Someone said, "Calculate the total delta H of the reaction and subtract the total delta H of the product from it. If you get a positive value, it radiates heat which means it's exothermic. If it is negative, heat is needed to get a reaction so it is endothermic."
...shouldn't it be the other way around?? If you get a positive value, the enthalpy change is added to the reactants' side of the equation, i.e. more heat is absorbed, and therefore it's endothermic, and vice versa for exothermic ?
You are correct in that the statement you mentioned is actually the opposite of the correct explanation. Allow me to clarify the concept of enthalpy change and how it relates to exothermic and endothermic reactions.
Enthalpy change, denoted as ΔH, is a measure of the heat energy absorbed or released in a chemical reaction. It can be positive or negative, depending on the nature of the reaction.
In an exothermic reaction, heat is released to the surroundings. This means that the products have lower enthalpy (heat energy) than the reactants, resulting in a negative ΔH value. Exothermic reactions often feel warm to the touch, as they are transferring heat energy to the environment.
On the other hand, in an endothermic reaction, heat is absorbed from the surroundings. In these reactions, the products have higher enthalpy than the reactants, leading to a positive ΔH value. Endothermic reactions often feel cool to the touch, as they are taking in heat energy from the surroundings.
To determine whether a reaction is exothermic or endothermic, you should compare the enthalpy of the products to that of the reactants. If the enthalpy of the products is lower than the enthalpy of the reactants (exothermic), the ΔH value will be negative. Conversely, if the enthalpy of the products is higher than the enthalpy of the reactants (endothermic), the ΔH value will be positive.
In summary, for an exothermic reaction, the ΔH value will be negative since heat is released, while for an endothermic reaction, the ΔH value will be positive because heat is absorbed.