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March 28, 2024, 08:31:49 pm

Author Topic: HSC Chemistry Question Thread  (Read 1040515 times)  Share 

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DrShellgon

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Re: Chemistry Question Thread
« Reply #180 on: March 21, 2016, 09:31:04 pm »
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Hi, I was wondering, why do pure liquids not affect equilibrium? Isn't there a possibility that the pure liquid, say it's like a pure acid, could react with the reactants or the products in an equilibrium? How about pure water? Won't this potentially dilute either the reactants or the products, causing a change in concentration and causing the equilibrium to shift?

How about pure solids? Why do pure solids not affect equilibrium? E.g. what if we put in a block of pure sodium, or lithium, won't it react with a particular side of the equilibrium and cause some sort of shift?

Also thanks for your previous answer :)

RuiAce

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Re: Chemistry Question Thread
« Reply #181 on: March 21, 2016, 09:35:15 pm »
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Hi, I was wondering, why do pure liquids not affect equilibrium? Isn't there a possibility that the pure liquid, say it's like a pure acid, could react with the reactants or the products in an equilibrium? How about pure water? Won't this potentially dilute either the reactants or the products, causing a change in concentration and causing the equilibrium to shift?

How about pure solids? Why do pure solids not affect equilibrium? E.g. what if we put in a block of pure sodium, or lithium, won't it react with a particular side of the equilibrium and cause some sort of shift?

Also thanks for your previous answer :)

Solids don't carry much kinetic energy for them to start reacting with other things by themselves. All that you do by adding a solid is somewhat alter the surface area for the solid to react; solid molecules aren't going to bump into other liquid/gaseous/aqueous substances just like that.

Now, what exactly do you mean by pure liquids? Do you mean something such as 18M sulfuric acid (H2SO4(l))?
__________________

In terms of dilutions.

Aqueous solution substances are called that because, they are, dissolved water. All water does is reduce the concentration of the substance (out of the definition of dilution).
« Last Edit: March 21, 2016, 09:38:19 pm by RuiAce »

liiz

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Re: Chemistry Question Thread
« Reply #182 on: March 22, 2016, 06:26:30 pm »
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hello :) wondering if someone could please help explain this question to me "Explain why ethanol will dissolve in water and in pentane?" - I know it's a simple question but I'm still struggling to understand the whole polar and non-polar concept! Thanks so much

Johny1234567

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Re: Chemistry Question Thread
« Reply #183 on: March 22, 2016, 07:01:36 pm »
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hello :) wondering if someone could please help explain this question to me "Explain why ethanol will dissolve in water and in pentane?" - I know it's a simple question but I'm still struggling to understand the whole polar and non-polar concept! Thanks so much
Firstly, hydrocarbons are non-polar substances. Water are polar substances. Remember the theory behind 'like dissolves like', i.e polar substances dissolve other polar substances and vice versa.

Ethanol, if you draw out the structure, has a non-polar carbon end, as well as a polar hydoxyl functional group. Because ethanol has both non-polar and polar-ends, it will dissolve in water (polar) and pentane (non-polar).

Johny1234567

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Re: Chemistry Question Thread
« Reply #184 on: March 22, 2016, 07:05:14 pm »
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Attached is a question in chemical monitoring and management. I've done up to e) but am stuck on f). Could someone please help me c:

P.S Not entirely necessary but would prefer if someone completed the entire question so I can confirm my answers.

Thanks!

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #185 on: March 22, 2016, 07:26:49 pm »
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hello :) wondering if someone could please help explain this question to me "Explain why ethanol will dissolve in water and in pentane?" - I know it's a simple question but I'm still struggling to understand the whole polar and non-polar concept! Thanks so much

Hey Liiz!

Sure thing! Ethanol will dissolve in water because of its hydrophilic (means water-liking) -OH group which allows it to interact with water molecules through hydrogen bonding. Ethanol will dissolve in pentane because of its non-polar hydrocarbon group which is able to interact with other non-polar substances such as pentane through dispersion forces.

