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March 28, 2024, 09:48:11 pm

Author Topic: HSC Chemistry Question Thread  (Read 1040530 times)  Share 

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anotherworld2b

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Re: Chemistry Question Thread
« Reply #585 on: August 04, 2016, 10:29:45 pm »
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The question asks for the maximum amount. This makes me question as to whether they mean maximum amount so that the atoms are "happy", or the actual maximum amount.

Why do I say this? Oxygen technically only needs (and ever wants) 2 covalent bonds to be happy. Sulfur, on the other hand, can take 2, 4 or even 6.

I feel that this was a badly worded question due to its lack of clarity.

For part c wouldnt N have a valence of 5 eletrons? So it needs 3 more electrons? But the answer says 5?

RuiAce

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Re: Chemistry Question Thread
« Reply #586 on: August 04, 2016, 10:31:42 pm »
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For part c wouldnt N have a valence of 5 eletrons? So it needs 3 more electrons? But the answer says 5?
Just as I suspected.

They want to know what is the maximum amount of covalent bonds that can form. That is always the amount of electrons in the valence shell.

For nitrogen, this is 5.

(In reality will it form 5? Never. But that's not the point of the question.)

anotherworld2b

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Re: Chemistry Question Thread
« Reply #587 on: August 04, 2016, 10:35:55 pm »
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Just as I suspected.

They want to know what is the maximum amount of covalent bonds that can form. That is always the amount of electrons in the valence shell.

For nitrogen, this is 5.

(In reality will it form 5? Never. But that's not the point of the question.)

Oh okay  ;D
Thank you i think it get it now  :D

anotherworld2b

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Re: Chemistry Question Thread
« Reply #588 on: August 04, 2016, 10:51:42 pm »
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I struggle with drawing lewis structures and was wondering if i could get some tips kn how to draw them
An example im struggling with is
SO4 ^2-

Jakeybaby

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Re: Chemistry Question Thread
« Reply #589 on: August 04, 2016, 11:18:20 pm »
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I struggle with drawing lewis structures and was wondering if i could get some tips kn how to draw them
An example im struggling with is
SO4 ^2-
Initially, we will count the number of valence electrons:

We know that Sulfur has 6, as does Oxygen.
Therefore:
Number of valence electrons = Valence Electrons(sulfur) + 4*Valence Electrons(oxygen) + 2(negative 2 charge shows 2 electrons are present)
Number of valence electrons = 6 + 6(4) + 2
= 32

Drawing the ion:
Spoiler

We need to ensure that the outer atoms have octets, therefore, we need to draw 6 dots around each of the oxygen atoms, and counting the number of valence electrons present for each atom: it can be seen that all the oxygens have 8, as does the centre sulfur atom.
Spoiler

But hang on, we're not done:
The formal charges need to be looked at:
Formal Charge = Valence Electrons - Non-Bonding Valence Electrons - Bonding Electrons/2

Therefore:
Formal ChargeOxygen = 6 - 6 - 2/2 = -1
Formal ChargeSulfur = 6 - 0 - 8/2 =  +2

We want the formal charges to be as close to 0 as possible on each atom, hence, to do this, we can take 2 valence electrons from 2 seperate oxygen atoms, to make some double bonds:
Spoiler
Now calculating the formal charges again:
Formal ChargeSulfur = 6 - 0 - 12/2 =  0
Formal ChargeBlack Oxygen = 6 - 6 - 2/2 =  -1
Formal ChargeBlue Oxygen = 6 - 4 - 4/2 =  0
Therefore, as we cannot get the formal charges any closer to 0, we have the lewis structure of the sulfate ion.

Spoiler
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RuiAce

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Re: Chemistry Question Thread
« Reply #590 on: August 04, 2016, 11:41:33 pm »
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I think the Lewis structure for the sulfate ion is not in the HSC course.

The concept of "formal charges" is not taught in the HSC nor preliminary courses. At most, the octet rule is.

sire123

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Re: Chemistry Question Thread
« Reply #591 on: August 06, 2016, 03:06:01 am »
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Can you guys explain to me why for the bromine water prac, it's either Br2 or HOBr? Can you show both word and structural equations for the reactions that take place with cyclohexene + bromine water, and cyclohexene + bromine water?

