# chem

a considerable amount of heat is required for the decomposition of aluminum oxide.
2Al2O3-> 4Al(s)+ 3O2(g)
change in heat= 3352KJ
a. what is the heat change for the formation of 1 mol of aluminum oxide from its elements?
i got this one. my answer is -1676.0KJ
but i didn't get part b.
b. is this reaction exothermic or endothermic?
i assumed that this would be exothermic because the change in heat is -1676.oKJ.
but the answer is endothermic. Help?

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1. The heat given in the problem is for the reaction as written so it is endothermic because the sign is +. That's part b.
Part a, the problem asks for the heat of FORMATION which is the other direction so the heat is -3352 kJ/2 mol which makes it -1676 kJ/mol.
I think you assumed the b part was asking for the same reaction as part a which is the formation. I don't read it that way.

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2. oh okay thanks!

also what would be considered as a free element?
would 4Al be one?

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3. yes
Al is a free element. Oxygen is a free element. ANY element in the periodic table is a "free" element. When it is combined with another element (or elements), it isn't free any more.

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