroleum an example of a fossil fuel?

RuiAce

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Re: Chemistry Question Thread
« Reply #966 on: October 27, 2016, 08:02:08 am »
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Hi! I think I'm just a wreck right now but if someone could explain this to me I would really appreciate it! Like I thought because strong acids ionise more H+ ions, that means they would need more base (i.e more OH ions) to neutralise but I guess not
The NaOH drives the reaction to completion.

Recall that only the reaction between a weak base and an weak acid is reversible, i.e. it goes into equilibrium. The presence of at least one of a strong acid, or a strong base, will force the reaction to go to the end.
This is because, in the case that both substances are partially ionised (weak), there's only so many H+ and OH- to react. So this system will stabilise out. Whereas if you have something strong, the NaOH here, the presence of the OH- ions will continuously drive the equilibrium (by LCP) so that the reaction ultimately goes to completion.

We note that the concentration of NaOH and both the volume AND concentration of the acids have been fixed. So the fact that we have all those OH- there from the NaOH will drive the reaction regardless of the acids. Hence, if the same outcome is to be achieved, the last thing to be fixed is the volume of NaOH.
is petroleum an example of a fossil fuel?
Yes

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Re: Chemistry Question Thread
« Reply #967 on: October 27, 2016, 08:42:18 am »
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SHIPWRECKS

For question b)(ii): In this reaction - I'm a bit confused. Why do both metals undergo their own seperate chemical reactions (i.e. two different systems). My first impression was that magnesium would just rust, whereas the iron wouldn't - acting as a sacrificial anode. BUT apparently, the Mg will oxidise, its electrons will react with Fe2+ from the electrolyte solution and thus a solid iron coating will form on the Mg, with the electrolyte solution turning less green (less Fe2+ ions). Thus, the iron nail is left to just rust itself, and do its own little thing (standard process of rusting: Fe --> Fe2+ --> Fe3+...).

What I don't understand, is why they don't effect each other even if they're in such close proximity? I think im being misled its all one system because it's in one single beaker...do sacrificial anodes need to PHYSICALLY TOUCH? I don't even know how it works. ._.
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nimasha.w

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Re: Chemistry Question Thread
« Reply #968 on: October 27, 2016, 08:49:34 am »
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hey! in the 2013 HSC paper, question 24b, it says that adding a catalyst with have no visible effect on the reaction mixture because it will increase the reaction rate of both the forward and reverse reactions. Does this mean with such things like the Haber process, the adding of the magnetite catalyst does not increase the reaction rate, it only lowers the activation energy?

RuiAce

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Re: Chemistry Question Thread
« Reply #969 on: October 27, 2016, 08:54:24 am »
+1
hey! in the 2013 HSC paper, question 24b, it says that adding a catalyst with have no visible effect on the reaction mixture because it will increase the reaction rate of both the forward and reverse reactions. Does this mean with such things like the Haber process, the adding of the magnetite catalyst does not increase the reaction rate, it only lowers the activation energy?
Lowering the activation energy DOES increase the rate.

What they mean that it has no visible effect on the reaction mixture is that the final equilibrium CONCENTRATIONS are the same. i.e. The role of the catalyst speeds up the rate equilibrium is achieved, however does nothing on the equilibrium ITSELF.

massive

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Re: Chemistry Question Thread
« Reply #970 on: October 27, 2016, 09:56:34 am »
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what would be the answer to part b?

jakesilove

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Re: Chemistry Question Thread
« Reply #971 on: October 27, 2016, 09:59:08 am »
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SHIPWRECKS

For question b)(ii): In this reaction - I'm a bit confused. Why do both metals undergo their own seperate chemical reactions (i.e. two different systems). My first impression was that magnesium would just rust, whereas the iron wouldn't - acting as a sacrificial anode. BUT apparently, the Mg will oxidise, its electrons will react with Fe2+ from the electrolyte solution and thus a solid iron coating will form on the Mg, with the electrolyte solution turning less green (less Fe2+ ions). Thus, the iron nail is left to just rust itself, and do its own little thing (standard process of rusting: Fe --> Fe2+ --> Fe3+...).

What I don't understand, is why they don't effect each other even if they're in such close proximity? I think im being misled its all one system because it's in one single beaker...do sacrificial anodes need to PHYSICALLY TOUCH? I don't even know how it works. ._.

Hey! This is a beyond stupid question. Normally, you'd be 100% right; there are two dissimilar metals, and there is seemingly an electrolyte between the two. The trick is that there are TWO different solutions combined together; one relevant to the Iron, one relevant to the Magnesium. This essentially 'requires' each metal to do their own thing, as the solutions won't interact with each other. Extremely stupid question, and it won't teach you anything unless exactly the same question, so learn from your mistake and move on because it is 99% unlikely to come up again.
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jakesilove

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Re: Chemistry Question Thread
« Reply #972 on: October 27, 2016, 10:02:17 am »
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what would be the answer to part b?

Hey! If a reaction is 'endothermic', it requires energy for the reaction to move forward. It ABSORBS energy from it's surroundings. Therefore, if we add heaps of energy (through heat), the reaction will more forward at a greater rate! The answer here is basically just that the reaction is endothermic :)
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wesadora

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Re: Chemistry Question Thread
« Reply #973 on: October 27, 2016, 10:19:27 am »
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Hey! This is a beyond stupid question. Normally, you'd be 100% right; there are two dissimilar metals, and there is seemingly an electrolyte between the two. The trick is that there are TWO different solutions combined together; one relevant to the Iron, one relevant to the Magnesium. This essentially 'requires' each metal to do their own thing, as the solutions won't interact with each other. Extremely stupid question, and it won't teach you anything unless exactly the same question, so learn from your mistake and move on because it is 99% unlikely to come up again.

I see. Hold on...so do sacrificial anodes have to be in physical contact? (e.g. magnesium wrapped around an iron nail)
Or say, could a magnesium and an iron strip be next to each other (on opposite sides) of a petri dish with like...an NaCl electrolyte - and the magnesium would still rust preferentially instead of the iron (forcing iron to be a cathode), or would both just do their own things? O.O
« Last Edit: October 27, 2016, 10:21:45 am by wesadora »
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Sssssrr

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Re: Chemistry Question Thread
« Reply #974 on: October 27, 2016, 10:29:01 am »
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hi, does changing concentrations of products/reactants have any effect on reaction rate in an equilibrium reaction, or does it only impact yield?