Heat in Equations

[yazzs]

Hello,
Is there a way to figure out if a reaction is endothermic or exothermic by just looking at the reaction? What is the minimum information needed to determine this?
Thanks.

[dylani]

Hi! While I am not quite sure as well, I believe that most of the questions we will encounter have the change in heat listed. Now with the change in heat (delta H with a degrees symbol) you can determine whether the reaction is endothermic or exothermic. For example for a forward reaction, if the change in H is positive, then the reaction is endothermic and adding heat will favor the product forming, whereas if the change in H is negative, then the reaction is exothermic (gives off heat) and adding heat will favor reactants forming. The change in Heat is most likely the minimal amount of information we will need to determine whether a reaction is endothermic or exothermic for this course. However, if not given the change in heat, you can draw out the structure/Lewis diagrams for a reaction to see if a bond breaks or forms during the reaction. For example in 2NO₂ --> N₂O₄, a bond is formed which releases heat, making the reaction exothermic. If a bond were to be broken, this requires heat, which would make the reaction exothermic.

[dylani]

Hi, I just wanted to revise some of statement before as we have started thermodynamics and I realize that I referred to the delta H symbol as change in heat, but it is actually enthalpy and wanted to make this correction. The reasoning is still the same but did not want to cause confusion as enthalpy is the amount of heat released or absorbed at a constant pressure, which is denoted by the delta H symbol, it is not simply just change in heat, though it can be thought of in this simplified way.

[Casey Fineberg]

In general, the form of the equation does not indicate whether a process is exothermic or endothermic. You must either measure the heating or cooling or be familiar with similar types of responses. For example, I can guess that H2 -> 2H is endothermic because H2 has less energy than the 2 H's. However, in more complex equations, you won't necessarily be able to tell.