VCE Chemistry Question Thread

[Seno72]

According to the VCAA:From a quick read of the study design, I see concepts mentioned in this particular AOS that I have been taught throughout organic chemistry in both VCE and uni, so I'd assume some organic chemistry knowledge is needed.

Thanks Insanipi. I know some stuff about Organic so I guess I'll have an advantage over others in my class.

[apa29]

Could someone please help me with this question?

[f0od]

i need some help on these questions – if anyone could help me, that'd be much appreciated! thank youu

1) A copper pan of mass 750g at a temperature of 25ºC is placed on a stovetop and heated with the addition of 12.5 kJ of heat energy. Given that the heat capacity of copper is 0.39Jg-1ºC-1, calculate the final temperature of the pan.

2) What is the minimum temperature required to dissolve 40 g of AgNO3 in 25 g of water?

[Jim_Bob]

So the generic equation to find latent heat is q=c x m x deltaT

In this case ur given q = 12500(12.5kJ into J)
c = 0.39
m = 750

rearrange the equation to have deltaT as ur subject:
DeltaT = q/c x m

substitute those variables in = 42.7
The trick here is you have to add the 42.7 to the original heat temperature of 25
So 42.7 +25 = 67.7 degrees or 68 rounded up

[f0od]

So the generic equation to find latent heat is q=c x m x deltaT

In this case ur given q = 12500(12.5kJ into J)
c = 0.39
m = 750

rearrange the equation to have deltaT as ur subject:
DeltaT = q/c x m

substitute those variables in = 42.7
The trick here is you have to add the 42.7 to the original heat temperature of 25
So 42.7 +25 = 67.7 degrees or 68 rounded up

Thank you so much!!!

[Rowena1307]

Hi all,
I have been working through the key knowledge points from the current study design(2017-2021) and I'm a little confused about the wording if one of the points. The point is 'the comparison of the energy transformations occurring in spontaneous exothermic redox reactions involving direct contact between reactants (transformation of chemical energy to heat energy) compared with those occurring when the reactants are separated in galvanic cells (transformation of chemical energy to electrical energy).' Is this talking about comparing galvanic cells to combustion reactions?
Thanks 

[sweetcheeks]

Hi all,
I have been working through the key knowledge points from the current study design(2017-2021) and I'm a little confused about the wording if one of the points. The point is 'the comparison of the energy transformations occurring in spontaneous exothermic redox reactions involving direct contact between reactants (transformation of chemical energy to heat energy) compared with those occurring when the reactants are separated in galvanic cells (transformation of chemical energy to electrical energy).' Is this talking about comparing galvanic cells to combustion reactions?
Thanks 

I've always thought of it about how we can use a galvanic cell to harness electrical energy from a redox reaction (like a copper zinc cell) vs. simply placing a block of zinc in a copper (ii) sulfate solution (we can't harness the electrical energy, you will still get heat energy). Combustion of methanol vs. a methanol fuel cell is another example (heat vs. electrical energy).

Its not necessarily comparing two different reactions, but comparing the same reaction under different conditions/setups.

[OHASI]

Hi,
I'm new so haven't worked well if this is the place to ask.
But I have a general question about IR spectroscopy. My Chem teacher said that the fingerprint region in an IR spectroscopy is unique to each molecule. If that is so, why do we need to analyse IR spectroscopy (-after 1400 cm) and find functional groups?

[sweetcheeks]

Hi,
I'm new so haven't worked well if this is the place to ask.
But I have a general question about IR spectroscopy. My Chem teacher said that the fingerprint region in an IR spectroscopy is unique to each molecule. If that is so, why do we need to analyse IR spectroscopy (-after 1400 cm) and find functional groups?

