Are Combination Reactions Always Exothermic?

[Miike012]

Homework Statement

During a combination reaction such as..
P4(s) + 5O2(g) = P4O10(s) : DeltaH = -3010..

And for the decomposition reaction...

P4O10(s) = P4(s) + 5O2(g) DeltaH = 3010

Questions
1. Is it true that for any combination reaction that Enthalpy will be neg, and decomposition reactions that enthalpy will be positive?
2. when enthalpy is negative, this means that energy is being released? and when something requires energy, enthalpy will be pos?

 

[QuarkCharmer]

I don't really know how to go about helping you without giving you the answer outright, but I will try.

If the enthalpy is negative, it is an exothermic reaction. So the real question is, "Are all combination reactions exothermic?", and "Are all Decomposition reactions endothermic?".

For the most part that is true, but there are some exceptions. Dicyanoacetylene, for example, has such a high endothermic formation heat, it can sometimes turn into carbon and nitrogen, which would be exothermic.

For question #2, the very definition of exothermic is that the reaction gives off heat. If the deltaH is negative, wouldn't that mean that energy is being released in the form of heat?