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March 29, 2024, 01:45:34 am

Author Topic: HSC Chemistry Question Thread  (Read 1040570 times)  Share 

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RuiAce

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Re: Chemistry Question Thread
« Reply #1710 on: February 21, 2017, 10:09:52 pm »
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Can someone please explain validity in experiments to me?? I am so confused

and

For heat of combustion of alkaniols experiment - if this was done at school would this automatically be considered invalid if so why??

Any help would be great thanks
Validity - Does your method address the aim?

Your experiment in the laboratory is very poor, as the method doesn't properly test what we want to at all. It's both invalid and inaccurate (although can be very reliable), because you have heat losses occurring everywhere - heat absorbed into the calorimeter, heat released in the environment, the specific heat capacity of the calorimeter not being concerned in calculations etc. all affecting the final values.
By not having the most efficient heat transfer, our results are no longer reliable as such a big portion of what our results depended on was unreliable.

I hope that makes sense. Essentially you need to know that the heat losses to the surroundings significantly decreases the reliability of the experiment. Let me know if you want me to clarify the above further.
Everything is good up to here. Reliability is specifically concerned with consistency.

You can have an experiment that's quite trashy, but still reliable because you consistently got the same wrong results every time you did it.

bsdfjnlkasn

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Re: Chemistry Question Thread
« Reply #1711 on: February 21, 2017, 10:16:54 pm »
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So for this question, we're trying to convert heat of combustion from kJ/g to kJ/mol. Molar mass is in g/mol. Looking at the units, if we times heat of combustion in kJ/g by molar mass in mol/g, we'll get kJ/mol, which is what we want!

Does that make sense? So to find heat of combustion in kJ/mol, you would multiply the values in the table by their respective molar masses :)

Hey thanks for giving that conversion, the units make more sense. Would it be ok to assumed that m = 1g? How would we then proceed with the rest of the calculations?

kiwiberry

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Re: Chemistry Question Thread
« Reply #1712 on: February 21, 2017, 10:28:37 pm »
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Hey thanks for giving that conversion, the units make more sense. Would it be ok to assumed that m = 1g? How would we then proceed with the rest of the calculations?

We're using the molar mass, so the mass of one mole of each substance. So for propanol (C3H8O):
(I guess they rounded the molar mass because you get 2019 if you don't)
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RuiAce

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Re: Chemistry Question Thread
« Reply #1713 on: February 21, 2017, 10:30:55 pm »
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Tbh, because I was worried, I always remembered this "hack":

The heat change per mole is in general, ALWAYS greater than that for the mass. And I know that n=m/MM so the molar mass is what distinguishes between the masses and moles.

Because the molar masses are always numbers greater than 1, and the heat change per mole is the larger one:


Given heat change per mole: Divide by MM to get heat change per gram
Given heat change per gram: Multiply by MM to get heat change per mole

(However, thanks for the proper explanation kiwiberry. This is just my quick way out.)

anotherworld2b

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Re: Chemistry Question Thread
« Reply #1714 on: February 21, 2017, 10:51:51 pm »
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Ah ok thank you  :D
Water has more energy than ice simply because water flows while ice doesn't. This means water must have kinetic energy which MAKES it flow. Also steam has more energy than water and ice simply as to create steam, you need to ADD heat to water just as how you need to ADD heat to ice to make it water. :D

Idek how you can freeze ice?!?

Yes as far as what you know now, the reactant is whats being consumed (obv...). When solid water i.e. ice is transformed into liquid water i.e. water, the ice i.e. the reactant in this case is the one being consumed :D

anotherworld2b

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Re: Chemistry Question Thread
« Reply #1715 on: February 21, 2017, 10:52:52 pm »
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I was also wondering how to do this question

Mathew587

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Re: Chemistry Question Thread
« Reply #1716 on: February 21, 2017, 11:15:59 pm »
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I was also wondering how to do this question

tip- kinetic energy. revise that bit and try again :)
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anotherworld2b

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Re: Chemistry Question Thread
« Reply #1717 on: February 21, 2017, 11:23:18 pm »
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ah I see. I am bit confused about ii. why pure hydrogen gas cannot cause the hydrogen it contains to ignite.
tip- kinetic energy. revise that bit and try again :)

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Re: Chemistry Question Thread
« Reply #1718 on: February 22, 2017, 07:59:12 am »
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Hi,
This might be a silly question. I understand that Sulfuric acid is diprotic, and Hydrochloric acid is monoprotic as you can see this in their equations. But I was doing a practise HSC question, and in the answers it stated that Citric Acid is triprotic. I was just wondering, if I am supposed to have learnt this?, and how do you know that it is triprotic, as to me it is not obvious through its equation?
Thank you

jakesilove

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Re: Chemistry Question Thread
« Reply #1719 on: February 22, 2017, 10:27:15 am »
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Hi,
This might be a silly question. I understand that Sulfuric acid is diprotic, and Hydrochloric acid is monoprotic as you can see this in their equations. But I was doing a practise HSC question, and in the answers it stated that Citric Acid is triprotic. I was just wondering, if I am supposed to have learnt this?, and how do you know that it is triprotic, as to me it is not obvious through its equation?
Thank you

