Please Help-Chem Questions.

[bec]

Q: Why does the presense of chloride ions in potassium chloride have no effect on the flame colour?
A: The energy required to promote an electron in a chlorine atom is much higher than for the potassium atom. As a consequence, the flame of a Bunsen burner excites relatively few chlorine atoms. Furthermore, energies emitted as electrons in the excited atoms return to their lowest energy levels are mainly outside the energies of visible light.

in english, anyone? i don't understand a word

[midas_touch]

Basically, its saying that not many Chlorine atoms become excited, i.e. not many electrons that belong to the Chlorine atoms get promoted to higher energy states. Of these electrons that get promoted, when they return to the ground (unexcited) states, the energy emitted in the form of a photon corresponds to a wavelength which is outside the visible light range, and so there is no effect on the flame colour.

[bec]

ok thanks, i think i understand...
why though? what is it about the electrons in chloride ions (can i generalise and say all anions?) that makes them require more energy to jump to higher energy states?
is it to do with the electron configuration?

[Collin Li]

It's because the electronegativity of elements far to the right of the periodic table is high. This means that the energy required to distance an electron from its initial position will be higher.

The high energy wavelengths that are emitted will be in the ultraviolet region of the UV-visible spectrum, and that is why you cannot perform flame tests on non-metals.

[bec]

thanks!

[bec]

here's yet another question...

Q: Name a substance, other than copper sulfate solution, whose concentration can be determined directly by UV–visible spectrometry.
A: Any coloured solution can be chosen, for example chlorophyll solution.

Why any coloured solution? I thought the whole thing with UV-visible spectrometry was that solutions didn't have to absorb wavelengths in the visible region?
Can the concentration of chlorophyll (which i'm guessing is green?) be determined using regular atomic absorption spectrometry?

[Collin Li]

Yeah, I'm pretty sure you can do anything that absorbs UV-visible. It doesn't need to be coloured. That's for old fashioned colorimetry, they're living in the dark ages.

AAS is based on the cathode lamps (restricted to metallic atoms), so unless chlorophyll contains some metallic atoms in it, I doubt it is possible.

[bec]

coblin i heart your explanations

Atomic emission spectroscopy (AES) is regarded as superior to flame tests because, in AES, more elements produce emission spectra in the hotter flame, and a much smaller quantity of an element needs to be present in order for it to be detected. Using AES, the amount of the element present in a sample can be accurately determined, even in the presence of larger amounts of other elements that would mask the flame colour of the element to the naked eye.

Ok, so i understand that aes is better than flame tests because
1) a smaller quantity of an element is needed
2) can be used for quantitative analysis
3) uses a machine, therefore more accurate than naked eye thinking that something is "kind of bluey-green"

but....what's this "hotter flame" they're talking about? isn't the flame in aes only there so the substance can be atomised?
thanksss

[iamdan08]

They use a hotter flame in AES than in simple flame tests. This hotter flame provides more energy, which as a result excites the electrons in a wider range of elements.

[bec]

so what if the analyte also emits energy because of the flame? doesn't that interfere with the results?
how do we know which light comes from electrons excited in the flame, and which comes from electrons excited because of the light derived from the cathode lamp?

does that make sense?

[Mao]

mmm you are talking about the cathode lamp so i'm assuming AAS?

the measurements are done on specific frequencies.
much like each atom has its own emission spectrum (back in unit 1), the complementary spectrum to this is their absorption spectrum.

Each cathode lamp is calibrated to a particular frequency for a particular metal (an AAS have a lamp for each type of metal, thats why they cost so darn much), and only that metal will absorb that frequency.

if that made sense?

[Collin Li]

AES is not the same as AAS. You seem to be getting them two confused.

AES is superior to flame tests in every way. Flame tests only give you a macroscopic "average" view of the emission spectra, and plus the flame may be too weak - even if it is strong enough, the emitted wavelengths will be invisible to the naked eye (UV - the high energy wavelengths of light).

For AAS, the flame in the atomiser is supposed to bring the atoms to their ground state (not 100% sure how, but you're supposed to assume this happens). I haven't seen an official explanation against why the emission caused by the flame does not affect the final results in the AAS, but I suspect this is due to the monochromator placed after the absorption, which selects a particular wavelength of light (the one that is being absorbed), ignoring the other emitted wavelengths (I'm aware this isn't a complete explanation). It's a very good question that my friend asked TSFX, who didn't have an answer for it. You probably don't have to worry about it.

[bec]

ahhh thank you both of you! you're right, i was getting mixed up between AAS and AES...and i was confused because of a combo of that and the fact that i get so incredibly hung up on details in chem that i get NOWHERE.
thanks...more to come i'm sure!

[bec]

hey does anyone know if colorimetry is on the course? it's not covered in any detail in my text book but i'm doing a whole unit on it at school - what's everyone else doing?

[iamdan08]

What text are you using? On the VCAA website in the study design it says:

-Discuss the principles of colorimetry, and the relationship between concentration and absorption

-Use second-hand colorimetry data to construct a calibration curve and determine the concentration of an ingredient in a consumer product, concentration of phosphate ions in laundry products

-perform a colorimetric analysis such as the phosphate content of washing powders

So it looks like it is.