This would usually be a 2 mark question and if you just write what I wrote above in the exam they are easy 2 marks to get! If you want me to expand on what I said, please dont hesitate to tell me! :)

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curious.egg

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Re: Chemistry Question Thread
« Reply #186 on: March 22, 2016, 08:33:03 pm »
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Hellooo!!

For the prac where we had to make a natural indicator, my school used purple cabbage and in the procedure, we added salt and ethanol when we were crushing it. Why was salt and ethanol added? Please help!

RuiAce

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Re: Chemistry Question Thread
« Reply #187 on: March 22, 2016, 08:48:41 pm »
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Attached is a question in chemical monitoring and management. I've done up to e) but am stuck on f). Could someone please help me c:

P.S Not entirely necessary but would prefer if someone completed the entire question so I can confirm my answers.

Thanks!
I have never seen the term "reaction efficiency" coined before in the HSC. Such a question will not be asked in this course.

a) Hydrogen and nitrogen introduced into the system. According to the reversible equation of the production of ammonia, as we initially has a concentration of ammonia of 0 mol L-1 it's unsurprising that ammonia begins to get produced. The system then reaches equilibrium.

N2(g) + 3 H2(g) ⇌ 2 NH3(g)

b) and c) A downward spike in the concentration of all 3 substances suggests that the volume of the system has been increased, which is equivalent to a decrease in pressure. As there are 4 moles of gas on the left and 0 on the right, LCP predicts that the equilibrium shifts to the left, which explains the gradual increase in the concentrations of nitrogen and hydrogen.

d) and e) An upward spike in only the concentration of hydrogen means that excess hydrogen was introduced into the system. Trivially, LCP predicts that the equilibrium will shift to the right to minimise this disturbance.

« Last Edit: March 23, 2016, 07:59:29 am by RuiAce »

RuiAce

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Re: Chemistry Question Thread
« Reply #188 on: March 22, 2016, 08:50:07 pm »
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Hellooo!!

For the prac where we had to make a natural indicator, my school used purple cabbage and in the procedure, we added salt and ethanol when we were crushing it. Why was salt and ethanol added? Please help!

This is a highly specific question to which you will have to ask your teacher to determine what exactly were the impacts of the substances.

I performed this experiment using red cabbage and I did not require the addition of any of these reagents.

DrShellgon

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Re: Chemistry Question Thread
« Reply #189 on: March 22, 2016, 10:39:16 pm »
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Solids don't carry much kinetic energy for them to start reacting with other things by themselves. All that you do by adding a solid is somewhat alter the surface area for the solid to react; solid molecules aren't going to bump into other liquid/gaseous/aqueous substances just like that.

Now, what exactly do you mean by pure liquids? Do you mean something such as 18M sulfuric acid (H2SO4(l))?
__________________

In terms of dilutions.

Aqueous solution substances are called that because, they are, dissolved water. All water does is reduce the concentration of the substance (out of the definition of dilution).

Yep, by pure liquids, I meant adding reagants like the H2SO4 you mentioned, I'm just wondering, won't it react with a particular side of the equilibrium and thus cause a shift?

Also with adding just normal water to the equilibrium, am I correct to assume that since it's just water, the dilution it causes to both sides of the equilibrium is equal and thus it will overall have no effect on the equilibrium, similar to how a catalyst, when added, simply speeds up the rate of reaction and has no overall effect on the equilibrium?

Sine

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Re: Chemistry Question Thread
« Reply #190 on: March 23, 2016, 12:07:01 am »
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experimental procedure errors for a back titration?

is incorrect identification of end point okay (and If you have any others)

RuiAce

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Re: Chemistry Question Thread
« Reply #191 on: March 23, 2016, 07:58:56 am »
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Yep, by pure liquids, I meant adding reagants like the H2SO4 you mentioned, I'm just wondering, won't it react with a particular side of the equilibrium and thus cause a shift?