Hey! This is just a stupid Chemistry thing that you'll have to memorise; Citric acid is triprotic. If you look at the structural formula, it sort of makes sense (there are three 'loose' Hydrogen). But, that doesn't matter; just memorise the point!
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strawberriesarekewl

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Re: Chemistry Question Thread
« Reply #1720 on: February 22, 2017, 10:27:59 am »
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What are some good tips on answering short answer and extended responses? I know we need to address the verb (i suck at that) and make a plan of what we are going to write but what else is required for full marks


anotherworld2b

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Re: Chemistry Question Thread
« Reply #1721 on: February 22, 2017, 11:23:13 am »
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I was wondering if my answers are correct. I am a bit confused about how to know what colour the reaction will turn into

kiwiberry

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Re: Chemistry Question Thread
« Reply #1722 on: February 22, 2017, 01:26:54 pm »
+1
What are some good tips on answering short answer and extended responses? I know we need to address the verb (i suck at that) and make a plan of what we are going to write but what else is required for full marks
-Equations and diagrams!!! Always always always include relevant equations where you can, and for prac questions especially, it is much easier to draw a quick diagram of the set-up than to describe it in words
-If you haven't already, take a look at the BOSTES verbs list! Remember that assess/evaluate questions always have an easy mark allocated to a judgement
-For long responses, using underlined headings to separate parts of your answer instead of having a page full of writing will help ensure that the marker doesn't miss any part of your answer, eg in assess/evaluate questions, I always write Judgement: before I write my judgement.
-And yes always plan long answers before you start writing them! It is very easy to start 'brain dumping' and go off on tangents unrelated to the question, teachers get really pissed off at this

Other than that, what helps for me is going through many past papers and looking at the marking criteria for different questions. Eventually, you'll have a better idea at how marks are allocated, and in exams this helps with knowing what to write to gain full marks :)
« Last Edit: February 22, 2017, 01:36:13 pm by kiwiberry »
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kiwiberry

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Re: Chemistry Question Thread
« Reply #1723 on: February 22, 2017, 02:03:39 pm »
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I was wondering if my answers are correct. I am a bit confused about how to know what colour the reaction will turn into

Hey! From your answers, I think you've got the right idea with the colours. Basically, if the equilibrium favours the forward reaction, the concentration of the products on the right will increase and thus their colour will become more prominent, vice versa

Q2 first photo- for the concentration of Cl- to increase, it will shift left

Q3 first photo- this is an exothermic reaction, meaning that heat is released. So when temperature is decreased, equilibrium will favour the exothermic reaction to produce more heat, which is the forward reaction in this case. Does that make sense?
Same goes for Q4 on the same photo, except this time it will favour the backward endothermic reaction since temperature is increased

Q5 first photo- you're right that OH- will react with H+ to form water! This reduces the concentration of H+. As a result, equilibrium will shift left to oppose the change and produce more H+

I'll have a look at the second photo and edit this post in a bit! Unless someone beats me to it haha.
Edit: ok I'm back!

Q3 second photo- there is actually no change to equilibrium here. Remember that the three factors affecting equilibrium are concentration, temperature and pressure. By removing solid iodine, its concentration/density isn't affected! Therefore, there will be no shift. Same goes for adding a solid. Good to note that Le Chatelier's Principle does not apply to solids

Q4 second photo- pressure will only affect gases in equilibrium. So because iodine is in solid form on the right, the mole ratio of gases is actually 1:0. Therefore, when volume is increased, pressure decreases, so equilibrium will shift to the reaction which produces more moles of gas, ie the backward reaction

Q5 second photo- same with 3&4 of the first photo, the equilibrium will shift to favour the backward endothermic reaction to reduce the temperature

Hope this helps :) Let me know if anything doesn't make sense!
« Last Edit: February 22, 2017, 03:24:30 pm by kiwiberry »
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strawberriesarekewl

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Re: Chemistry Question Thread
« Reply #1724 on: February 22, 2017, 03:05:13 pm »
+1
-Equations and diagrams!!! Always always always include relevant equations where you can, and for prac questions especially, it is much easier to draw a quick diagram of the set-up than to describe it in words
-If you haven't already, take a look at the BOSTES verbs list! Remember that assess/evaluate questions always have an easy mark allocated to a judgement
-For long responses, using underlined headings to separate parts of your answer instead of having a page full of writing will help ensure that the marker doesn't miss any part of your answer, eg in assess/evaluate questions, I always write Judgement: before I write my judgement.
-And yes always plan long answers before you start writing them! It is very easy to start 'brain dumping' and go off on tangents unrelated to the question, teachers get really pissed off at this

Other than that, what helps for me is going through many past papers and looking at the marking criteria for different questions. Eventually, you'll have a better idea at how marks are allocated, and in exams this helps with knowing what to write to gain full marks :)

Thanks Kiwiberry