What is always true of a combustion reaction?

"oxygen gas must be one of the reactants"
The statement is always true.
Combustion is rapid reaction with oxygen.

"carbon dioxide will be produced"
The statement is NOT always true.
For example, combustion of hydrogen give water as the only product, but no carbon dioxide is produced.

"the reaction absorbs energy"
The statement is false.
Combustion releases energy.

"one reactant must contain carbon"
The statement is false.
For example, hydrazine (N₂H₄) is combustible, but it does not contain carbon.


The answer: oxygen gas must be one of the reactants

None of these statements are true of "combustion."

Oxygen need not be present for a "combustion" reaction. Fluorine and chlorine will also support combustion. Combustion is defined as "rapid oxidation", but oxygen need not be present for oxidation to occur. Oxidation is defined as the increase in oxidation state. For instance, when sodium metal is placed into a container of chlorine gas it will undergo rapid oxidation, producing sodium chloride. The sodium has been oxidized. The sodium has undergone combustion in chlorine.

More substances will undergo combustion in fluorine than in oxygen. It has been suggested that oxygen will even burn in fluorine.

There is a tendency to confuse the general term "combustion" with a specific kind of reaction, the "complete combustion of a hydrocarbon." The combustion of a hydrocarbon is assumed to be the hydrocarbon burning in oxygen to produce carbon dioxide and water vapor.
CH4(g) + 2O2(g) --> CO2(g) + 2H2O(g) --- complete combustion of a hydrocarbon

But other substances can undergo combustion. A very fine iron wire (iron wool) can undergo combustion in oxygen.
3Fe(s) + 2O2(g) --> Fe3O4(s)

these are all combustion reactions
.. (1).. 2 H2(g) + 1 O2(g) ---> 2 H2O(g) + heat
.. (2).. 1 H2(g) + 1 Cl2(g) ---> 2 HCl(g) + heat
.. (3).. 1 CH4(g) + 2 O2(g) ---> 1 CO2(g) + 2 H2O(g) + heat

your choices
.. oxygen gas must be one of the reactants... FALSE... See (2)
.. carbon dioxide will be produced... . . ... . .. .FALSE.. .see (1) and (2)
.. the reaction absorbs energy... ... .. .. .. .. .. FALSE....see all 3
.. one reactant must contain carbon.. ... ... ... FALSE.. see (1) and (2)

all the choices are false.


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