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Because delta H = enthalpy of final(or product) - enthalpy of initial(or reactant). Exothermic reactions release energy, therefore the enthalpy of the product is less than the enthalpy of the reactants. A small number - a bigger number results in a negative value for delta H. And vice versa for endothermic reactions (absorb energy). I hope this helps!
Exothermic reactions have a negative overall enthalpy because energy is released when bonds are formed. Thus the products require less chemical potential energy than reactants, to hold atoms together. For endothermic reactions, the overall enthalpy of the reaction is positive because energy is required to break bonds. The reactants require less chemical energy than the products do, to hold atoms together.
If you think about it logically, it requires energy to break chemical bonds. If it takes energy to break bonds, energy must be absorbed by the reactants to break those bonds and so that energy comes in the form of heat. If the enthalpy of the reaction or delta H is positive, it means more energy is absorbed or taken into the system than is released to the surroundings, meaning the reaction is endothermic. Vice versa
H is calculated by taking final-initial, if the final has less H than the initial (-), then that energy had to go somewhere, it was released into the world, exothermic. If the final has greater H than the initial, then it needed energy from somewhere and was absorbing energy from the outside world (+), endothermic.
delta H has a negative or positive value because it is whether a reaction is absorbing heat (breaking down molecules) or giving off heat (consolidating molecules). If you're going from a solid to gas it will be endothermic and delta H will be positive because you need to break down the solid to transition to a gas phase. On the other hand if you're going from a liquid to a solid delta H would be negative because the molecules need to bind together and become a solid. It helps to think of an Ice cube, if you have water and put it in the freezer it becomes a solid but then warm it up and it becomes a liquid. Thats a visual and theoretical explanation. Delta H is defined mathematically as Heat final- Heat initial, so if Heat final is less than delta H will be negative because the reaction gave off heat and so on.
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