hihi2345
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How does UV break a Cl2 bond? I'd guess that the molecule absorbs the radiation causing it to vibrate and the bonds to break? Is this right?

Can why can't UV do this to other bonds? like a carbon chain or C-H bond? Is it because they're too stonrg?
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charco
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(Original post by hihi2345)
How does UV break a Cl2 bond? I'd guess that the molecule absorbs the radiation causing it to vibrate and the bonds to break? Is this right?

Can why can't UV do this to other bonds? like a carbon chain or C-H bond? Is it because they're too stonrg?
Yes, yes, yes

The chlorine - chlorine bond has a strength of 242 kJ mol-1

This means that each Cl-Cl bond has a bond energy of 242000/6.02 x 1023 J = 4.02 x 10-19 J

E = hc/l

4.02 x 10-19 J = 6.626 x 10-34 x 2.99 x 108/wavelength

Hence, wavelength = 4.93 x 10-7 m

= 493 nm

Visible light has a range of 400 nm to 700 nm (approx)

Hence UV light is able to break the Cl-Cl bond as it has more energy than the bond energy. In fact the minimum energy required to break the bond is actually in the visible region of the spectrum.
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RudraDubey
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(Original post by charco)
Yes, yes, yes

The chlorine - chlorine bond has a strength of 242 kJ mol-1

This means that each Cl-Cl bond has a bond energy of 242000/6.02 x 1023 J = 4.02 x 10-19 J

E = hc/l

4.02 x 10-19 J = 6.626 x 10-34 x 2.99 x 108/wavelength

Hence, wavelength = 4.93 x 10-7 m

= 493 nm

Visible light has a range of 400 nm to 700 nm (approx)

Hence UV light is able to break the Cl-Cl bond as it has more energy than the bond energy. In fact the minimum energy required to break the bond is actually in the visible region of the spectrum.
So its just the photoelectric effect on a covalent bond?
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scimus63
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When Einstein carried out expt for photoelectric it is the energy needed to eject electrons from a metal surface, not break covalent bonds. Photoelectric effect provided idea of wave particle duality, carried on from Planck and idea that energy is quantised
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RudraDubey
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(Original post by scimus63)
When Einstein carried out expt for photoelectric it is the energy needed to eject electrons from a metal surface, not break covalent bonds. Photoelectric effect provided idea of wave particle duality, carried on from Planck and idea that energy is quantised
Okay but doesn't the electron in the covalent bond still absorb the photon and then is energized to eject the photon or am i looking too deep into it?
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scimus63
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The bond I imagine as the attraction of the negative electrons for the protons in the nucleus, I imagine that the photon will supply enough energy to overcome this attractive force, that is break the bond.

Thats just what I think, does not mean its correct!
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jack_harrison
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The way it works requires quite a bit of undergrad level knowledge. It can be explained by molecular orbital theory; the idea that when you put atoms together to form a molecule, their atomic orbitals overlap to make a set of new (molecular) orbitals.

The gist is that an electron absorbs the photon and is excited from one molecular orbital to a higher energy one. If this new molecular electron configuration is unstable enough (has a bond order of 0; effectively no bond), the molecule enters a 'dissociative state' and the bond breaks on the timescale of a molecular vibration.

The stronger the bond, the more energy the electron transition described above requires so the higher energy of photon is needed to break the bond. Cl2 and a lot of other molecules with weaker bonds have bond energies that correspond to the UV/Visible regions.

If you want to know more, you could have a quick look at molecular orbital theory, and the mechanism of the reaction you describe is photodissociation.
Last edited by jack_harrison; 1 month ago
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