Ask A Scientist Chemistry Archive

name         Jane C.
status       educator
age          40s

Question -   Is the formation of rust considered an exothermic or endothermic reaction? Where does the 
activation energy come from to start the reaction?
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Jane,

With respect to activation energy, all reactions are at least partially endoenergetic. However, 
rusting (oxidation of iron) is overall very exoenergetic. Even though under normal conditions the 
oxidation reaction proceeds very slowly, there is sufficient energy to initiate the process at room 
temperature. Once begun, the process can sustain and slightly accelerate itself by using the heat 
released from the reaction and that initially available from room-temperature to provide the 
necessary activation energy.

Have you ever seen the shower of sparks that occur when steel is cut with an oxyacetylene torch? When 
steel is cut with an oxyacetylene torch, small pre-heat flames emerging from the torch nozzle are used 
to bring the metal to a high enough temperature that it is able to "burn" in a stream of pure oxygen 
gas that is delivered to the metal through a center orifice in the torch tip. Once the cut has been 
initiated, the heat released from this very exoenergetic combustion of iron is sufficient to carry 
the cutting process even if the pre-heat flames are turned off.


Regards,
ProfHoff 675
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"Rust" is really a family of iron/oxygen compounds with various ratios of: Fe, O, and H (as in water). 
All the "rusting" reactions are exothermic to the tune of from about -60 to -190 kcal/mol That is they 
all liberate a substantial amount of heat. In fact one can generalize that for most metals oxidation 
is exothermic. Now the "activation" energy to which you refer has to do with just how "fast" the 
reaction occurs. In the case of  iron. Do not have the numbers in front of me but from qualitative 
observation I would expect it is quite small because rust occurs about equally fast at any reasonable 
temperature interval.

Vince Calder
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