Chemistry 3/4 2013 Thread

[Alwin]

In UV-visible spectroscopy, what part of the substance actually absorbs the electromagnetic radiation? My textbook implies that it is the electrons, but my teacher told us it's actually the bonds. I'm a tad confused. :S

From the textbook:

UV–visible spectra arise because photons in this region of the electromagnetic spectrum have sufficient energy to promote electrons from low energy levels to higher energy levels. Such electron transitions may occur in atoms, ions or molecules. Because different substances have many different energy levels, the energy and wavelength of the light required to promote the electrons vary. The spectrum can therefore be used to assist in identifying a substance.

What it means is that not only electrons in an atom can absorb energy, but also electrons in bonds can absorb energy. I think this is what your teacher was referring to as "bonds"

[lzxnl]

Sorry, but wrong. What you sir found was the the particle with the LARGEST radius. You want the SMALLEST radius since more deflection! The correct answer is the particle with the lowest mass divided by charge.

Another way, just to visualise it is say a a canon ball and a water gun are shot in to a cross wind. Obviously the canon ball will be deflected much less than the water droplets, because it is much much heavier. Dividing by charge allows you to factor in the effects of interactions between the field and the charge.

Yeah you're right. I'm stupid. Mindfreeze -.-

[Stick]

From the textbook:
What it means is that not only electrons in an atom can absorb energy, but also electrons in bonds can absorb energy. I think this is what your teacher was referring to as "bonds"

OK, that's a tad unusual. Thanks.

[9_7]

Hey guys just a simple question on volumetric analysis. Why do you wash volumetric/conical flasks with distilled water?

[Patches]

Because when you make the solution up to the mark (with distilled water), it doesn't matter if there's extra distilled water in the flask since it will still total to 250ml or whatever. If you washed it with something else, whatever is dissolved in it would affect the concentration of your standard solution.

[Bad Student]

Can someone please explain what multiplets represent on an NMR spectrum?

[thushan]

Can someone please explain what multiplets represent on an NMR spectrum?

Just means you have a splitting that looks really weird that you can't count it easily. Often occurs when you have splitting of peaks that have already been split, because if the neighbouring carbons are in different environments, then the protons of each carbon cause their own splitting of the peak - so one carbon's protons could cause splitting of the peak, and the other carbon's protons cause splitting of EACH of the split peaks.

Confusing - sorry - don't worry about it for Year 12.

[jono88]

So i have a sac on NMR and its out of 40marks. Done all the past vcaa questions relating to NMR, analysed various compounds already, i don't know what else to do lol...

[jono88]

Oh and made notes on NMR as well

[Bad Student]

Just means you have a splitting that looks really weird that you can't count it easily. Often occurs when you have splitting of peaks that have already been split, because if the neighbouring carbons are in different environments, then the protons of each carbon cause their own splitting of the peak - so one carbon's protons could cause splitting of the peak, and the other carbon's protons cause splitting of EACH of the split peaks.

Confusing - sorry - don't worry about it for Year 12.

My teacher's been teaching us this stuff. He says it will help us understand the topic better. I just get confused though.

[Bad Student]

So i have a sac on NMR and its out of 40marks. Done all the past vcaa questions relating to NMR, analysed various compounds already, i don't know what else to do lol...

Teach your cat NMR. You'll gain a better understanding when you explain to others in simple terms.

[brightsky]

Teach your cat NMR. You'll gain a better understanding when you explain to others in simple terms.

this is gold.

[memarani]

Can someone help me with this question please?

14. Explain why the following statements are true.

a. Alkanoic acids are more soluble in water than alcohols.

b. Alcohols are weaker acids than alkanoic acids.

[lzxnl]

a

By definition, an alkanoic acid has a double bond from carbon to oxygen. This double bond makes the carbon quite positive. Also, the carbon has a single bond to the oxygen. This further increases the positive charge on the carbon. Therefore, there is quite a strong dipole down the end of the carboxylic acid functional group, allowing for stronger dipole-dipole and hydrogen bonding forces with water molecules, increasing solubility. Alkanols, in contrast, don't have the extra polarity afforded by the C=O bond.

b

The strength of an acid is directly related to how stable the conjugate base is. Hydrochloric acid is a strong acid because its conjugate base, the chloride ion, is quite stable with its octet of electrons. With alkanoic acids, if you deprotonate them to form the conjugate base, we have a negative charge on an oxygen atom. However, the excess electron density actually spreads over both oxygen atoms; they share the charge, producing a stabilizing effect. In contrast, alkanols, if deprotonated, leave ions with the negative charge solely on the oxygen atom which isn't nearly as stabilizing. Therefore, carboxylic acids are much stronger acids than alcohols.

[brightsky]

a) carboxylic acids are in general more polar than the alcohols. alcohols only have available a -OH group to form hydrogen bonds with water. carboxylic acids however have not only an -OH group, but also a C=O group. this means that carboxylic acid can form more extensive H-bonds with water, entailing higher solubility.

b) alcohols are really really weak acids. when alcohols lose a proton, they form what is called an alkoxide ion. the oxygen atom from which the hydrogen detached will be highly negatively charged due to the electron originally belonging to the hydrogen floating around it, meaning that it will readily grab a hydrogen and reform alcohol. so the position of equilibrium is actually far to the left. when carboxylic acids lose a proton, however, they form the renowned alkanoate ion. the electron originally belonging to the hydrogen is 'delocalised' over the entire -COO(-) group, so the negative charge is kind of smeared throughout a larger region. for this reason, the alkanoate ion is much less attractive to protons floating in its vicinity, and so the position of equilibrium will not lie as far to the left. thus alcohols are much weaker acids than carboxylic acids. the word acid in the latter pretty much gives it away.