some more qs (1 Viewer)

[Sirius Black]

Q1 Which of the following is not soluble in ethanol?
a) water
b)ammonia
c)ethylene
d) CO
I chose (d) coz ethanol is supposed to be able to dissolve all polar substances and some organic liquid right? but the answer is (c) can any1 explain this?

Q2. Which one of the following is a physical property of ethanol?
a)polarity
b)soluble in choloroform
c)react with Na to produce hydrogen
d)lower melting pts than alkanes
Well, choices (c) & (d) are obviously wrong and the answer is (a)-but for (b) isnt this due to the polarity of ethanol thus it can dissolve in choloform? why isn't it a physical prop?

thanx

 

[*ashlea*]

for your second question, i think that the only correct answer is a) polarity because solubility in one specific substance is not counted as a physical property, as then there'd b way 2 many properties to account for.. whereas polarity is a clear cut physical property, its either polar or its not- end of story.

 

[Sirius Black]

OKAY THANX i got it now-so basically properites hav to be generic right?
what about Q1 ?

 

[funking_you]

Q1). All of the others (except c) are polar molecules. Ethylene or simple ethene is a non-polar molecule. Polar substances dissolve only other polar substances, polar substances cannot dissolve non-polar substances.

Q2). Melting and boiling points are both physical properties.
Solubility is not the most obvious properties to classify, however, the do believe your teacher expected that you should known that bp & mp are physical properties.
What is a physical property? it is a property that is a consequence of the types of atoms and their physical arrangement in space.
A chemical property, is one that is a consequence of a chemical reaction.



G

 

[Sirius Black]

Q1). All of the others (except c) are polar molecules. Ethylene or simple ethene is a non-polar molecule. Polar substances dissolve only other polar substances, polar substances cannot dissolve non-polar substances.

This would make sense to me if i am still in yr 11 but I think ethanol is a special case coz in syllabus we actually need to discuss its polar -OH end and non-polar ethyl end which makes ethanol a good solvent for both polar and non-polar substances

 

[funking_you]

Hi again,

Well, the issue you have brought up is a very good one.

Simply put,
Ethanol is a polar molecule, due to the presence of the OH group. It is able dissolve
with other polar substances, including water.
This is easily understood, as all that it requires is an understanding of whether a molecule is polar or not, and you need to recall the 'rule' that like dissolves like, i.e. polar substances dissolve polar substances, and vice versa.

These are concepts that are developed from year 11, hence it is resonable to assume a year 12 student can recall and learn this.

However the syllabus gets it wrong when it asks students to "describe and account for the many uses of ethanol as a solvent for polar and non-polar substances"
Why? because it requires a much greater understand and appreciation of the solubility process, which is beyond that which the syllabus teaches.
Also, this 'dual' role of ethanol as a solvent is quite limited in definition...

Here is a real explaination of its due role as a solvent, which will teach you in what instances it will dissolve what type of substances.

ChemCoach Answers:

• The hydroxyl group (-OH) generally makes the alcohol molecule polar. Those groups can form hydrogen bonds to one another and to other compounds.

• Two opposing solubility trends in alcohols are: the tendency of the polar OH to promote solubility in water, and of the carbon chain to resist it.

• Thus, methanol, ethanol, and propanol are miscible in water because the hydroxyl group predominates.

• Butanol is moderately soluble because of a balance between the two trends.

• Pentanol and branched butanols are effectively insolble because of the hydrocarbon chain's dominance.

So, in your case, your teacher is simply going with the more simplified definition, in which ethanol will only dissolve other polar substances, and hence why the answer is (c).

I strongly believe that the HSC syllabus must examine the concept of intermolecular forces, so students can trully understand points such as 'explaining trends in bp of hydrocarbons', and explaining the 'role of ethanol as a solvent'
The BoS assumes students will bring an understanding of IMF from year 11 (which , in my experience, rarely happens).


Conclusion: the syllabus is not perfect, but study the above dot-points i gave you, and always go with the simple definition of ethanol as a solvent (will only dissolve polar substances) unless the questions requires explaination of the dual nature, at which point, refer to the dot-points given above.

Best of Luck,
George

 

[Dreamerish*~]

thanks for that chemcoach
my tutor said that too when we did revision yesterday. she said we should talk about the intermolecular forces when describing ethanol as a solvent. and name the intermolecular forces too.

 

[meLoncoLLie]

what our chem teacher gave us was:

ethanol
1. dissolves polar substances because it has a polar end (OH)
2. is miscible with water because it forms both dipole-dipole bonds and hydrogen bonds with water molecules
2. dissolves non-polar substances because its non-polar end (the C-H chain) attracts non-polar molecules by dispersion forces.

is this correct?

 

[lilkatie]

ok i'm understanding now
i was getting confused about how the sylabus said it was polar and non polar

 

[l-mercedes-l]

Sorry, this is unrelated but i have a question

Sorry guys i wasnt sure where to put this;

If was thinking today that if RESPIRATION uses oxygen and the products are carbon dioxide and water does that mean that it is a combustion reaction? Because glucose is kind of like a hydrocarbon?

This is random eh? but does anyone know if this is right?

-Saedie

 

[Sirius Black]

Thanks to ChemCoach

Thanx for ur detailed explaination,ChemCoach!
However, i got new evidence from a website, it says "[CO is]about 7 times more soluble in methanol and ethanol than in water.
So I think this is able to prove the fact that CO DOES hav good solubility in ethanol
Plz see the site: http://www.coheadquarters.com/coproperties.htm

 

[xiao1985]

yes, fundamentally... yet respiration is a much more complicated process whereby the energy of combustion are transferred from chemical energy to heat energy (bi product) and chemical energy in other sorts of chemicals (ATP)... then ATP travels around and releases the stored energy where needed

 

[l-mercedes-l]

thanks

thanks for answering my question... im glad i was sorta kinda right

-saedie