Name: Jane C.
Is the formation of rust considered an exothermic or endothermic reaction? Where does the
activation energy come from to start the reaction?
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.
"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
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Update: June 2012