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?
2007-03-04 14:59:28 · 3 answers · asked by helllooooo 2 in Science & Mathematics ➔ Chemistry
Enthalpy of an exothermic RXN is negative. The reason is that the products have a lower stored-energy content than the reactants did. Energy was released when the RXN occurred.
2007-03-04 15:03:38 · answer #1 · answered by physandchemteach 7 · 0⤊ 0⤋
The practical considration is as follows:
Since in exothermic reactions, the heat leaves the system, it has a negative sign. In an endothermic reaction, the system gains heat, so it has a positive sign.
In calculating the heat of a reaction, we subtract the potential energy of the reactans from the products. In an exotherm, the reactants have a higher energythan the products, so products minus reactants gives a negative number. Consistent witht e above paragraph. Similarly in an endothermic reaction, products minus reactants gives a positive result.
The units of enthalpy are the units of energy. Joules are the SIU. Calories are also used in mteric for heat. The English system uses BTU's.
2007-03-04 15:06:10 · answer #2 · answered by reb1240 7 · 0⤊ 0⤋
It's negative, as the enthalpy of the products is lower than that of the reactants.
2007-03-04 15:02:37 · answer #3 · answered by TheOnlyBeldin 7 · 0⤊ 0⤋