I am struggling to resolve the definitions of heat and enthalpy of reaction. This is based on research I have been doing around the production of course materials. I have found the following definition of heat:
“If a process is accompanied by a change in the internal energy of the system, this change is the result of some kind of interaction between the system and its surroundings. If this interaction is only the result o fa temperature difference between system and surroundings it is denominated as heat. All other, adiabatic, kinds of interaction are known as work.” (From: ‘”Work” and “Heat” in Teaching Thermodynamics’, Peter van Roon, from Empirical Research in Chemistry and Physics Education International Council of Associations for Science Education, 1992.)
Next, I have found the following definition of enthalpy of reaction: “The enthalpy of reaction is the amount of heat released or absorbed in a reaction carried out at constant pressure.” (From: Evaluating of Preservice Science Teachers’ Understanding of General Chemistry Concepts by Using Two Tier Diagnostic Test, Ayfer Mutlu, Burçin Acar Şeşen, Journal of Baltic Science Education, Vol. 15, No. 1, 2016)
Since van Roon reports how commonplace it is for students to confuse heat and work, I am keen to firm up my own understanding. Since the temperature change accompanying a chemical reaction does not result from heat transfer via conduction, convection or radiation, can an enthalpy change be accurately described in terms of “heat” absorbed or released? If not, should it more accurately be described as work, as per van Roon’s definition? It may not seem like an important point, but I am keen to ensure that I am using language precisely.
Best Answer
"[…]Since the temperature change accompanying a chemical reaction does not result from heat transfer via conduction, convection or radiation […]"
I think there's some confusion here. It is confusing to ascribe the temperature change to a flow of heat. The point is, surely, that in exothermic reactions heat escapes from the system of reactants, usually by conduction through the walls of the conducting vessel. The heat flow takes place because, due to the reaction, the temperature of the reactants/products becomes greater than that of the vessel and its surroundings.
In thermodynamics, heat is not a property of a system, but is energy in transit to or from the system. However enthalpy is a property of the system. We can equate the enthalpy change to the heat flow only when the pressure is constant.