VCE Chemistry Question Thread

[cooldude123]

Can dispersion forces exist in polar molecules? And if so, would that include molecules with hydrogen bonding existing as well?

All molecules have dispersion forces regardless of polarity. (This occurs due to temporary dipoles which occur randomly in molecules).

Polar molecules also experience dipole forces which is basically the attractive force between adjacent polar molecules. Hydrogen bonding is a strong type of dipole force caused by differences in electronegativity.

[lzxnl]

All molecules have dispersion forces regardless of polarity. (This occurs due to temporary dipoles which occur randomly in molecules).

Polar molecules also experience dipole forces which is basically the attractive force between adjacent polar molecules. Hydrogen bonding is a strong type of dipole force caused by differences in electronegativity.

I disagree with the last point. Hydrogen bonding only really occurs between hydrogens and one of N, O or F. The condition is that the hydrogen must be covalently bonded to a N, O, F, the N, O, F must be covalently bonded to another hydrogen and the N, O or F must have a lone pair available for bonding. A hydrogen bond is then a bond in which the N, O or F uses one of its lone pairs to make a 'bond' with the hydrogen atom. As the N, O or F is relatively negative from its bond to the hydrogen and because the hydrogen is positive from its bond with a N, O or F, there is a rather strong attraction. Also, N, O and F are small, which allows the atoms to get closer to hydrogen atoms.

The classic example of why hydrogen bonding requires N, O or F only and not any electronegative atom is seen by comparing ammonia, hydrogen fluoride and hydrogen chloride. Ammonia boils at 240 K. Anhydrous HF boils at around 293 K and HCl boils at 188 K. HF and HCl are in the same group, but due to the small size and higher electronegativity of the fluorine atom, HF has a MUCH higher boiling point. NH3 and HCl have atoms with similar electronegativities, but the N-H hydrogen bonding is stronger than the H-Cl dipole interaction, so again there is a large difference in boiling point.

[chocolate.cake.1]

Why can alkanes only undergo substitution reactions with molecules in the form X2 (e.g. Cl2) and not other molecules like, for example, water? Is it something to do with the fact that water is polar?

Thanks

[Fullmethyl Alchemist]

Why can alkanes only undergo substitution reactions with molecules in the form X2 (e.g. Cl2) and not other molecules like, for example, water? Is it something to do with the fact that water is polar?

Thanks

Halogenation reactions have two parts- the breaking of the C-H bond, and the breaking of the X-X bond. The current theory is that latter bond breaks under heat and light, and the reactive halogen atom will try to complete its octet, and will break the C-H bonds to form a C-X bond in order to do so. Most other bonds don't break under light/heat- water, in fact, is insanely heat-stable.

[lzxnl]

Why can alkanes only undergo substitution reactions with molecules in the form X2 (e.g. Cl2) and not other molecules like, for example, water? Is it something to do with the fact that water is polar?

Thanks

Alkanes are pretty stable. I don't know of many general reactions that can actually use an alkane. The only general alkane reactions I'm aware of include halogenation and combustion. Combustion works because you're reacting the alkane with a strong oxidant that is kinetically inhibited (as long as you start a combustion reaction, it keeps itself going as the energy it provides maintains the breaking of the activation energy barrier).

Halogenation, on the other hand, works because you're using light to the break the X-X bond to make individual X radicals. Radicals are, well, insanely reactive and reactive with pretty much anything they come across. Now with chlorine atoms, they're quite electronegative and now they have 7 valence electrons. They're not going to be too happy are happy?

[RazzMeTazz]

In Atomic Emission Spectroscopy, what is the purpose of the slit, through which the light passes before passing through the prism?

Would it just be to collect the light in a single stream, so that it can enter the prism?

[I am a unicorn]

Hi everyone

I was doing some chemistry textbook questions and after checking my answers this question confused me:
Draw the structural formulas and name all the possible products that can be formed by the reaction of chlorine with ethane.

