Applying Oxidation Numbers to Al + KClO4

[vertciel]

Hello:

I am having some trouble relating to the following question to the concept of oxidation numbers as required by my practice problem, and would appreciate any help.

Thank you!

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1. Explain why it is dangerous to grind a mixture of Al with KClO₄ in a mortar and pestle.

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I calculated the oxidation numbers for each element: Al = 0, K = +1, Cl = +7, and O = -2.

I know that aluminium + potassium perchlorate would produce flash power through an explosive reaction, and that powdered Al would have much more surface area on which the reaction could take place. The mortar and pestle additionally would cause the reaction to occur at a faster rate since there is more contact between the particles, but I do not see how I can apply the concept of oxidation numbers to this reaction.

 

[Kushal]

metals are reducing agents and the perchlorate is an oxidising agent.

 

[vertciel]

Thanks for your reply, Kushal.

Could you please explain how potassium perchlorate is an oxidising agent?

 

[Kushal]

i'm afraid i may not be an exact source for the equation of perchlorate as an oxidising agent.

but, i know that Cl can have oxidation states: -1, +1, +3, +5 and + 7.

if the perchlorate (+7) is reduced to chlorate (+5), then the equation is:

2H⁺ + ClO₄^- + 2e^- ------> ClO₃^- + H₂O

i placed H⁺ in so as to be able to balance the equation.
OR, perchlorate (+7) can be reduced to chlorite (+3)

4H⁺ + ClO₄^- + 4e^- ------> ClO₂^- + 2H₂OOR, perchlorate can be reduced to hypochlorite, OR to chloride...

so i have no idea which reduction reaction will occur, unless i know the electrode potential values of the reactions. someone else might know which is the actual reduction reaction...

 

[chemisttree]

Chlorine goes from +7 to -1, a change of eight electrons per atom of chlorine. Each aluminum atom goes from 0 to +3.

The electrode potential of chlorine going from +7 to -1 is:

Cl⁺⁷ + 8 e- + 8H⁺ ------> Cl^- + 4H₂O = 1.389V