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March 29, 2024, 01:02:10 am

Author Topic: VCE Chemistry Question Thread  (Read 2313172 times)  Share 

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ArtyDreams

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Re: VCE Chemistry Question Thread
« Reply #8445 on: February 14, 2020, 05:27:41 pm »
+7
I'm not very good at explaining things, but heres my short explanation:

When an atom is heated up, the atom absorbs energy. As this occurs, the electrons get 'excited' and jump to a higher energy level. After a while, they jump back down to ground state, and they release the energy they have absorbed as a photon of light.

The various shells in an atom vary in energy levels. Electrons must occupy one of these energy levels.  To have a transition between the levels requires an amount of energy exactly equal to the energy difference between the two levels. This has a large effect on the absorption and emission spectra of an element. Because the atom can only absorb specific amounts of energy, only certain wavelengths of light will be absorbed.

i.e. The electrons in an atom can only occupy certain allowed energy levels.

Hope this helps!


Fashionmonarch

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Re: VCE Chemistry Question Thread
« Reply #8446 on: February 16, 2020, 07:04:46 pm »
0
Hey guys! Could i please have some help with this question?
If 25.00 mL of NaOH was added to 15.00 mL of H2SO4, what is the final pH?
2NaOH (aq) + H2SO4(aq) --> Na2SO4(aq) + 2H2O(l)

sweetcheeks

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Re: VCE Chemistry Question Thread
« Reply #8447 on: February 16, 2020, 08:22:51 pm »
+2
Hey guys! Could i please have some help with this question?
If 25.00 mL of NaOH was added to 15.00 mL of H2SO4, what is the final pH?
2NaOH (aq) + H2SO4(aq) --> Na2SO4(aq) + 2H2O(l)


Before we answer it, can you please post what you've done so far?

Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8448 on: February 17, 2020, 08:52:20 pm »
+3
Hey guys! Could i please have some help with this question?
If 25.00 mL of NaOH was added to 15.00 mL of H2SO4, what is the final pH?
2NaOH (aq) + H2SO4(aq) --> Na2SO4(aq) + 2H2O(l)

Welcome to the forums Fashionmonarch!
Like sweetcheeks has mentioned above, it would be great if you could include your current thought process/working out so that we can better understand what you are actually stuggling with and give feedback based off that  :) (otherwise we might answer the question whithout actually helping you understand where you went wrong).
This being said, I think some information is missing from this question - to work this out we would want to know the concentration of each solution (the sodium hyudroxide solution and the sulphuric acid solution).
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Fashionmonarch

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Re: VCE Chemistry Question Thread
« Reply #8449 on: February 17, 2020, 09:52:00 pm »
0
Before we answer it, can you please post what you've done so far?

Would I be able to find the moles of the reactants by using the 1ml= 1g ? as they don't give any information about concentration in the question. From there I'm guessing i work out which is excess and limiting? But i'm not sure how I go about the rest of the question.

miyukiaura

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Re: VCE Chemistry Question Thread
« Reply #8450 on: February 18, 2020, 05:16:16 pm »
0
Not sure if this question is relevant here or anything (just started 1/2 chem), but how do you figure out how many valence electrons there are in an element in the d block? Obviously if it's in the p block you would look at the group number but I'm not sure about those in the d block. My teacher said that the term 'valence' wouldn't apply here as it can vary. Thanks.
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ssillyssnakes

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Re: VCE Chemistry Question Thread
« Reply #8451 on: February 18, 2020, 08:31:21 pm »
+3
Not sure if this question is relevant here or anything (just started 1/2 chem), but how do you figure out how many valence electrons there are in an element in the d block? Obviously if it's in the p block you would look at the group number but I'm not sure about those in the d block. My teacher said that the term 'valence' wouldn't apply here as it can vary. Thanks.
With transition metals it is difficult to know for sure, especially with only a high school chemistry level intuition. I believe that many have 2 valence electrons (meaning that the 4s orbital is full) but there are also notable exceptions so idk.

I don't know when you would need to know this for VCE chemistry - if a question requires the knowledge of the number of valence electrons in a transition metal, I'd be surprised to see it not be given in the stem of the question honestly

If anyone else can help more - please do, as I'm just trying my best lmao
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colline

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Re: VCE Chemistry Question Thread
« Reply #8452 on: February 19, 2020, 08:48:17 pm »
+1
I don't know when you would need to know this for VCE chemistry - if a question requires the knowledge of the number of valence electrons in a transition metal, I'd be surprised to see it not be given in the stem of the question honestly
I don't know about 1/2, but you definitely won't need to know this for 3/4 chem.

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Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8453 on: February 22, 2020, 05:45:40 pm »
+2
Would I be able to find the moles of the reactants by using the 1ml= 1g ? as they don't give any information about concentration in the question. From there I'm guessing i work out which is excess and limiting? But i'm not sure how I go about the rest of the question.

