If enthalpy is the heat for a reaction, it must have a sign as well as a magnitude. What sign should the enthalpy for an exothermic reaction have? Why?

We know that exothermic reactions give off heat. Therefore the sign of the enthalpy would be negative because we’re losing it.

You can think of it like this

Endothermic + h

Reactant (s) + h - > product (s)

Exothermic - h

Reactant (s) - > product (s) + h