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March 28, 2024, 11:42:33 pm

Author Topic: QCE Chemistry Questions Thread  (Read 31182 times)

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jasmine24

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Re: QCE Chemistry Questions Thread
« Reply #90 on: January 19, 2021, 04:05:57 pm »
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Hi, I was wondering how to identify the endpoint on a titration curve. I know its when the indicator changes colour but im not sure how this could be identified by just looking at the curve
Thanks!

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #91 on: January 19, 2021, 04:09:08 pm »
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Hi, I was wondering how to identify the endpoint on a titration curve. I know its when the indicator changes colour but im not sure how this could be identified by just looking at the curve
Thanks!

Hi!

You know that bit where the curve goes really steep? Like it's basically straight-up? That's what you're looking for

no worries :)

jasmine24

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Re: QCE Chemistry Questions Thread
« Reply #92 on: January 19, 2021, 07:34:22 pm »
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Hi!

You know that bit where the curve goes really steep? Like it's basically straight-up? That's what you're looking for

no worries :)
So is it the whole steep part? I was working on a question from my textbook with a graph sort of like the one i attached and it said the endpoint was at the point i labelled and I'm confused as to how it is
Thank you

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #93 on: January 19, 2021, 07:38:58 pm »
+5
The equivalence point is in the "middle" of the steep part and the end point is a little bit past that (slightly more volume). Like how when you wait for the colour change, we tend to approximate the equivalence point and end point as being achieved by the same volume

jasmine24

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Re: QCE Chemistry Questions Thread
« Reply #94 on: February 02, 2021, 06:29:32 pm »
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Hi, would anyone be able to explain dataset 1 - item 3. Im not sure how pKa = 14 - 9.2 as i thought at the half-equivalence point pKa = pH
TIA :)
Question: https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia1_smple_ass_inst.pdf
Answers: https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia1_smple_m_scheme.pdf

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #95 on: February 04, 2021, 01:44:32 pm »
+9
Hi, would anyone be able to explain dataset 1 - item 3. Im not sure how pKa = 14 - 9.2 as i thought at the half-equivalence point pKa = pH
TIA :)
Question: https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia1_smple_ass_inst.pdf
Answers: https://www.qcaa.qld.edu.au/downloads/senior-qce/sciences/snr_chemistry_19_ia1_smple_m_scheme.pdf

Hey!

The first thing that draws my attention here is is:
Ammonia is a base and is the substance of unknown concentration (analyte)
We are adding to this our HCl, an acid and our substance of known concentration (titrant)

For all of this I'm assuming 25 C as per usual in QCE chem pH questions

Remember we started with a base and we're adding acid, at this stage we are still on the basic side. At the half-equivalence point we have that half of the base has reacted with the HCl, so pKb = pOH  by the exact same logic that gets you to the more commonly discussed pH = pKa. 

Remembering that: pKa + pKb = 14  = pH + pOH, we then just do a quick conversion to get the desired value.

We can think of it like this:
An ammonium ion (right hand side of the equation) has a pKa value of roughly 9.2. We want to get the reverse-direction value, ammonia's one, so we do 14 - pk[ammonium] = 14 - 9.2.

We can also think of it like this:
We have pH of 9.2. We want pOH since we're looking at the constant for a base. 14 - pH = 14 - 9.2


TLDR; QCAA used pKa even though they were asking you about a base, which makes things confusing.

Let me know if this clears things up :)

jasmine24

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Re: QCE Chemistry Questions Thread
« Reply #96 on: February 04, 2021, 08:16:17 pm »
+6
Hey!

The first thing that draws my attention here is is:
Ammonia is a base and is the substance of unknown concentration (analyte)
We are adding to this our HCl, an acid and our substance of known concentration (titrant)

For all of this I'm assuming 25 C as per usual in QCE chem pH questions

Remember we started with a base and we're adding acid, at this stage we are still on the basic side. At the half-equivalence point we have that half of the base has reacted with the HCl, so pKb = pOH  by the exact same logic that gets you to the more commonly discussed pH = pKa. 

Remembering that: pKa + pKb = 14  = pH + pOH, we then just do a quick conversion to get the desired value.

We can think of it like this:
An ammonium ion (right hand side of the equation) has a pKa value of roughly 9.2. We want to get the reverse-direction value, ammonia's one, so we do 14 - pk[ammonium] = 14 - 9.2.

We can also think of it like this:
We have pH of 9.2. We want pOH since we're looking at the constant for a base. 14 - pH = 14 - 9.2


TLDR; QCAA used pKa even though they were asking you about a base, which makes things confusing.

