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Author Topic: VCE Chemistry Question Thread  (Read 2313231 times)  Share 

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tay75

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Re: VCE Chemistry Question Thread
« Reply #8370 on: December 19, 2019, 09:19:43 am »
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Hi tay 75!
Welcome to the forums!

I am fairly sure that for VCE we can assume redox reactions like these are taking place in water (unless otherwise stated).
Some Ionic compounds are water soluble (as the hydrogen bonds the ions form with water are stronger than the ionic bond between the ions). These water-soluble ionic compounds will disassociate (break apart) into their constituent ions when in water.
For question 1, FeI2 is a water soluble compound, meaning you can treat it as an Iron (II) cation (Fe2+) and Iodide anion (I-). As such you can think of the equation as being: Fe + I2 -> Fe2+ + 2I- since FeI2 essentially breaks up and exists as separate Fe2+ and I- ions. This means its more appropriate to sau the redox conjugate pairs are Fe/Fe2+ and I2/I-
The compounds in example 2 are also water-soluble, so the same logic applies.

However, some ionic compounds are water insoluble. These will not disassociate into constituent ions and thus when writing the conjugate redox pairs, it will be most appropriate to note the whole insoluble compound

the last two questions are dealing with covalently bonded compounds/ions. Polyatomic ions such as MnO4- contain covalent bonds (bonding the oxygen to the Manganese). These bonds are not broken apart in water and as such these covalent compounds/ polyatomic ions do not disassociate themselves, thus you must write the whole MnO4- ion in the redox conjugate pairs. The same is the case for the last question.
As a side note: If MnO4- was bonded to a potassium as KMnO4, this would be a water soluble ionic compounds that would disassociate in water to give the two constituent ions: K+ and MnO4-, however MnO4- does not itself break apart in water as its atoms are covalently bonded.

Hopefully this explanation helps you understand and hopefully I didn't get anything wrong (feel free to ask/correct me if you think I have made a mistake or if there's something that you still don't understand)

That explanation was very comprehensive but still concise, I pretty much understand all of it now. Thank you so much!

Just a quick question on the solubility of ionic compounds; I remember in Unit 1/2 we determined solubility using the solubility table. Obviously we don't have that in the data book any longer, but I remember my teacher saying that there's another way to determine it in 3/4. Will it take any memorising or is there a method to it?

Thanks again for your time and warm welcome.

ArtyDreams

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Re: VCE Chemistry Question Thread
« Reply #8371 on: December 19, 2019, 09:49:33 am »
+2
That explanation was very comprehensive but still concise, I pretty much understand all of it now. Thank you so much!

Just a quick question on the solubility of ionic compounds; I remember in Unit 1/2 we determined solubility using the solubility table. Obviously we don't have that in the data book any longer, but I remember my teacher saying that there's another way to determine it in 3/4. Will it take any memorising or is there a method to it?

Thanks again for your time and warm welcome.

Yep - you will have to memorise it for Year 12. I'm sure there are a few different ways of memorising it (its worth checking this out on google) , my teacher taught us something called 'PANES' & 'CHOPS' that helps determine the solubility of compounds. I'd write what it means, but I really dont remeber ha ha. I think one of those words gives soluble and the other gives exceptions or something else like that.

Hope this helps!

tay75

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Re: VCE Chemistry Question Thread
« Reply #8372 on: December 19, 2019, 01:52:09 pm »
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Yep - you will have to memorise it for Year 12. I'm sure there are a few different ways of memorising it (its worth checking this out on google) , my teacher taught us something called 'PANES' & 'CHOPS' that helps determine the solubility of compounds. I'd write what it means, but I really dont remeber ha ha. I think one of those words gives soluble and the other gives exceptions or something else like that.

Hope this helps!

Yup, this looks like what I'm looking for. People have already made quizlets for it so lucky me!

Cheers ArtyDreams

Bri MT

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Re: VCE Chemistry Question Thread
« Reply #8373 on: December 19, 2019, 04:53:43 pm »
+3
Yep - you will have to memorise it for Year 12. I'm sure there are a few different ways of memorising it (its worth checking this out on google) , my teacher taught us something called 'PANES' & 'CHOPS' that helps determine the solubility of compounds. I'd write what it means, but I really dont remeber ha ha. I think one of those words gives soluble and the other gives exceptions or something else like that.

Hope this helps!

Small note: at my school we were taught SNAPE (as in the fictional potions teacher) which I personally think is much more memorable than PANES

tay75

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Re: VCE Chemistry Question Thread
« Reply #8374 on: December 19, 2019, 08:05:41 pm »
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Small note: at my school we were taught SNAPE (as in the fictional potions teacher) which I personally think is much more memorable than PANES

Yeah our school uses SNAPE as well. I will use that instead and maybe add CHOPS to my arsenal too. Thanks for the tip.

redleafbun

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Re: VCE Chemistry Question Thread
« Reply #8375 on: December 19, 2019, 08:32:12 pm »
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When you look at the species in the equations crossed out, you will see that you lack the reactants present for it to be used; there is no oxygen gas or hydrogen peroxide as reactants given that the system is closed, thus you cannot use them!
To break down what Bri mentioned earlier about how the strongest reductand will react with the strongest oxidant present:
Oxidants are shown on the left hand side of the equations (they are reduced and gain electrons) and the strongest oxidant is the one closest to the top - In this case this is Nickle 2+ ions. Reductants are shown on the right (they are oxidised and gain electyrons) and the strongest reductant is the onle closest to the bottom - In this case solid copper. As such, we know that nickle ions will react with copper metal.
help visualising the increasing reductand and oxidant strengths
The oxidisation of water (the top equation crossed out) does not occur becuase theres is no hydrogen peroxide to be reduced (if hydrogen peroxide was present this reaction would occur) and becuase water is a weaker reductant than copper metal (if all the copper was used up then this reaction would  occur). The other crossed out equation doesn't occur since oxygen gas is not present (for this to be the case we would have to be told that ocygen gas is being bubled into the solution), and since hydroxide ions are not present (and even if they were they are a weaker reductant than the copper metal)

Hopefully this helps clarify some things, but let me know if this is still confusing. Understanding electrochemical reactions can be quite tricky!

