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Exothermic reactions have a negative delta H because the enthalpy of the products have less enthalpy than the reactants, and delta H is the difference between the products and the reactants, making the delta H negative. In endothermic reactions the products have a higher H than the reactants, making delta H positive.
Because an exothermic reaction releases energy and the energy of the products is lower than the energy of the reactants, the enthalpy is negative. An endothermic reaction absorbs energy so the energy of the products is higher than the energy of the reactants, so the enthalpy is positive.
Exothermic reactions have a negative value because it releases energy so products are lower than reactants. Vice versa for endothermic reactions which have a positive value because it requires energy so the reactants are lower than the products.
The enthalpy for an exothermic reaction is negative because the system/reaction loses energy, and the surroundings gain energy. Similarly, an endothermic reaction requires energy, so the enthalpy value is positive.
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