Enthalpy H is a measure of the energy content of a system at constant pressure.
ID: 782218 • Letter: E
Question
Enthalpy H is a measure of the energy content of a system at constant pressure. Chemical reactions involve changes in enthalpy,
DeltaH , which can be measured and calculated:
where the subscript "rxn" is for "enthalpy of reaction" and "f" is for "enthalpy of formation" and m and n represent the appropriate stoichiometric coefficients for each substance.
The following table lists some enthalpy of formation values for selected substances.
**************Consider the reaction
2Al(OH)3(s) ----> Al2O3(s)+3H2O(l)
with enthalpy of reaction
DeltaHrxno=21.00kJ/mol
What is the enthalpy of formation of Al2O3(s) ?
Substance DeltaHf (kJ/mol) HCl(g) -92 Al(OH)3(s) -1277 H2O(l) -285.8 AlCl3(s) -705.6 H2O(g) -241.8Explanation / Answer
2Al(OH)3 ---> Al2O3 + 3H2O
dH reaction = dH Al2O3 + 3dH H2O -2dH Al(OH)3
21 = dH Al2O3 + 3( 285.8) -2(1277)
dH Al2O3 = 1717.6 KJ/mol
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