Ahhhh I understand now! Thank you guys for those explanations ;D

IThinkIFailed

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Re: VCE Chemistry Question Thread
« Reply #8376 on: December 20, 2019, 04:11:06 pm »
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Does LPG and petroleum gas have Sulfur and nitrogen in it? Can’t seem to find a solid answer on google...
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sweetcheeks

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Re: VCE Chemistry Question Thread
« Reply #8377 on: December 20, 2019, 04:26:34 pm »
+4
Does LPG and petroleum gas have Sulfur and nitrogen in it? Can’t seem to find a solid answer on google...

LPG and petroleum gases are composed mostly of small hydrocarbons such as propane and butane. They may contain trace levels of sulfur and nitrogen due to the presence of impurities. Sulfur containing compounds such as ethanethiol (CH3CH2SH) are also added so that leaks can be detected (we can't smell hydrocarbons but we can smell many sulfur containing compounds at extremely low levels.

IThinkIFailed

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Re: VCE Chemistry Question Thread
« Reply #8378 on: December 20, 2019, 08:34:57 pm »
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LPG and petroleum gases are composed mostly of small hydrocarbons such as propane and butane. They may contain trace levels of sulfur and nitrogen due to the presence of impurities. Sulfur containing compounds such as ethanethiol (CH3CH2SH) are also added so that leaks can be detected (we can't smell hydrocarbons but we can smell many sulfur containing compounds at extremely low levels.
Thank you!

Do we need to memorise the energy content of all the different fuels?
« Last Edit: December 20, 2019, 08:36:40 pm by IThinkIFailed »
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Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8379 on: December 20, 2019, 09:03:03 pm »
+4
Thank you!

Do we need to memorise the energy content of all the different fuels?
To quote the study design,
You need to know "the comparison of fossil fuels (coal, crude oil, petroleum gas, coal seam gas) and biofuels (biogas, bioethanol,
biodiesel) with reference to energy content".
You won't need to memorise different energy content values for different fuels, but you will need to be able to compare them (I.e. Diesel has greater energy content then biodiesel)
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jnlfs2010

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Re: VCE Chemistry Question Thread
« Reply #8380 on: December 25, 2019, 05:51:51 pm »
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The textbook says "As the temperature of the engine increases, the viscosity of the engine oil decreases due to the disruption of the intermolecular forces."

Why is this the case? I'm a little confuzzled. Thanks for any help
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IThinkIFailed

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Re: VCE Chemistry Question Thread
« Reply #8381 on: December 25, 2019, 06:16:39 pm »
+3
The textbook says "As the temperature of the engine increases, the viscosity of the engine oil decreases due to the disruption of the intermolecular forces."

Why is this the case? I'm a little confuzzled. Thanks for any help

My Chem knowledge is very weak, but I’m gonna have a go at this

When there is higher temperatures, intermolecular bonds will likely begin to break, and as a result the atoms of the engine oil will be able to move more freely, and as a result, become more inclined to flow, reducing viscosity.



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jnlfs2010

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Re: VCE Chemistry Question Thread
« Reply #8382 on: December 25, 2019, 06:32:44 pm »
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My Chem knowledge is very weak, but I’m gonna have a go at this

When there is higher temperatures, intermolecular bonds will likely begin to break, and as a result the atoms of the engine oil will be able to move more freely, and as a result, become more inclined to flow, reducing viscosity.







THanks for the help. Whoops I think I had a mind blank.
« Last Edit: December 25, 2019, 06:36:48 pm by jnlfs2010 »
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Erutepa

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Re: VCE Chemistry Question Thread
« Reply #8383 on: December 25, 2019, 08:26:15 pm »
+4


My Chem knowledge is very weak, but I’m gonna have a go at this

When there is higher temperatures, intermolecular bonds will likely begin to break, and as a result the atoms of the engine oil will be able to move more freely, and as a result, become more inclined to flow, reducing viscosity.
Chemistry on Christmas!
I sure like the sound of this!

I think theory is good  :D - however there are a couple little bits that I think can be improved.
Firstly, Its more appropriate to say 'intermolecular forces of attraction' or just 'intermolecular forces' (the latter is what the study design uses) instead of 'intermolecular bonds'. This is because these inter molecular forces of attraction are much weaker and much more readily broken then chemical bonds which are lasting attractions that hold atoms together to form molecules. Despite its name, hydrogen bonding is really an inter molecular force of attraction rather than a true chemical bond due to its relatively weak force of attraction.
Also, when referring to the decreasing viscosity of the engine oil, it would be better to say that the molecules of the engine oil will be able to more freely move, rather than saying the atoms will be able to more freely move. While the atoms will have a greater kinetic energy and will be vibrating to a greater extent, however the decrease in viscosity is more so due to the greater ability of the molecules of oil to move past each other.

Other than these (perhaps nit-picky) details, your response is really good!
« Last Edit: December 25, 2019, 09:04:26 pm by Erutepa »
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IThinkIFailed

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Re: VCE Chemistry Question Thread
« Reply #8384 on: December 26, 2019, 01:07:51 pm »
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Biodiesel by itself is not classified as a fatty acid right?
Rather it’s made up of long ester molecules which are classified as fatty acids?
I’m a bit stumped about this...
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