You cannot assume that 1ml of HCL solution (for example) = 1 gram of HCL. This is becuase the actual amount of HCL in this solution would depend on the concentration of the solution.
 To work out this question you will need more information than given. If that is all that is given in the question, then I would be inclined to say that the question is flawed - where was this question from?
« Last Edit: February 22, 2020, 05:47:23 pm by Erutepa »
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coldairballoon

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Re: VCE Chemistry Question Thread
« Reply #8454 on: February 23, 2020, 07:29:48 pm »
0
How do you know the state of water as the product of a combustion reaction? And do the data book's combustion enthalpy values consider water as a liquid or gas?
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dream chaser

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Re: VCE Chemistry Question Thread
« Reply #8455 on: February 23, 2020, 07:49:01 pm »
+1
How do you know the state of water as the product of a combustion reaction? And do the data book's combustion enthalpy values consider water as a liquid or gas?

Hi coldairballoon,

I am pretty sure the state of water can either be liquid or gas. I don't think you will be marked down for writing either state. Best to ask your chemistry teacher about this as well.

Hope this helps  :)

whys

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Re: VCE Chemistry Question Thread
« Reply #8456 on: February 23, 2020, 07:56:10 pm »
+3
How do you know the state of water as the product of a combustion reaction? And do the data book's combustion enthalpy values consider water as a liquid or gas?

Gas. Combustion occurs when a substance is burned (hence oxygen is used in the reaction) so the water in such high temperatures remains a gas as water vapour. I am under the impression VCAA accepts both gas and liquid states for water in combustion reactions though, someone please correct me if I'm wrong. Not sure about enthalpy values. However, in the combustion of a hydrocarbon, the product is liquid water as the water vapour produced by the reaction is condensed into a liquid.

EDIT:
if water is in liquid form that means it releases more heat energy vs gas form releases less heat energy
« Last Edit: February 23, 2020, 07:58:08 pm by whys »
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Evolio

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Re: VCE Chemistry Question Thread
« Reply #8457 on: February 24, 2020, 03:25:16 pm »
+2
Quote
How do you know the state of water as the product of a combustion reaction? And do the data book's combustion enthalpy values consider water as a liquid or gas?
Hello. The data book's heat of combustion values consider water as a liquid as the heats of combustions were calculated at 25 degrees celsius where water is a liquid. My teacher told us to write water as a liquid for this reason.

Also, I had a few questions to ask.
I'm a bit confused about the concept of salt bridges in galvanic cells. So, I know that salt bridges contain cations which flow to the cathode and anions that flow to the anode, thus allowing the charges to be neutral at each of the half cells. However, I was wondering why it was a bad thing if they're not neutral? Why is it bad if one half-cell is negatively charged (cathode where reduction is happening) and one half-cell is positively charged where oxidation is happening?
Also, I'm confused about the concept of 'salt bridges completing the circuit'. What does that mean and why is it important in producing electrical energy?

Also from the VCAA 2010 Exam, question 7 d, I'm a bit confused about why the Lithium ions move as well? I thought only the ions ions in the electrolyte moved to achieve neutrality?




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Re: VCE Chemistry Question Thread
« Reply #8458 on: February 24, 2020, 05:49:34 pm »
+4
Also, I had a few questions to ask.
I'm a bit confused about the concept of salt bridges in galvanic cells. So, I know that salt bridges contain cations which flow to the cathode and anions that flow to the anode, thus allowing the charges to be neutral at each of the half cells. However, I was wondering why it was a bad thing if they're not neutral? Why is it bad if one half-cell is negatively charged (cathode where reduction is happening) and one half-cell is positively charged where oxidation is happening?
Also, I'm confused about the concept of 'salt bridges completing the circuit'. What does that mean and why is it important in producing electrical energy?

Also from the VCAA 2010 Exam, question 7 d, I'm a bit confused about why the Lithium ions move as well? I thought only the ions ions in the electrolyte moved to achieve neutrality?
Key to remember electrostatic attraction here! Electrons are attracted to positive hence if there was no salt bridge the cathode would become negative (from lack of cations) and thus electrons would be repelled from going there. Consequently this would mean electrons would fail to flow from the anode across the wire to the cathode, hence no longer produce electricity!!!!!

For the VCAA 2010 exam, I think as it is an electrolytic cell rather than just a galvanic, lithium ions would need to move back to anode so that the cell has enough lithium in the anode to discharge. both equations involve lithium ions as well so the ions would be constantly moving from when they are created in oxidation to when they are consumed in reduction.
Sorry if this doesn't make sense, majority of chem knowledge went poof after my exam last year XD

Ionic Doc

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Re: VCE Chemistry Question Thread
« Reply #8459 on: February 24, 2020, 08:24:48 pm »
+1
Benjamin has purchased a new camp stove that runs on propanol and decides to test it out.
He adds 146.25g of fuel to the stove and heats up 2L of water for 3 minutes, during which time the temperature of the water rises from 20.2 degrees to 94.6 degrees (celsius). When Benjamin reweighs the fuel there is 99.13g remaining.

Calculate the heat of combustion (KJ/g) for the propanol.

Okay so first I figured out how much energy was used (using the q=mcAT formula)
Therefore Q = (2000g) *(4.18)*(74.4)
= 622 KJ

Next step all I did was

622 / 47.12g

47.12g as that is how much was used up in the reaction.

=13.2 KJ G-1

Just wondering if I did this correctly.
thanks
« Last Edit: February 25, 2020, 07:13:00 am by Ionic Doc »
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