# How does UV break a Cl2 bond?

Watch
Announcements
Thread starter 8 months ago
#1
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?
1
reply
8 months ago
#2
(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.
0
reply
1 month ago
#3
(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?
0
reply
1 month ago
#4
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
0
reply
1 month ago
#5
(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?
0
reply
1 month ago
#6
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!
0
reply
1 month ago
#7
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
0
reply
X

Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### Poll: What factors affect your mental health most right now? Post-lockdown edition

Anxiousness about restrictions easing (30)
5.32%
Uncertainty around my education (65)
11.52%
Uncertainty around my future career prospects (66)
11.7%
Lack of purpose or motivation (75)
13.3%
Lack of support system (eg. teachers, counsellors, delays in care) (32)
5.67%
Impact lockdown had on physical health (28)
4.96%
Social worries (incl. loneliness/making friends) (60)
10.64%
Financial worries (35)
6.21%
Concern about myself or my loves ones getting/having been ill (22)
3.9%
Exposure to negative news/social media (34)
6.03%
Difficulty accessing real life entertainment (15)
2.66%
Lack of confidence in making big life decisions (56)
9.93%
Worry about missed opportunities during the pandemic (46)
8.16%

View All
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.