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randommscience117

Sat Feb 11 2012, 06:33AM

Registered Member #4274 Joined: Mon Dec 19 2011, 03:10AM
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Posts: 47

So I learned in chemistry today that Hydrogen does not combust, it reacts. Combustion only occurs with carbon containing fuels and therefore releases CO2 as a byproduct, which is not the case with Hydrogen. What I am wondering is if this is really true. Elsewhere on the internet it says that Hydrogen combusts (e.g. wikipedia - combustion), but it makes sense that it only reacts.

Combustion of Methane
CH4 + 2 O2 â†’ CO2 + 2 H2O + energy
vs.
Reaction of Hydrogen
2 H2 + O2 â†’ 2 H2O(g) + heat

Thanks!

randommscience117

Sat Feb 11 2012, 06:37AM

Registered Member #4274 Joined: Mon Dec 19 2011, 03:10AM
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Posts: 47

Also, reading on wikipedia, it says "Combustion need not involve oxygen; e.g., hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion."
Combustion involves oxygen according to the first sentence of the page "Combustion, or burning, is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species."

Forty

Sun Feb 12 2012, 05:04AM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

ugh. chemists and their damn words.
(eg. you don't need oxygen to oxidize something)

if you mix hydrogen and oxygen, nothing really happens at all. It just sits there and takes up space (~NkT/P space.)
If you add ignition, it explodes.

gasoline and oxygen does the same thing and it's called combustion.
I don't see any reason why combustion need refer only to carbon fuels.

and I certainly don't think they'd put a sticker on a hydrogen tank that says "caution: may react in the presence of oxygen to form water and heat"

randommscience117

Sun Feb 12 2012, 05:44AM

Registered Member #4274 Joined: Mon Dec 19 2011, 03:10AM
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Posts: 47

A combustion reaction yields CO2 and H20, and without a carbon containing fuel, there is no CO2 formed from the reaction, therefore Hydrogen does not combust. Hydrogen instead is a synthesis reaction, A + B -> AB.

Hydrogen and oxygen do not necessarily explode when ignited, as in a hydrogen torch. The 'catalyst' for the reaction of hydrogen is heat, so obviously hydrogen and oxygen don't do anything until a 'catalyst' is added.

Fulmen

Sun Feb 12 2012, 09:38AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Sounds like useless semantics to me.
Both are redox (reduction-oxidation) reactions where the fuel reduces the oxidizer and the oxidizer oxidizes (duh) the fuel.

klugesmith

Sun Feb 12 2012, 02:44PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

randommscience117 wrote ...
...Combustion only occurs with carbon containing fuels and therefore releases CO2 as a byproduct ...

That definition of combustion is new to me -- for about 50 years I thought it just meant burning.

Who or what told you carbon is required? Can you find a reference on the Internet? Not that that would make it correct. It's OK to question the authority of your teachers, textbooks, and parents. Sometimes not out loud.

On the other hand, words often have special meanings within limited fields of study or craft. Someone already pointed out that to chemists, oxidation need not involve oxygen. In a battery being charged, is the negative terminal being the anode?

randommscience117

Mon Feb 13 2012, 03:27AM

Registered Member #4274 Joined: Mon Dec 19 2011, 03:10AM
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Posts: 47

There are six types of chemical reactions:
Combustion - A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide
Synthesis - A synthesis reaction is when two or more simple compounds combine to form a more complicated one
Decomposition - A decomposition reaction is the opposite of a synthesis reaction - a complex molecule breaks down to make simpler ones
Single Replacement - This is when one element trades places with another element in a compound
Double Replacement - This is when the anions and cations of two different molecules switch places, forming two entirely different compounds
Acid-Base - This is a special kind of double displacement reaction that takes place when an acid and base react with each other

The definition of a combustion reaction is one that produces CO2, and only carbon containing fuels can produce CO2 when burned, meaning that hydrogen by definition does not combust.

This isn't "useless semantics" for someone who is trying to learn the most from their Chemistry course. There are two points from burning hydrogen that are all over the internet that don't make a lot of sense: hydrogen combusting, and the "smoke" from burning hydrogen in a rocket that isn't smoke, it's steam. I want to know more about hydrogen so I don't sound confused when I talk about it.

Fulmen

Mon Feb 13 2012, 09:55AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

randommscience117 wrote ...
This isn't "useless semantics" for someone who is trying to learn the most from their Chemistry course.

I disagree (somewhat). It IS semantics (from Greek, the study of meaning), and while your definitions might be true I find them somewhat confusing. The chemistry involved is exactly the same no matter what fuel we're talking about, they are all redox reactions. Now if your chemistry course defines combustion this way you have to accept that, but you should also understand that the difference is purely semantic. If I have a choice between knowing the right term or understanding the actual phenomena I choose understanding every time.

FYI, this is the Oxford Dictionary definition of combustion: rapid chemical combination of a substance with oxygen, involving the production of heat and light.

klugesmith

Mon Feb 13 2012, 03:32PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

randommscience117 wrote ...
There are six types of chemical reactions:
Combustion ...
Synthesis ...

You still haven't told us who taught you that. Please don't be narrow-minded. Teachers get a break: sometimes they have to simplify things, so most students get most of it.
A few minutes of Internet searching, for your seven-word initial statement, turns up all manner of categories for chemical reactions.

says there are six, but the list does not include combustion.

says there are only four, but it gets extra credit for naming its author.

strikes me as relatively authoritative. It starts with a proper and modest qualification,

There are several different types of chemical reactions and more than one way of classifying them. Here are some common reaction types:

Then lists nine, including:

Combustion
A combustion reaction is a type of redox reaction in which a combustible material combines with an oxidizer to form oxidized products and generate heat (exothermic reaction). Usually in a combustion reaction oxygen combines with another compound to form carbon dioxide and water.

Good thing they say Usually, which implies that there are exceptions. Otherwise, how could spontaneous human combustion exist? Our N, P, Na, Cl, etc. have to go somewhere!

Bored Chemist

Mon Feb 13 2012, 07:38PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Great! Coal, which contains sulphur and nitrogen, is logically incombustible because burning it produces things like SO2 rather than just CO2 and water. The same goes for diesel, natural gas and gasoline.

Either there's something fundamentally wrong with that definition, or the world has got it wrong.

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