Chemistry 3/4 2013 Thread

[Bad Student]

So far so good. The topics I'm learning are still familiar from last year, but the questions are significantly more complicated. Once I reach the new stuff (chromatography, spectroscopy etc.), it should start to get really challenging.

[Jaswinder]

Could someone please explain why the first shell has the lowest energy level and the furthest has the highest energy level?

[thushan]

Could someone please explain why the first shell has the lowest energy level and the furthest has the highest energy level?

The first shell is most stable; the attractive force between the nucleus and the electron is strongest here.

It takes energy to yank the electron from the first to the second shell against the attractive force of the nucleus; hence, we have inputted potential energy into the electron by moving it up a shell.

[Edward21]

Wow, we dived into chromatography from the 1st lesson  and almost up to IR spectoscopy..

[Bad Student]

Wow, we dived into chromatography from the 1st lesson  and almost up to IR spectoscopy..

Are you going to go back to volumetric and gravimetric analysis? Or are you going to skip it completely?

[Edward21]

Are you going to go back to volumetric and gravimetric analysis? Or are you going to skip it completely?

Not really, that was headstart stuff and holiday homework because it's year 11 stuff, but we'll have to come back to back titrations though 

[Jaswinder]

Could someone explain the principles behind nmr, ir, and mass spectroscopy. I'm having trouble understanding those techniques. Thanks

[teletubbies_95]

Hey!
I think the best resource is to use Thushan's notes which are available in the "Resources" section of Atarnotes! It's really good in explaining the concepts!

[Edward21]

Could someone explain the principles behind nmr, ir, and mass spectroscopy. I'm having trouble understanding those techniques. Thanks

I'll explain IR, just because I feel most confident in it, I might get this wrong though, anyone can correct! OK: infrared waves are not as high a frequency as the electromagnetic wavelengths of the visible, UV and gamma rays. The infrared radiation CANNOT alter the valence electrons in the atoms of a molecule. What they do alter is the bonds between atoms, where as you would know they are constantly vibrating, and as each bond is different thus absorbs a SPECIFIC wavelength or infrared (IR) radiation. For an easy example: if 10 chocolate bars come through (the specific wavelenth of IR radiation), 2 come out (transmittance to the detector) then it must be so, that there are bonds in our sample that have absorbed 8 chocolate bars (the specific wavelength!). The graph/spectra of IR measures: TRANSMITTANCE (how much comes through) with inverted peaks signifying what has been absorbed at that specific wavelength, if less of that SPECIFIC wavelength is being detected after that wavelength is passed through the sample, it means it has absorbed it, the lower the inverted peak the more STRONGLY it has absorbed that. This type of spectrum measures transmittance, how much comes through the detector, NOT absorbance where: nothing being absorbed would see a line of no absorbance at that wavelength down the bottom, as IR measures transmittance, nothing being absorbed would have ALL the wavelength coming through the detector indicating 100% transmittance (therefore 0% absorbance) and a line across the top. Effectively, if IR spectroscopy measures transmittance, and inverted peaks means less of that specific wavelength reaches the detector indicating absorption....THEN if one were to flip the spectrum upside down, we would see a standard spectrum of absorption, as now the inverted peaks that measured what wasn't coming through the detector indicating what has been absorbed, are now peaks of absorption. This spectrum of ethanol shows how different wavelengths of IR are absorbed by the bonds, meaning that less of that wavelength comes through the other side of the sample, thus an inverted peak/trough now appears. Hope that explained the logistics of this technique, are you doing 3/4 chemistry in Year 11, or just curious?

[hardworker]

OK i'm really stressing out and need some help please. 2 and a half weeks from now i have my first chem SAC and its on 'titration' teacher hasn't really specified what sort  of titration. Overall the SAC is an extended experimental investigation. what should i study and read up because i really need some guidance.

[thushan]

OK i'm really stressing out and need some help please. 2 and a half weeks from now i have my first chem SAC and its on 'titration' teacher hasn't really specified what sort  of titration. Overall the SAC is an extended experimental investigation. what should i study and read up because i really need some guidance.

Hey -

1) Revise the chapter on Volumetric Analysis, acid-base chemistry and redox chemistry. Ensure you understand not only the theory, but the correct lab technique.

2) Spam as many questions as you can from your various resources on Volumetric Analysis.

[hardworker]

Thanks T-Rex will do its just with chem i feel so bogged down by some of the questions.

[hardworker]

Just wondering T-Rex what are the  correct lab technique.

[Bad Student]

A student conducts two titration experiments. In experiment 1, a 20.00 mL sample of 0.100 M hydrochloric acid was placed in a conical flask, and a 0.100 M sodium hydroxide solution was slowly added until a total of 50.0 mL had been added. In experiment 2, a 20.00 mL sample of 0.100 M ethanoic acid was used instead of hydrochloric acid.

Would the volume of NaOH needed for complete reaction in experiment 2 be equal to, greater than or less than the value in experiment 1? Explain your choice.


I know that ethanoic acid is a weak acid, so you would have to add in extra. But there seems to be an excess amount of NaOH solution added to the hydrochloric acid in experiment 1.

[Stick]

^ I believe that's just a technicality. They want you to just think about the consequences on the titration if a weak acid is used instead. This sort of question pops up quite a bit so make sure you understand it.