[jwxie]

Hi guys.

Carbon tetrachloride is prepared by the reaction of natural gas (CH4) and chlorine gas in the presence of uv light.
Write the balanced equation of that.

CH4 + 4Cl2 -> CCl4 + 4HCl

How do I know that HCl must be present in this equation? But in general, how do I determine all the possible products? The question does not tell us that HCl is a product. Of course, H must be used up. Oxidation, Reduction, etc. But for a more complex equation, some compounds formation may not exist.

UV light might be a hint, but do all light reaction produces gas (photosynthesis is a typical case).

In a chemical reaction, can we ever end up with one of the products being the original (in a perfect, ideal balanced chemical reaction)? I am not sure if I can take heat in combustion as a reactant.

Thank you.

[fledarmus]

Heh - you work lots and lots of reactions until you can recognize reaction types.

Some hints in your reaction - if you just consider the formation of carbon tetrachloride from natural gas and chlorine gas (and don't know the reaction equation that you presented), what are the oxidation states of the various atoms? What is missing from that type of reaction?

[Arkcon]

Hi guys.

Carbon tetrachloride is prepared by the reaction of natural gas (CH4) and chlorine gas in the presence of uv light.
Write the balanced equation of that.

CH4 + 4Cl2 -> CCl4 + 4HCl

How do I know that HCl must be present in this equation? But in general, how do I determine all the possible products? The question does not tell us that HCl is a product. Of course, H must be used up. Oxidation, Reduction, etc.

That's how you know, in this particular case.  Good job, on your part, right here.

But for a more complex equation, some compounds formation may not exist.

That is correct, for very complex reactions, it may have to be determined experimentally.

UV light might be a hint, but do all light reaction produces gas (photosynthesis is a typical case).

No.  Barking up wrong tree you are.  Says Yoda.

In a chemical reaction, can we ever end up with one of the products being the original (in a perfect, ideal balanced chemical reaction)? I am not sure if I can take heat in combustion as a reactant.

No, generally we don't.

Thank you.

[OnePound]

The uv light part is crucial. When exposed to uv, Cl2 splits to form Cl* (Cl dot) free radicals. When CH4 is added, the following happens:

Cl2 --> 2Cl* -- two chlorine free radicals are formed
Cl* + CH4 --> *CH3 + HCl -- the chlorine free radical reacts with CH4 to produce another free radical, and HCl
*CH3 + Cl2 --> Cl* + CH3Cl -- *CH3 then reacts with more Cl2, regenerating the Cl* and forming the product (CH3Cl).

For more help:
http://www.chemguide.co.uk/mechanisms/freerad/whatis.html

[CrimpJiggler]

If you're learning organic chemistry then I recommend you learn about this particular class of reactions. You're right, the UV light is a hint. UV light causes Cl₂ to split into 2 Cl free radicals. This starts a chain reaction. Free radicals are extremely reactive so the Cl radical reacts with CH₄ and takes one of its hydrogens forming 1 molecule of HCl and 1 methyl (CH₃) radical.

[gratianmasu]

If you're learning organic chemistry then I recommend you learn about this particular class of reactions. You're right, the UV light is a hint. UV light causes Cl₂ to split into 2 Cl free radicals. This starts a chain reaction. Free radicals are extremely reactive so the Cl radical reacts with CH₄ and takes one of its hydrogens forming 1 molecule of HCl and 1 methyl (CH₃) radical.

You mentioned good point here, But it would be nice if you provide few more information.

[fledarmus]

You mentioned good point here, But it would be nice if you provide few more information.

Look up free radical halogenation in a good organic textbook. Or try:

http://en.wikipedia.org/wiki/Free_radical_halogenation
http://en.wikipedia.org/wiki/Free_radical_reaction

UV light in the reaction, especially at the undergraduate level, is usually a hint that it is a free radical reaction.

[gratianmasu]

You mentioned good point here, But it would be nice if you provide few more information.

Look up free radical halogenation in a good organic textbook. Or try:

http://en.wikipedia.org/wiki/Free_radical_halogenation
http://en.wikipedia.org/wiki/Free_radical_reaction

UV light in the reaction, especially at the undergraduate level, is usually a hint that it is a free radical reaction.

Thanks fledarmus.It's really provide good help for me.