Sign changes

[Nicole Garrido 2I]

When adding two enthalpies can the total change signs, such as if the enthalpy of the first is positive and you add a negative one and the negative one is higher and what would that mean for the reaction?

[Cynthia Ulloa]

yes, the signs can definitely change when adding them up to get the total. As for the negative and positive signs, they let us know whether the reaction was endothermic, absorbing heat, or exothermic, releasing heat.

[Theodore_Herring_1A]

Yes they can, and if the enthalpy is negative it is an exothermic reaction, while if the enthalpy is positive it is an endothermic reaction.

[caseygilles 1E]

Yes, the total can change signs depending on whether it is endothermic or exothermic. A negative change in enthalpy indicates a release of heat through forming more bonds than breaking and a positive change in enthalpy indicates that heat was used to break bonds. Remember that when products are formed, the change in enthalpy of formation should always be negative, therefore we need to change the sign from positive to negative of the value given in the textbook for deltaHf of that molecule. Also remember deltaHf of elements is zero.