Also with adding just normal water to the equilibrium, am I correct to assume that since it's just water, the dilution it causes to both sides of the equilibrium is equal and thus it will overall have no effect on the equilibrium, similar to how a catalyst, when added, simply speeds up the rate of reaction and has no overall effect on the equilibrium?

Ok I had to do a bit of research to determine a formal answer.

The whole basis of Le Chatelier's Principle and all factors of dynamic equilibrium do not strictly relate to the moles of the substance, rather, the concentrations of the substances.

Effectively, gas concentrations can be measured as they will essentially be the moles of the substance over the volume of the vessel.
An aqueous substance's concentration can also be measured as it is the moles of the substance over the volume of the entire fluid they're dissolved in.

Solids and liquids, on the other hand, cannot be defined in terms of a concentration. You can add more moles of solid or liquid to a system, but this doesn't change their concentrations. The reason for this is, what is there to change the concentration with respect to?
- For solids, just adding more of them in really doesn't do much. They'll just sit there.
- For liquids, if you add more liquid what can you take the concentration of liquid with respect to? You increase the moles of the liquid, but you don't have something to take a volume with respect of, to find a concentration.

If your school elects to take up the industrial chemistry option topic, you will also be introduced to the equilibrium constant of a system, and find that in this constant (determines the ratio of reactants to products) no solid or liquid is included either.

RuiAce

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Re: Chemistry Question Thread
« Reply #192 on: March 23, 2016, 08:04:06 am »
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experimental procedure errors for a back titration?

is incorrect identification of end point okay (and If you have any others)
Back titration specifically is not something important in the HSC course.

Due to the nature of what it is compared to regular titration, the experimental procedure errors are similar to that of normal titration. This would include the incorrect identification of the endpoint as well as anything else (problematic method - equipment washed wrongly).

Maz

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Re: Chemistry Question Thread
« Reply #193 on: March 23, 2016, 09:41:18 pm »
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hey, can i please get some help with this question?
when you suffer from an acid stomach, the fluid in your stomach can contain hydrochloric acid at
a concentration of 2.5x10-4 mol L-1
to reduce the amount of acid in your stomach you are advised to take an antacid tablet that contains 5mg of aluminium hydroxide and 5mg of magnesium hydroxide. if at this time you have 1.5L of fluid in your stomach calculate the concentration of hydrogen ions and hydroxide ions in you stomach after you have taken the tablet.
if you took a second antacid tablet what would the hydrogen and hydroxide ion concentration be now?
i'd really appreciate the help...
thankyou :)
2016: Methods | Chem | Physics | Accounting | Literature

katherine123

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Re: Chemistry Question Thread
« Reply #194 on: March 24, 2016, 06:45:09 pm »
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Hi can u check my 6 marks response? question is attached below

Le Chatalier’s principle applies in step 1 and 2 since reactions are in equilibrium, but it is not applied in step 3 since reaction goes to completion.
 At step 1, low pressure and low temperature will cause reaction to shift towards the formation of NO(g). When heat is removed, reaction, being exothermic, will shift to the right to make up some of the reduced heat. When pressure is reduced, the reaction shifts right as 9 moles of gas react to form 10 moles of gas which results in increase in pressure.

The answer include “these conditions reduce the rate of reaction”  which I don’t really understand
 
 At step 2, the production of NO2(g) is favoured at low temperature and high pressure. This is because the removal of heat will cause the exothermic reaction to shift right to counteract some of the lost heat, and increase in pressure cause reaction to shift right since 3 moles of gas reacts to form 2 moles of gas which reduce some of the increased pressure.
At step 3, high pressure and high temperature will favour the formation of HNO3. The increase in pressure also increases the collision between gas molecules and water, and increase in temperature will increase the average kinetic energy of particles which leads to more successful collisions, hence reaction will shift right.
I don’t get this bit included in the answer  “as this step is not an equilibrium, the highest yield and rate of formation of nitric acid will occur when reactants undergo more collisions per second. “

-Doesn’t change in pressure, temperature, concentration or volume have no effect on reactions that go to completion?
-Pressure only applies for gas molecules right?