Let me know if this clears things up :)
Thank you so much, this def cleared things up! Since they asked for the pKa i just assumed it was an acid

justsomerandom21

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Re: QCE Chemistry Questions Thread
« Reply #97 on: July 13, 2021, 05:32:51 pm »
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Hi just a quick question about Chem, but would anyone recommend memorising all the notes that we are given in class by our teachers or just the syllabus dot points. How about reading the textbook? Thanks in advance.

jinx_58

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Re: QCE Chemistry Questions Thread
« Reply #98 on: August 05, 2021, 08:09:02 pm »
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Hey.

Experiments were conducted to see the reaction between sodium thiosulphate and hydrochloric acid.

At times, the sulfur dioxide was lost to the atmosphere whereas sometimes it was trapped in the reaction.

Would this affect the reaction rate?

Many thanks,
jinx_58
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Currently doing Unit 4: QCE
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My Year 12 QCE Journal!

jasmine24

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Re: QCE Chemistry Questions Thread
« Reply #99 on: August 22, 2021, 01:38:40 pm »
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can someone help me with this syllubus objective?
"appreciate that each class of organic compound displays characteristic chemical properties and undergoes specific reactions based on the functional group present; these reactions, including acid-base and oxidation reactions, can be used to identify the class of the organic compound"

I cant find any notes on it  :(

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #100 on: August 24, 2021, 10:14:40 am »
+1
can someone help me with this syllubus objective?
"appreciate that each class of organic compound displays characteristic chemical properties and undergoes specific reactions based on the functional group present; these reactions, including acid-base and oxidation reactions, can be used to identify the class of the organic compound"

I cant find any notes on it  :(


Hey!

This is is about understanding molecules will react differently depending on their functional groups. For example, alkenes can undergo addition reactions but alkanes can't since alkanes only have single carbon-carbon bonds. Therefore, if we are unsure whether a molecule is an alkane or alkene we can test this by seeing whether it is able to undergo addition reactions (i.e. using the bromine test). Similarly, we know that primary or secondary (but not tertiary) alcohols can experience oxidation and that aldehydes (but not ketones) can be oxidised to carboxylic acids.

I hope this helps :)

justsomerandom21

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Re: QCE Chemistry Questions Thread
« Reply #101 on: March 09, 2022, 10:42:08 pm »
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Howdy,
Do any kind souls have any advice on good titrations IA2s (student experiment)? Any advice on things that you did that went well would be appreciated too :).



Thanks,
justsomerandom21





jinx_58

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Re: QCE Chemistry Questions Thread
« Reply #102 on: March 14, 2022, 09:47:26 am »
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Howdy,
Do any kind souls have any advice on good titrations IA2s (student experiment)? Any advice on things that you did that went well would be appreciated too :).



Thanks,
justsomerandom21

Greetings!
I am doing the same thing right now. I went through the Deadly EEI website (http://seniorchem.com/eei.html), and found some cool pracs there.

Hope this helps,
 - jinx_58
Class of 2022!
Currently doing Unit 4: QCE
Physics || Chemistry ||Methods || General English || Ancient History || Religion & Ethics
My Year 12 QCE Journal!

tr0206

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Re: QCE Chemistry Questions Thread
« Reply #103 on: April 19, 2022, 12:51:07 am »
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Hi,
Just a quick question for a Year 11 assignment; I'm investigating PEM fuel cells against Lithium-Ion Batteries, and am trying to find a meaningful way to compare their energy outputs.

As far as I can understand, a Fuel Cell rated as having a 250kW capacity would have an equivalent capacity of 250kWh assuming it had enough fuel (Hydrogen) for one hour of travel at (x) speed. I thought this because internet research told me that kWh = kW x (Time). Is this logic accurate for comparing kWh to kW between Fuel Cells and Batteries? If not, could anyone recommend another method of comparing these units.
Thanks.

priya22

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Re: QCE Chemistry Questions Thread
« Reply #104 on: May 07, 2022, 11:04:38 am »
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hi people,
For chemistry I did the galvanic cell prac. My group used magnesium sulphate (with aluminum electrode) and copper sulphate (copper electrode). We tested different concentrations and had so many issues with getting the voltage but, we eventually got our results.

The results are pretty all over the place (doesn't show any correlation- the number are like {it is in order}; 0.08, 0.05, 0.14, 0.53, 0.15)  and now i'm stuck trying to figure out how to write for the rest of the report and the kind of analysis required to get some good analysis done.
I would like some help with what I can actually write.