RuiAce

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Re: Chemistry Question Thread
« Reply #592 on: August 06, 2016, 08:34:39 am »
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Can you guys explain to me why for the bromine water prac, it's either Br2 or HOBr? Can you show both word and structural equations for the reactions that take place with cyclohexene + bromine water, and cyclohexene + bromine water?
For your first question, it does not matter whether we use Br2 or HOBr. However, for reference sake, this is what actually happens.

Generally speaking, rather than using liquid bromine we mix the bromine into the water. A reaction takes place:
Br2(l) + H2O(l) -> HBr(aq) + HOBr(aq)

Note that the hydrobromic acid does nothing.

(Not too sure but I think bromine water is preferred over actual bromine because it oxidises more easily.)
____________________

To understand the reactions that take place you need to consider what exactly are substitution and addition reactions. Substitution reactions are exactly that - certain atoms effectively displace other atoms. Addition reactions are when bonds are broken and the atoms form new bonds.

So say you went from ethane to ethanol through reaction with water. Then two of the hydrogens are going to have to be substituted for the H and OH from water.
If you went form ethene to ethanol though, the double bond breaks and the water molecule just adds on. The H goes to one carbon whereas the OH goes to the other.

Now, cyclohexene has formula C6H10. We can't really assign it a number as to where the double bond is (unlike say, 1-hexene), but we know that there is a double bond there. It has a high electron density and is very reactive, so when the HOBr is introduced it will readily add on.

(Note that HOBr is what has the brown or whatever colour. The bromine water is what gets decolourised, not the cyclohexene.)

C6H10(l) + HOBr(aq) -> C6H10BrOH(l)

On the contrary, cyclohexane only contains single bonds. For a substitution reaction to take place on top of any of those single bonds we need to provide much more energy. In fact, this can only be done so through adding UV radiation.

C6H12(l) + HOBr(aq) -(UV)-> C6H10BrOH(l) + H2(g)

Under laboratory conditions, you're not going to find that much UV at all. So you will see the cyclohexene decolourise it, and not the cyclohexane. That is all.

Structural diagrams can be found at EasyChem

sire123

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Re: Chemistry Question Thread
« Reply #593 on: August 06, 2016, 10:14:51 am »
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Wow cheers Rui! I have a couple more questions.

Well first one is a 6 marker "Discuss the process of fermenting glucose into ethanol. Include a chemical equation and a chemical structure of ethanol." How would you tackle and structure this q? Is it asking for the prac, and all the conditions needed to make this work...

And what is the difference between Biopol, polydroxybutanoate, and polyhydroxybutyrate? Are they all the same?

jakesilove

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Re: Chemistry Question Thread
« Reply #594 on: August 06, 2016, 01:14:53 pm »
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Wow cheers Rui! I have a couple more questions.

Well first one is a 6 marker "Discuss the process of fermenting glucose into ethanol. Include a chemical equation and a chemical structure of ethanol." How would you tackle and structure this q? Is it asking for the prac, and all the conditions needed to make this work...

And what is the difference between Biopol, polydroxybutanoate, and polyhydroxybutyrate? Are they all the same?

Hey Sire!

Let's start with that 6 marker: "Discuss the process of fermenting glucose into ethanol. Include a chemical equation and a chemical structure of ethanol."

Okay, clearly we need a chemical equation and the chemical structure of ethanol. I would probably smash that out right at the start, so that you don't forget!


Make sure to include Yeast as a catalyst as well! As for the chemical structure of Ethanol:


Easy! Okay, let's try to answer the actual question. We don't really need to be talking about the prac specifically (ie. you don't need to write a method). Instead, I would be listing as much information as possible. I would be writing something like this.

The fermentation of Glucose to Ethanol can be described chemically by the equation given above. However, to achieve the greatest yield of Ethanol, certain conditions must be satisfied.

