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Author Topic: VCE Chemistry Question Thread  (Read 2313375 times)  Share 

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caffinatedloz

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Re: VCE Chemistry Question Thread
« Reply #8415 on: January 11, 2020, 07:58:51 am »
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Hey guys! Was just wondering if anyone could help me out. Do all metals produce a colour when sprayed into a flame? Is this colour always within the visible spectrum of light (something that we can see)?

I understand that many metals produce colours that are distinct (although our eyes may not be able to distinguish between all of them) and that some non-metals produce UV light that cannot be observed (noble gasses).

I have read through this old AN thread and this website among other things but I'm still a little confused. Any help would be greatly appreciated.

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Re: VCE Chemistry Question Thread
« Reply #8416 on: January 11, 2020, 09:58:11 am »
+3
Hey guys! Was just wondering if anyone could help me out. Do all metals produce a colour when sprayed into a flame? Is this colour always within the visible spectrum of light (something that we can see)?

I understand that many metals produce colours that are distinct (although our eyes may not be able to distinguish between all of them) and that some non-metals produce UV light that cannot be observed (noble gasses).

I have read through this old AN thread and this website among other things but I'm still a little confused. Any help would be greatly appreciated.

In an atom you have electrons whizzing around at particular energy levels. These energy levels are linked to what "shell" the electrons are in. The valence electrons (by definition) are in the furthest shell.

In the flame test, metals are heated and valence electrons temporarily gain energy. When they crash back down to a lower energy state the excess energy is released in the form of light.

With light, different frequencies correspond to different energy levels. Visually, our eyes interprer frequency as colour. For example, red light is lower frequency and energy than blue light.

The energy jump for electrons in a metal determines the frequency of light that is emitted. Some of these frequencies are within the visible spectrum & some are not. If the frequencies are in the visible spectrum we interpret that frequency as colour.

Hope this helps!

Edit: typos
« Last Edit: January 11, 2020, 09:59:44 am by Bri MT »

jnlfs2010

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Re: VCE Chemistry Question Thread
« Reply #8417 on: January 11, 2020, 10:04:54 am »
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Hi jnlfs2010 again!
Good work on explaining your thought process so far!

Its helpfull to start by writing out the equation to begin with:
CxHyOz + ?O2 --> ?CO2 + ?H2O

This next bit I did find a littly tricky and had to do a quick bit of googling- It might just be me but I do think this is a tad bit beyond a VCE level (would love others inputs on it though)
The resulting gas mixture will be consisting of carbon dioxide gas, water vapour, and excess oxygen. When this gas mixtuture is passed though CaCl2, this anhydrous calcium chloride (meaning a calcium chloride compound without associated water molecules) readily absorbs the water vapour. As such, the change in madd of the CaCl2 represents the mass of water produced in the combustion of Terephthalic acid. Passing the gas mixture though NaOH would then cause the CO2 in the gas to react with the NaOH to result in the formation of a sodium bicarbonate (NaHCO3) and as such the change in NaOH mass would represent the mass of the CO2 produced from the combustiuon of the Terephthalic acid.
Now that you know the mass of the products, you can figure out the mole amount of each product and do the math from there to get the empiracal formula of the acid - I will let you try to do this on your own though.
let me know if you can work it out (if not I can help you further)

As I said, I think the question expects a bit too much interms of wanting you to know how each of the products will react with the CaCl2 and the NaOH, but I would love to hear what others think in terms of how hard it is.

yeah, I'm just confused about how the CO2 and H2O will react with the CaCl2 or NaOH, otherwise, the question isn't too hard yes. Is there reason why co2 reacts with naoh and h2o reacts with cacl2?
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caffinatedloz

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Re: VCE Chemistry Question Thread
« Reply #8418 on: January 11, 2020, 10:05:58 am »
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thanks bri! that makes a lot of sense!

Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8419 on: January 11, 2020, 10:26:13 am »
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In an atom you have electrons whizzing around at particular energy levels. These energy levels are linked to what "shell" the electrons are in. The valence electrons (by definition) are in the furthest shell.

