ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: jackinthepatch on March 25, 2009, 06:37:27 pm

Title: Questions
Post by: jackinthepatch on March 25, 2009, 06:37:27 pm: What is the fingerprint region in IR spectroscopy? I sort of kind of know the basic answer, just can't think of a good way to put it into words...

Oh and also, what is the relationship between wavelength, wavenumber and frequency?

Thanks in advance :)
Title: Re: Questions
Post by: dekoyl on March 25, 2009, 06:53:42 pm: The fingerprint region shows unique absorbance pattern for each compound under the $1500 cm^{-1}$ .
Title: Re: Questions
Post by: shinny on March 25, 2009, 07:03:41 pm: From my notes;
"The region of wavenumbers below 1500 cm-1 tends to correspond to the vibration and rotation of the entire molecule and hence is unique to each molecule. This region is known as the ‘fingerprint region’, and is useful for identifying compounds by comparing against other spectra."

The reason why the rest of the spectra isn't as useful is because they tend to only correspond to vibration of individual bonds, such as OH bonds etc. Many molecules have these so it doesn't do particularly much in terms of identifying the entire molecule, and problems especially come up when you get to isomers and such.

For wavenumber, also in my notes is "Wavenumber is the reciprocal of the wavelength corresponding to a particular peak. As such, increasing wavenumber corresponds to increasing frequency and decreasing wavelength." As for the others, someone who does physics answer this as it's definitely not my strong point.
Title: Re: Questions
Post by: cns1511 on March 25, 2009, 08:09:14 pm: Wavenumber is the inverse of wavelength.
That is: Wavelength is $\lambda$ and wavenumber is $\frac{1}{\lambda}$
Title: Re: Questions
Post by: Mao on March 25, 2009, 08:44:47 pm: frequency is $\frac{c}{\lambda} = c\times \mbox{wavenumber}\times 100$ where wavenumber is in cm^-1
Title: Re: Questions
Post by: jackinthepatch on April 14, 2009, 08:39:24 am: Sorry for reviving an old thread but here's another question lol.

a. Write a half equation for the conversion of Mn²⁺ to MnO₄^- in acidic aqueous solution.

b. Write a half equation for the conversion of IO₄^- to IO₃^- in acidic aqueous solution.

c. Write the overall chemical equation.

I kinda sorta know the answer...but apparently I'm wrong. So I'm just checking lol.
Title: Re: Questions
Post by: Dark Horse on April 14, 2009, 11:00:43 am: a) Mn2+ + 4H₂O --> MnO₄^- + 8H⁺ + 5e^-
b) IO₄^- + 2H⁺ + 2e^- ---> IO₃^- + H₂0
c) 2Mn2⁺ + 5IO₃^- .... (remember to cancel the waters and H⁺)

i think... :)
Title: Re: Questions
Post by: jackinthepatch on April 14, 2009, 11:10:15 am: Ah yeah the overall equation is what got me...
And what do you mean by cancelling the waters and H⁺?
Sorry lol.
Title: Re: Questions
Post by: d0minicz on April 14, 2009, 01:00:24 pm: 2Mn^2+ + 8H2O + 5IO3 + 10H^+ ---> 5IO3^- +5H2O + 2MnO4^- + 16H^+
is the overall equation right
just simplify the waters and H+ ions
so it will become
2Mn^2 +3H2O +5IO3 ---> 5IO3^- +2MnO4^- +6H^+
Title: Re: Questions
Post by: jackinthepatch on April 14, 2009, 01:10:59 pm: Oh ok...yeah so it's just as easy as that haha silly me.

Thanks guys.
Title: Re: Questions
Post by: kurrymuncher on April 14, 2009, 01:11:19 pm: Why does the heinemann book say the fingerprint region is before 1000. Is it 1000 or 1500??
Title: Re: Questions
Post by: dekoyl on April 14, 2009, 02:36:35 pm: Quote from: kurrymuncher on April 14, 2009, 01:11:19 pm
Why does the heinemann book say the fingerprint region is before 1000. Is it 1000 or 1500??
Ah yeah I saw that. Most sources say 1500 cm^-1, including my teacher so I go with 1500 cm^-1.
Title: Re: Questions
Post by: pHysiX on April 15, 2009, 06:47:16 pm: my teacher taught me 1400cm^-1 XD
Title: Re: Questions
Post by: d0minicz on April 15, 2009, 08:00:26 pm: i got taught 1400cm^-1 too haha
Title: Re: Questions
Post by: kurrymuncher on April 15, 2009, 08:39:48 pm: lol, so what is the actual value??
Title: Re: Questions
Post by: Collin Li on April 16, 2009, 08:43:55 am: It doesn't really matter, but it's in the range of 1000-1500 where it starts.