- Enzymes to convert the Glucose to Ethanol (specifically, Yeast)
- Moderate temperatures (about 37 degrees Celsius)
- A mixture of Glucose in solution
- Anaerobic conditions (ie. low oxygen)
- An Ethanol concentration <15% (or else the Yeast will die)


Great! So we've identified a whole bunch of important factors, we've explained how the process works, we're nearly done! Finally, I would talk about the fact that if we want to USE the ethanol, we need to use multiple iterations of fractional distillation. This is because the Ethanol/Glucose solution will only have a concentration of about 15%. As such, we need to purify it! This allows it to be used.

That should be enough to satisfy the 6 marks! You can always add some extra info, but I don't think you need more than that :)

As for your Biopol/polydroxybutanoate/polyhydroxybutyrate conundrum, basically Biopol is the brand name of PHB (like Panadol is the brand name, but Paracetamol is the drug in it!). I don't think that polydroxybutanoate exists, but polyhydroxybutyrate is the full chemical name of PHB (I would use this last term).

Let me know if you need any more help! Great questions :)

Jake
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jakesilove

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Re: Chemistry Question Thread
« Reply #595 on: August 06, 2016, 07:23:57 pm »
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I'm just posting a couple of questions for another user; feel free to write up some solutions, otherwise I'll get to an answer shortly!






Jake
« Last Edit: August 06, 2016, 07:25:33 pm by jakesilove »
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jakesilove

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Re: Chemistry Question Thread
« Reply #596 on: August 06, 2016, 07:31:49 pm »
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I'm just posting a couple of questions for another user; feel free to write up some solutions, otherwise I'll get to an answer shortly!

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Jake

For the first question, you need to start by stating Le Chatelier's Principle. Something along the lines of "When a system undergoes some kind of change, the equilibrium will shift to try to minimise that change". Then, identify that the equilibrium is exothermic (as delta H is negative). As such, an increase in temperature will shift the equilibrium to the left (towards the reactants). Elaborate on this a little, and this will explain the results observed!
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jakesilove

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Re: Chemistry Question Thread
« Reply #597 on: August 06, 2016, 07:35:03 pm »
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I'm just posting a couple of questions for another user; feel free to write up some solutions, otherwise I'll get to an answer shortly!

(Image removed from quote.)
(Image removed from quote.)
(Image removed from quote.)
(Image removed from quote.)

Jake

The third question is to do with Hydrogen bonds and dipole-dipole interaction. Identify that Ethane/Butane etc. will only have dispersion inter-molecular forces. These are the weakest inter molecular forces. However, the identified compounds will have dipole-dipole interactions (as the hydroxide groups are electronegative) and hydrogen bonding (between hydrogen and Oxygen). Dipole-dipole interactions are stronger than dispersion, and Hydrogen bonding is stronger still. Therefore, as the intermolecular bonding is much stronger, the boiling point will be much higher (more energy required to break the bonds). This explains the boiling points!
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jakesilove

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Re: Chemistry Question Thread
« Reply #598 on: August 06, 2016, 07:38:55 pm »
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I'm just posting a couple of questions for another user; feel free to write up some solutions, otherwise I'll get to an answer shortly!

(Image removed from quote.)
(Image removed from quote.)
(Image removed from quote.)
(Image removed from quote.)

Jake

For the final question, let's break down the two marks into 1) Improving validity and 2) Improving reliability.

Validity

This method does not take into account the fact that water is evaporating as the carbon dioxide is being released. A more valid experiment would be filling a similar can with water, and weighing the water evaporation over time simultaneously. Then, the water evaporation can be subtracted from the changes over time, and the carbon dioxide can be isolated.

Reliability

This one is easy; just do it multiple times! Potentially, take more measurements as well. Nothing too tricky here
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sire123

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Re: Chemistry Question Thread
« Reply #599 on: August 06, 2016, 08:09:49 pm »
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Can you do the second image as well? And continuing that q it says "Justify the conditions used during he Haber synthesis of ammonia - High temperature (reactor) and High pressure". But isn't 400 degrees celsius and 200 atm not that high?