In the flame test, metals are heated and valence electrons temporarily gain energy. When they crash back down to a lower energy state the excess energy is released in the form of light.

With light, different frequencies correspond to different energy levels. Visually, our eyes interprer frequency as colour. For example, red light is lower frequency and energy than blue light.

The energy jump for electrons in a metal determines the frequency of light that is emitted. Some of these frequencies are within the visible spectrum & some are not. If the frequencies are in the visible spectrum we interpret that frequency as colour.

Hope this helps!

Edit: typos
To just add to this,
Emisstion spectra refers to all the different frequencies of light that atoms of a specific element can relsease when returning to lower energy levels. Some frequencies of light will be visable and some won't be.

All metals do have emission specta containing visable light, however not all metals produce characteristic flame colours. After a quick bit of reading it seems that this occurance is not fully understood, however the best guess is that for some metals (such as the noble metals including gold, silver, and platinum) the energy of the flame is insufficient to raise the atoms of that element to a higher energy state, thus it cannot emit its characteritic frequencies of light.

To summarise, not all metals produce a characteristic flame colour, but all metals can emit charactersitc frequencies of visable light (although metals can also produce non-visable light)


yeah, I'm just confused about how the CO2 and H2O will react with the CaCl2 or NaOH, otherwise, the question isn't too hard yes. Is there reason why co2 reacts with naoh and h2o reacts with cacl2?
(this is something I had to research)
CaCl2 readily absorbs water into its crystalline structure. Before the water vapour passes through the CaCl2, it is presumably anhydrous meaning doesn't have associated water molecules. As the water vapour passes through the CaCl2, it is absorbed by the compound and the compound becomes hydrated. We write this as: CaCl2 · nH2O (note that these absorbed water molecules aren't just floating around with the CaCl, they are part of its structure). Not all ionic compounds readily absorb water in this way to form hydrated compounds.

For the reaction of NaOH with CO2, there's really two different reactions occuring that can be combined to give the reaction:
NaOH + CO2 --> NaHCO3
These two reactions are:
the reaction of carbon dioxide with water to form carbonic acid:
CO2 + H2O --> H2CO3
and the reaction of the carbonic acid with the sodium hydroxide to form sodium bicarbonate and water:
H2CO3 + NaOH --> NaHCO3 + H2O

By combining these two seperate reactions and calcelling out the water apearing on both sides of the reaction, you get the overall equation mentioned earlier.

Hopefully this helps with understanding the reactions taking place - although I don't think you need to worry about memorising them as I think its a bit beyond VCE. That being said, knowing about hydrated compounds is helpfull as they do apear in the course (don't think you need to know specific examples of them though).
« Last Edit: January 11, 2020, 10:51:41 am by Erutepa »
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jnlfs2010

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Re: VCE Chemistry Question Thread
« Reply #8420 on: January 12, 2020, 08:28:22 pm »
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For the exam, how much in detail do we need to know about optical isomers?

Like do we need to know in detail about the different interaction with plane-polarized light? Or we just need to know optical isomers are a type of stereoisomerism where the atoms are connected in the same way but are arranged differently in 3D?
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sweetcheeks

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Re: VCE Chemistry Question Thread
« Reply #8421 on: January 12, 2020, 08:32:25 pm »
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For the exam, how much in detail do we need to know about optical isomers?

Like do we need to know in detail about the different interaction with plane-polarized light? Or we just need to know optical isomers are a type of stereoisomerism where the atoms are connected in the same way but are arranged differently in 3D?

