VCE Chemistry Question Thread

[Ancora_Imparo]

Slight correction. HCl is a polar covalent compound. In its pure gaseous form, it is a covalent molecule. When dissolved in water, HCl reacts with water in an acid base reaction to form H3O+ (or H+(aq)) and Cl-. HCl is a strong acid as chloride ion is quite stable (full octet of electrons) so you only get H+ and Cl- in an aqueous solution of HCl.
If dissolved in liquid HF, however, HCl is protonated by the stronger acid (liquid HF is a strong acid unlike aqueous HF) and acts as a base. The ionization of HCl is a result of its reaction with water, not because it is ionic.

NaOH is ionic though so when that dissolves it really does split apart into ions.

Whoops! Don't know what I was thinking when I said that. Message to take home: HCl is covalent, not ionic!

[DJA]

thanks guys.

tsfx states that typical uncertainties in volumetric analysis are:
* 20ml pipette: +/- 0.05ml
* burette: +/- 0.02ml

I thought the pipette was more accurate than the burette? o.o

from what I know that seems correct psy. I have always been taught that 20 ml pipette is +/- 0.05ml and burette is +/- 0.02ml.
Seems legit- please correct me anyone if its wrong

The only thin I know is that the 250mL volumetric flask is the one which is +/- 0.3ml. So far more inaccurate

[Sanguinne]

q60 in checkpoints in chemical analysis

The amount of copper in a solution of copper (II) sulfate  can be determined using atomic absorption spectroscopy. When a blue copper(II) sulfate solution is introduced into an AAS, a green flame is observed.
Consider the following statements.

I) A copper (II) sulfate solution appears blue because it absorbs red light.
II) The metal species undergoes oxidation in the flame.
III) The flame is green due to electron transitions from a higher energy state to lower energy state.
Which of the above statements are true?
I understand why II is not true and why III is true, but I dont understand why I is true. When would it absorb red light and why would it appear blue?
Thanks

[scribble]

it absorbs the light that is complementary to the colour that it appears. that is the colour that is opposite to it on a colour wheel.
the idea is, if we have "white" light, which is a mixture of all colours, and the copper absorbs the red light, then what we see is a mixture of all the colours of light except for red. this appears blue. (or more correctly, cyan)

and heres a picture of a colour wheel:
http://www.d.umn.edu/~mharvey/colorwheel.jpg

[Sanguinne]

it absorbs the light that is complementary to the colour that it appears. that is the colour that is opposite to it on a colour wheel.
the idea is, if we have "white" light, which is a mixture of all colours, and the copper absorbs the red light, then what we see is a mixture of all the colours of light except for red. this appears blue. (or more correctly, cyan)

and heres a picture of a colour wheel:
http://www.d.umn.edu/~mharvey/colorwheel.jpg

that makes sense, thanks.
do i have to know the colour wheel?

[Blondie21]

For short answer Q. 2 part b - http://www.vcaa.vic.edu.au/Documents/exams/chemistry/2012/2012chem1-w.pdf (Sorry I couldn't printscreen the question - it wouldn't fit!)
How do we find the answer and differentiate between the bottom two spots?

 I knew it was one of them but didn't know which !

[Ancora_Imparo]

So using the R_f values from solvent G, we know that alanine is one of the middle two dots (going from left to right), as its R_f value is also in the middle.

Using the R_f values from solvent F, we know that alanine had to have travelled the furthest downwards as it has the lowest R_f value. So, out of the two middle dots we were considering before, alanine would correspond to the lower of those two.

[psyxwar]

"The amount of additional potassium permangenate soln required to reach end point is minute. This excess will notionally increase the calculated amount of iron present but, in practice, the extra has no effect on the calculated amount. Why not?"

I actually have no idea. Only thing that comes to mind is the uncertainties of the burette.