The fingerprint region, being unique to a molecule, means that you would readily need to have available all the IR spectrums of the potential molecules (if analysing an unknown). By analysing after 1400 you can quickly identify what the molecule might be and what it might not be by identifying the functional groups. If you had to identify a molecule that could be one of many isomers, it would be extremely time-consuming to pull up all the potential molecules IR spectrums and try to match the fingerprint region (there is also no guarantee that there would be an exact match, unless the same equipment under the same conditions was used for both).

[Mattjbr2]

Hey guys.
I don't understand why butan-2-one has a higher boiling point than methyl ethanoate. It goes against what I've learnt from the textbook.
As you can see from the photo, the only physical difference between them is the part in yellow. In butan-2-one, a methyl group is replaced with an oxygen atom.
But why does this lower the boiling point? Shouldn't this increase the boiling point? Isn't there a hydrogen bond between the oxygen atom and water molecules, resulting in a higher BP?

[VanillaRice]

But why does this lower the boiling point? Shouldn't this increase the boiling point? Isn't there a hydrogen bond between the oxygen atom and water molecules, resulting in a higher BP?

Boiling point is related to the energy required to separate the molecules of that substance (i.e. two methyl ethanoate molecules) from each other, rather than from water molecules. Therefore, if we consider only methyl ethanoate, it cannot form hydrogen bonds with another methyl ethanoate molecule.

Hey guys.
I don't understand why butan-2-one has a higher boiling point than methyl ethanoate. It goes against what I've learnt from the textbook.
As you can see from the photo, the only physical difference between them is the part in yellow. In butan-2-one, a methyl group is replaced with an oxygen atom.

When comparing boiling points, we should consider the intermolecular forces, like you have done. The strongest intermolecular force in both cases is dipole-dipole interactions. In the case of butan-2-one, there is only one oxygen atom, compared to the two in methyl ethanoate. Therefore, the C=O dipole in butan-2-one is more distinguished, compared to methyl ethanoate, where there is another (highly electronegative) oxygen atom pulling electrons in a different direction. Since the dipole in butan-2-one is stronger (i.e. it is more polar), then it has a higher boiling point.

Hope that makes sense

[Mattjbr2]

Boiling point is related to the energy required to separate the molecules of that substance (i.e. two methyl ethanoate molecules) from each other, rather than from water molecules. Therefore, if we consider only methyl ethanoate, it cannot form hydrogen bonds with another methyl ethanoate molecule.
When comparing boiling points, we should consider the intermolecular forces, like you have done. The strongest intermolecular force in both cases is dipole-dipole interactions. In the case of butan-2-one, there is only one oxygen atom, compared to the two in methyl ethanoate. Therefore, the C=O dipole in butan-2-one is more distinguished, compared to methyl ethanoate, where there is another (highly electronegative) oxygen atom pulling electrons in a different direction. Since the dipole in butan-2-one is stronger (i.e. it is more polar), then it has a higher boiling point.

Hope that makes sense

Thank you!!

[Rowena1307]

I've always thought of it about how we can use a galvanic cell to harness electrical energy from a redox reaction (like a copper zinc cell) vs. simply placing a block of zinc in a copper (ii) sulfate solution (we can't harness the electrical energy, you will still get heat energy). Combustion of methanol vs. a methanol fuel cell is another example (heat vs. electrical energy).

Its not necessarily comparing two different reactions, but comparing the same reaction under different conditions/setups.

Thank you! 

[-_-zzz]

Hey guys,

Just wondering if I could get some help for part c and d of this question.

Thanks!

[Yertle the Turtle]

Hey guys,

Just wondering if I could get some help for part c and d of this question.

Thanks!

Hey,
For part (c) you use the n=m/M formula to discover the number of moles of the solute in the crystals, which you have just found the mass of. Then you use c=n/V, with n being the number of moles of the solute, found above, while V is 0.250L.

For part (d) you would find the number of moles of the solute in 50mL of the solution from part (c) using n=cV, and then find the molar concentration of the new solution using c=n/V again. Then you use the molar mass of the solute to convert between mol/L and g/L, given that m=nM, and I'll leave that up to you.

Hope this helps!