I believe that you just need to know that they are a type of stereoisomer and how to identify chiral centres of optical isomers. You should also be aware of optical isomers interactions with enzymes (most enzymes will be specific for one type of optical isomer)

jnlfs2010

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Re: VCE Chemistry Question Thread
« Reply #8422 on: January 12, 2020, 08:45:43 pm »
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I believe that you just need to know that they are a type of stereoisomer and how to identify chiral centres of optical isomers. You should also be aware of optical isomers interactions with enzymes (most enzymes will be specific for one type of optical isomer)

Thanks for the help!!
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Re: VCE Chemistry Question Thread
« Reply #8423 on: January 13, 2020, 02:49:05 pm »
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Hi
For combustion reactions, is the water product written in the gas or liquid state? Different reactions have different states. Does it matter, or is there a specific ruling by vcaa?

Sine

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Re: VCE Chemistry Question Thread
« Reply #8424 on: January 13, 2020, 03:02:52 pm »
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Hi
For combustion reactions, is the water product written in the gas or liquid state? Different reactions have different states. Does it matter, or is there a specific ruling by vcaa?
Good question!

Generally, the enthalpies of combustion are given at room temperature (25C). So enthalpy changes given in the databook assume liquid water. However, in the past VCAA has actually given the answer as gas (g) - so they have been "technically" incorrect. Although the assessors report doesn't always show every suitable answer.

Unless the question clearly shows what the actual product is it doesn't matter too much and VCAA should accept either.

jnlfs2010

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Re: VCE Chemistry Question Thread
« Reply #8425 on: January 16, 2020, 10:45:40 pm »
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Why are some biofuels such as ethanol and biodiesel have a lower energy content than their fossil fuel counterparts petrodiesel and petrol?
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Re: VCE Chemistry Question Thread
« Reply #8426 on: January 17, 2020, 08:21:51 am »
+5
Why are some biofuels such as ethanol and biodiesel have a lower energy content than their fossil fuel counterparts petrodiesel and petrol?

This is because ethanol and biodiesel contain polar functional groups, which attract water molecules. Esters are found in biodiesels, and due to the two oxygen atoms, this will be able to form hydrogen bonds with nearby water molecules. The same thing goes for ethanol molecules, due to the highly polar OH group.

In turn, the presence of H2O molecules in the fuel is going to affect the efficiency of combustion, because some energy will be directed towards the water molecule instead of the fuel.
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jnlfs2010

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Re: VCE Chemistry Question Thread
« Reply #8427 on: January 19, 2020, 12:31:21 pm »
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Why does biodiesel have a lower cloudpoint than biodiesel? Appreciate any help received!
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Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8428 on: January 19, 2020, 04:21:04 pm »
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Why does biodiesel have a lower cloudpoint than biodiesel? Appreciate any help received!
The cloud point is the temperature at which crystilisation of the fuel begins to occur. In other words, as a fuel gets colder, at some point crystals will begin to form - this point is called the cloud point.
This crystilisation of the fuel lowers its performance and high cloud points are considered a disadvantage for fuels as this means the fuel will begin to crystalise a higher temp (i.e. 5 degrees celcius rather than -10 for a fuel with a lower cloud point).

biodiesel actually has a higher cloud point, meaning it will crystalise at a high temperature than regular diesel - something which is particulalrly a disadvantage when the fuels are being used in colder climates as biodisel may posess lower performance as it may crystalise. Its higher cloud point is due to the stronger intermolecular attraction between biodiesel molecules compared to diesel molecules. Diesel consists only hydrocarbons, thus will only posess dispersion forces as an intermolecular form of attraction, while biodiesel is composed of fatty acid esters which are typically longer than diesel molecules, thus will posessstonger dispersion forces, and will additionally posess dipole dipole attractions due to the electronegative ester group present. As such, since biodiesel has stronger intermolecular attractions than regular diesel, it will crystalise more readily, reflected in its higher cloud point.

Hope this helps  :)
« Last Edit: January 19, 2020, 04:25:13 pm by Erutepa »
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jnlfs2010

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Re: VCE Chemistry Question Thread
« Reply #8429 on: January 21, 2020, 02:25:19 pm »
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Will increasing the temperature in HPLC decrease the retention time and increase degree of separation and why?



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2019: Accounting [45]
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