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Why do iron filings spark when thrown into a fire whereas a lump of iron do not?
Please put full answers. I am pretty dim so could you explain thoroughly. Thanks a lot.
To understand why iron fillings spark and a large piece of iron does not we need to understand the kinetics, mass transfer, heat transfer and thermodynamics of the reaction that is going on.
I am not certain of the exact rate equation but I suspect it looks like this (knowing the absolute equation is probably not necessary for this discussion):
r = k(T) * [Cfe]^a * [Co2]^b
where
r = rate of oxidation of iron
k(T) = rate constant as a function of temperature
= Ao * e^(-Ea/RT) (Arrhenius equation)
Ao = rate constant at a reference temperature
Ea = activation energy, in terms of J/mol
R = gas constant, 8.31451 J/mol K
T = Temperature in terms of Kelvin
Cfe = concentration of iron = which in this case I suspect is in terms of surface area of iron
Co2 = concentration, in terms of concentration in the air
a = unknown power factor that would have to be determined experimentally
b = unknown power factor that would have to be determined experimentally
Ok, with this rate equation there are a lot of unknowns. However, we can hypothesize that the rate of reaction will increase if the concentration of each reactant increases, if the rate constant as a function of temperature increases and if the temperature increases.
The iron concentration will increase if there is more surface area. Small iron particles have much more surface area than a large piece of mass. It could be over 1000 times if the particles are small enough! The reason why more surface area is required is due to mass transfer considerations. For the iron and oxygen to react they must be in contact with each other. The smaller particles allow quicker access to the iron than if the oxygen had to permeate through the metal and metal oxide film that forms on the large piece of metal.
The oxygen concentration is the same whether burning the large piece of metal or the iron fillings so this is not the cause of the difference.
The activation energy required to undergo the reaction can change with respect to particle size. If you look up dust explosions on the internet you will find that in general smaller particles require less activation energy than larger particles. Less activation energy required means the rate will be higher for the smaller particles.
The initial temperature of the fire is the same for both the iron filings and the large piece of metal. However, the reaction of iron with oxygen is exothermic. This is where the thermodynamics and heat transfer plays a role.
Since the smaller particles have more surface area and possibly lower activation energy, the rate of oxidation is expected to be much higher than on the large piece of metal. This higher rate of oxidation produces more heat within a given time which has to be dissipated. Since air does not transfer heat very well through conduction or convection then the temperature of the metal will rise to give off the heat by radiation. This along with the mass of iron fillings being less than the larger lump of iron causes the temperature of the iron fillings to get to a point where the radiation emitted is in the visual spectrum as the sparks you see.
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