brightsky's Chemistry Thread

[brightsky]

Warning: These Chem questions might seem ridiculously easy for all you Chem pros out there, but any help will be thoroughly appreciated!

[brightsky]

What do bond-dissociation energies actually describe? My intuitive understanding is that it refers to the amount of energy needed to break a certain chemical bond, but I fear that I'm completely wrong. Thanks in advance.

[huaxiadragon]

Bond-dissociation energy describes the strength of the bond. The greater the energy the stronger the bond. I don't think you'll be specifically tested for this in Year 12 exam, but it is very important in helping you to understand spectroscopy in Unit 3

[dc302]

What do bond-dissociation energies actually describe? My intuitive understanding is that it refers to the amount of energy needed to break a certain chemical bond, but I fear that I'm completely wrong. Thanks in advance.

You're right. It is also a way of measuring the strength of a chemical bond (by seeing how much energy is needed to break it).

[Mao]

Bond Dissociation Energy (BDE) is a concept very similar to bond energy.

The DBE is the change in enthalpy when you break a bond symmetrically:

HO-H --> HO. + H. (where . indicate a radical, i.e. unpaired electron)

The bond energy is the average of the bond dissociation energies, found as the following:

HO-H --> HO. + H. (ΔH1)
HO. --> O + H. (ΔH2) [note that ΔH2 and ΔH1 have different values]
Bond energy = 0.5*(ΔH1+ΔH2)

[brightsky]

what's the difference between shells and orbitals?

[Mao]

what's the difference between shells and orbitals?

Bohr's model used shells (before Schrodinger). When quantum mechanics kicked in, Schrodinger divided shells into orbitals. Shells are essentially an informal grouping of orbitals which share a common quantum number.

Each electron has 4 quantum numbers, which are parameters to the wave equation. These describe the 'shell', 'orbital', 'geometry' and 'spin' of the electron.

[brightsky]

what's the difference between shells and orbitals?

Bohr's model used shells (before Schrodinger). When quantum mechanics kicked in, Schrodinger divided shells into orbitals. Shells are essentially an informal grouping of orbitals which share a common quantum number.

Each electron has 4 quantum numbers, which are parameters to the wave equation. These describe the 'shell', 'orbital', 'geometry' and 'spin' of the electron.

hmm..you lost me at 'common quantum number'.

[dc302]

Quantum numbers are intrinsic properties that you won't understand until you actually start solving schrodinger's equation in 2nd/3rd year physics or chemistry. Until then they're kinda just like facts you have to 'accept'. If you want to know more you can wikipedia them.

[brightsky]

Quantum numbers are intrinsic properties that you won't understand until you actually start solving schrodinger's equation in 2nd/3rd year physics or chemistry. Until then they're kinda just like facts you have to understand. If you want to know more you can wikipedia them.

ahh i see. i'll have a glance at wikipedia (though most of its chem pages are full of undecipherable jargon). but what i want to know is, is it 'wrong' to say the 2s shell? or must you always refer to it as 2s orbital...or subshell?

[dc302]

You should say 2s orbital.

edit: for vce I think it is sufficient to know that the 2 is the shell number and the s represents the subshell. But again, I'm pretty sure all that is required is to say the '2s orbital' or '2p orbital' etc. Also note that the shell and subshell are the first 2 quantum numbers. The 3rd quantum number tells you about the orientation of the orbital, and as an example, there are 3 orientations for the 2p orbital. 2px, 2py, 2pz. But for VCE, you will only need to know that there are three 2p orbitals, and no more specific than that.

[brightsky]

You should say 2s orbital.

edit: for vce I think it is sufficient to know that the 2 is the shell number and the s represents the subshell. But again, I'm pretty sure all that is required is to say the '2s orbital' or '2p orbital' etc. Also note that the shell and subshell are the first 2 quantum numbers. The 3rd quantum number tells you about the orientation of the orbital, and as an example, there are 3 orientations for the 2p orbital. 2px, 2py, 2pz. But for VCE, you will only need to know that there are three 2p orbitals, and no more specific than that.

ahh yes i see...i've seen the p_x, p_y, etc. notation in a podcast, but never knew it related to quantum numbers. but the orbital table loses me when it gives notation like d_z^2, dxz, f_y(3x^2-y^2), etc. but yeah thanks for clearing that up!

[dc302]

Haha yeah, they can get pretty complicated, especially when you start learning about molecular orbitals. If you take chemistry in uni (first year) you'll be introduced to the theories of how things bond together using orbitals. It's quite interesting, but I can't say what you do after that as I only did 1st year chem (though I did do 2nd year quantum mechanics).

[Mao]

Haha yeah, they can get pretty complicated, especially when you start learning about molecular orbitals. If you take chemistry in uni (first year) you'll be introduced to the theories of how things bond together using orbitals. It's quite interesting, but I can't say what you do after that as I only did 1st year chem (though I did do 2nd year quantum mechanics).

Eventually they get you onto software which approximately solves Schrodinger's equation for larger molecules using methods such as DFT (you may have heard of GAUSSIAN). A lot of HOMO and LUMO scattered from second year.

[dc302]

If gaussian is a program then no I haven't heard of it, but I've heard of it plenty of times in maths etc, from the mathematician/physicist Gauss. Anyway, I always liked this area of chemistry, so it was a shame I couldn't take any further subjects in it.