# What is the precise relationship between enthalpy and heat?

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.

• Why was this downvoted? – Dominik Car May 12 '18 at 10:58
• It's been edited since the downvote, prior to which I gather it lacked clarity and or evidence of research. – thetada May 12 '18 at 11:11

"[…]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.

• Thanks Philip, appreciate it. So in terms of how I could accurately navigate this issue, it would be reasonable to say that an exothermic reaction leaves the products with a greater average kinetic energy than that of the reactants. This increased kinetic energy is then transferred to the neighbouring particles of the surroudings via conduction / convection / radiation. So the heat transfer is between the particles involved in the reaction and its surroundings. – thetada May 12 '18 at 11:10
• This, to me, gives the right intuitive sense of what's going on. I'd want to say "average random kinetic energy of particles" rather than just "average kinetic energy". [Note that (kelvin) temperature is proportional to average random kinetic energy of particles for an ideal gas, but that for real systems there is generally not strict proportionality. I don't think this makes your qualitative argument invalid.] – Philip Wood May 12 '18 at 11:40

I tend to define heat as

a transfer or transformation of energy by microscopic means

and in that framework I would note that the usual suspects of conduction, convenction and radiation are means of heat transfer. On the other hand a chemical reaction going on inside the system introduces a transformation.

For an exothermic reaction chemical energy is transformed (at least partly) into thermal energy, which is modeled in a thermodynamic framework in the same terms and symbols (usually $Q$) as heat transfers.