The answers (attached below) showed so many possible products, so why is it that sometimes more than one hydrogen atom in an alkane can be substituted? (I always thought only one H atom could be substituted?)

[I am a unicorn]

Uhhhh I'm back with another question

Can chloroalkanes undergo substitution with water molecules to produce alcohols? or is it only possible with OH- ions (from NaOH, etc.)
E.g. Would the following reaction possible: chloroethane + water --> ethanol + hydrochloric acid

Thankyou

[thushan]

Nah, that cannot happen; H2O is not a strong enough base to force that sort of reaction.

[thushan]

I disagree with the last point. Hydrogen bonding only really occurs between hydrogens and one of N, O or F. The condition is that the hydrogen must be covalently bonded to a N, O, F, the N, O, F must be covalently bonded to another hydrogen and the N, O or F must have a lone pair available for bonding. A hydrogen bond is then a bond in which the N, O or F uses one of its lone pairs to make a 'bond' with the hydrogen atom. As the N, O or F is relatively negative from its bond to the hydrogen and because the hydrogen is positive from its bond with a N, O or F, there is a rather strong attraction. Also, N, O and F are small, which allows the atoms to get closer to hydrogen atoms.

The classic example of why hydrogen bonding requires N, O or F only and not any electronegative atom is seen by comparing ammonia, hydrogen fluoride and hydrogen chloride. Ammonia boils at 240 K. Anhydrous HF boils at around 293 K and HCl boils at 188 K. HF and HCl are in the same group, but due to the small size and higher electronegativity of the fluorine atom, HF has a MUCH higher boiling point. NH3 and HCl have atoms with similar electronegativities, but the N-H hydrogen bonding is stronger than the H-Cl dipole interaction, so again there is a large difference in boiling point.

Whether it's just an extremely strong dipole-dipole interaction is a rather debatable point. Mao and I was discussing this a couple of years back - he's of the opinion that it is.

I'm too far out of uni-level Chemistry (been 4 years!) to remember the intricacies of this, unfortunately.

[cosine]

Can someone help me with spectroscopy? I am just struggling with the definition as a whole, specifically like what is electromagnetic radiation and how does it involve chemistry?

Thanks

[Redoxify]

What errors can arise from Uv visible?

[warya]

In Atomic Emission Spectroscopy, what is the purpose of the slit, through which the light passes before passing through the prism?

Would it just be to collect the light in a single stream, so that it can enter the prism?

I will let someone else answer this, but let me just say I just had a SAC on spectroscopy and chromatography, and all these details I was confused and stressing about, such as to what extent we need to know about nuclear shielding in NMR etc weren't really tested. The only theory questions related to what part of the atom/molecule was absorbing the specific energy type. Also, teachers mean it when they say we aren't going to be tested on instrumentation. Almost all questions were finding absorbance from a calibration curve, converting ppm to mol/L and stating which bonds are present given an IR spectra, its molar mass from MS and drawing the molecule- that's pretty much it- I really wish I spent less time worrying about the purpose of a monochromator and more time practicing dilution questions in checkpoints lol

[RazzMeTazz]

Would it be correct to say in NMR, the amount of energy required for every nucleus with an odd number of nucleons  to resonate is the  same (given that the strength of the external magnetic field is the same) however the reason the different nuclei absorb different wavelengths of radiowaves is due to their different chemical environments, resulting in nuclear shielding?

Also would the overall spin of a nucleus with an odd number of nucleons always be upwards, since there is always a greater number of nucleons aligned with the external magnetic field than against it?

Finally, I don't really understand why only nuclei with an odd number of nucleons works in NMR spectroscopy? If you had a nucleus which had an equal number of nucleons (say two nucleons in the up spin state and two in the down spin state, wouldn't they also be able to absorb energy and resonate?

Thanks

[cosine]

Can someone help me with spectroscopy? I am just struggling with the definition as a whole, specifically like what is electromagnetic radiation and how does it involve chemistry?

Thanks

Anyone