[Blondie21]

So using the R_f values from solvent G, we know that alanine is one of the middle two dots (going from left to right),

How do we know this? Sorry I don't really understand the question. What is happening as it is turned over and why are the dots represented differently? .... It's like on the first one there is one point of origin and on the second one there are 3 .. If that makes sense.. :|

[jgoudie]

This is two stage TLC.  First a chromatogram was ran in direction of solvent G, there two substances with the same Rf value, 0.51, one happens to be alanine the other threonine.  We don't know which is which so we need to seperate these two by using another solvent (solvent F).  The below diagram shows the end result.

You can see the first origin for solvent G is the red line, the origin for solvent F is the green line.  Here we can measure the Rf values for each solvent and thus identify the compounds.

Hope this makes a bit of sense.  In terms of why you get three dots and then four.  It is due to the Alanine and Threonine having a very similar affinity for solvent G (meaning they have the same Rf), where as these two must have a different affinity for solvent F (meaning they have the different Rf).

How do we know this? Sorry I don't really understand the question. What is happening as it is turned over and why are the dots represented differently? .... It's like on the first one there is one point of origin and on the second one there are 3 .. If that makes sense.. :|

[eagles]

Explain why both factors of low temperature and high pressure will cause a real gas to deviate from ideal gas behaviour.

My answer:
That low temperature should decrease the random movement of gas particles, as there is less kinetic energy generated from random collisions, and thereby reduce overall pressure.
When low temperature of a real gas induces high pressure, the real gas deviates from ideal gas behaviour.

Can you help me improve this? 

[lzxnl]

I am confused about this question:

Explain why low temperature and high pressure will cause a real gas to deviate from ideal gas behaviour.

A detailed explanation is appreciated. Cheers!

At low temperatures, gas molecules have less kinetic energy on average, so intermolecular forces have a stronger effect overall on molecular motions. As the intermolecular forces can no longer be neglected, the gases deviate from ideal gas behaviour.

At high pressures (i.e small space, or really high temperatures), gas molecules interact with each other a lot more as the space is relatively small for the amount of energy they have (end up getting closer to each other). Again, intermolecular forces have a greater effect overall.

Or, you can think about it this way. Reducing the temperature and pressurising a gas are the only ways of changing the state of a gas (unless you want to consider supercritical fluids and plasmas as different states of matter...but let's not go there). This implies that in reality, intermolecular forces must be able to hold a gas together if we keep reducing the temperature or pressurise the gas.

Explain why both factors of low temperature and high pressure will cause a real gas to deviate from ideal gas behaviour.

My answer:
That low temperature should decrease the random movement of gas particles, as there is less kinetic energy generated from random collisions, and thereby reduce overall pressure.
When low temperature of a real gas induces high pressure, the real gas deviates from ideal gas behaviour.

Can you help me improve this? 

(just because this is the updated version of the quote)

Low temperatures do not induce high pressure. Just saying. You've said that yourself, so you've contradicted yourself.
There is a difference between the two cases. Also, low temperatures do not "generate" less kinetic energy; the particles just have less kinetic energy overall. Your answer doesn't really tell me much.

[eagles]

At low temperatures, gas molecules have less kinetic energy on average, so intermolecular forces have a stronger effect overall on molecular motions. As the intermolecular forces can no longer be neglected, the gases deviate from ideal gas behaviour.

At high pressures (i.e small space, or really high temperatures), gas molecules interact with each other a lot more as the space is relatively small for the amount of energy they have (end up getting closer to each other). Again, intermolecular forces have a greater effect overall.

Low temperatures do not induce high pressure. Just saying. You've said that yourself, so you've contradicted yourself.
There is a difference between the two cases. Also, low temperatures do not "generate" less kinetic energy; the particles just have less kinetic energy overall. Your answer doesn't really tell me much.

Thank you! However, I've bolded the bits which I don't understand clearly from the explanation. I don't see the intended emphasis of the intermolecular forces.
Do you mind elaborating? Thanks!

[lzxnl]

The ideal gas model makes the assumption that intermolecular forces are negligible. Under the ideal gas model, gases cannot change state, for those require intermolecular forces. The fact that increasing pressures or decreasing temperatures allow gases to change state shows that the ideal gas model begins to break down there.

[eagles]

Yep, that makes sense now. Thanks!