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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: crowell on January 06, 2008, 06:18:32 PM

Title: Zn + O = combustion rxn or synthesis rxn ---- WHY?
Post by: crowell on January 06, 2008, 06:18:32 PM

Is the reaction of zinc and oxygen a combustion reaction or a synthesis reaction?  Is it possible to be categorized as both?

Thanks for your time

Title: Re: Zn + O = combustion rxn or synthesis rxn ---- WHY?
Post by: Arkcon on January 06, 2008, 06:24:05 PM

Zn + O₂ would probably be called a synthesis reaction in a general chemistry text.  The term "combustion reaction" may have meaning in jargon of some other discipline, but I've haven't heard of it in this case.  'Course, the zinc is combusting, so yeah, 's alright too.

Title: Re: Zn + O = combustion rxn or synthesis rxn ---- WHY?
Post by: crowell on January 06, 2008, 06:32:35 PM

Okay...so then I guess it is possible to for a reaction to be both a sythesis reaction and a combustion reaction at the same time and these terms are not mutually exclusive

Title: Re: Zn + O = combustion rxn or synthesis rxn ---- WHY?
Post by: Arkcon on January 06, 2008, 06:37:30 PM

Oops - I'm wrong. :o There is such a thing in general chem as a combustion reaction -- wikipedia (http://en.wikipedia.org/wiki/Chemical_reaction#Reaction_types)  I gotta go back to high school again, it seems.

Now it becomes hard to answer -- if a combustion reaction is defined as the oxidation of an entire molecule, as opposed to a single functional group on the molecule -- does it apply to a single atom, or not.  Meh, kinda of a pointless definition anyway, IMHO.

Title: Re: Zn + O = combustion rxn or synthesis rxn ---- WHY?
Post by: Alpha-Omega on January 06, 2008, 06:54:25 PM

Combustion RXNS:  When a compound containing carbon, hydrogen, and oxygen is burned in the
presence of excess oxygen, the products are carbon dioxide and water. 

By Definition:  The products of a combustion RXN are CO2 and H2O.

Example:  C8H18 (l) + 17O2 (g) è8CO2 (g) + 9H2O(l)

Title: Re: Zn + O = combustion rxn or synthesis rxn ---- WHY?
Post by: Alpha-Omega on January 06, 2008, 07:10:03 PM

Zinc oxide is an ionic compound made up of zinc cations Zn2+ and oxide O2- anions.  When uncharged zinc and oxygen atoms react to form zinc oxide, electrons are transferred from the zinc atoms to the oxygen atoms to form these ions.  Each zinc atom loses 2 electrons and each oxygen atom gains 2 electrons.

This is what happens:


OVERALL RXN:  2Zn(s)   + O2 (g)   ----> 2ZnO (s)

Zn Half RXN:  Zn ---->  ZN2+ + 2e-           or                2Zn ----> 2Zn@+ + 4e-

O Half RXN:  O + 2e- ----> O2-            or     O2  + 4e-    ---->   2O2-