Do decomposition reactions have positive enthalpy?

[inaccensa]

2AlCl3 (s)----> 2Al(s) + 3 Cl2 (g) delta H for AlCl3 at 25c = -164.2 kJ/mol

What is the enthalpy of reaction?

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[thebillsfan]

since delta H for AlCl3 is negative, the enthalpy for the opposite reaction (the one youre asking about) is simply +164.2. That doesnt mean all decomp rxns have positive enthalpy, but this one does.