[Hushai]

Hi,

I am a new Chemistry teacher and I am trying to understand heat.  In thermodynamics, heat is defined as a transfer of energy due to a difference in temperature.  In the Zumdahl textbook it says that in an exothermic reaction heat is released to the surrounding.  However it does not say that the system has a higher temperature than the surrounding.  "potential energy in chemical bonds has been converted to thermal energy (random kinetic energy) via heat" (page 231).

My questions are these:

Is the system actually always hotter than the surrounding in an exothermic reaction?  If the surroundings had a higher temperature than the system, would we get an exothermic reaction?

Could heat and work (force acting over a distance) be distinguished and defined as microscopic and macroscopic work respectively?

Thank you

[Borek]

Is the system actually always hotter than the surrounding in an exothermic reaction?  If the surroundings had a higher temperature than the system, would we get an exothermic reaction?

No, yes.

[Hushai]

Let me rephrase one of my questions:

in an exothermic reaction heat is said to be released to the surroundings even though the system might be colder than the surrounding.  Since heat is defined as a transfer of energy from a hotter system to a cooler system, why is the energy going out of system due to an exothermic reaction still called "heat"?

[Borek]

Heat is one of the forms of energy. Definition that takes into account heat transfer only can't be used to describe what happens when you convert one type of energy into another. At the same time we know that temperature change of something means it either absorbed or gave out some heat. If you take a reaction mixture and allow it to react, its temperature changes - that means heat was consumed or produced.

[Hushai]

I understand your answer.  Then why heat is always defined as energy transfered from a hottest body to a coolest body?
Could heat be simply defined as the kinetic energy of the random motion of particles?
Thank you

[Borek]

Could heat be simply defined as the kinetic energy of the random motion of particles?

I don't feel confident on this ground, but from what I remember other definitions lead to some problems and inconsistencies.

[rabolisk]

Let's be clear on some things. Heat is not a type of energy that a system possesses. It is a process of energy transfer. It would not be right to define heat as kinetic energy of random motion of particles, because this implies that heat is a state function.

If a system was at a lower temperature than its surrounding, it's implied that there is an adiabatic wall between the system and the surrounding. If an exothermic reaction happened within the system, no heat would be transferred from the system to the surrounding; the temperature of the system would just increase. In most cases, though, the system and the surrounding would be at equal temperature (diathermic), so the exothermic reaction would result in heat flowing out of the system (which is now at a higher temperature due to the reaction) to the surrounding, raising the temperature of the surrounding. Think about bomb calorimetry as an example.

[Borek]

Heat is not a type of energy that a system possesses. It is a process of energy transfer.

I know this is the way it is stated in many places, but I don't like this approach. When you have an object that has a kinetic energy (say a bullet) and it is stopped, kinetic energy disappears. As energy is conserved, some other form of energy must appear. If heat is not a form of energy, what happened to energy? Same problem with radiation absorbed by some object.

As I stated earlier, I seem to remember that heat is treated as a process to avoid some other problems, but it seems to me this approach produces its own problems. What am I missing?

Edit: I see this can be just problem with naming. Heat is exchanged in process, "heat stored in body" is a thermal energy.

[Hushai]

After reading Borek last post , I search on the internet "heat and thermal energy" and I found the website "ask a scientist". 
A post had the question:  "what is the difference between heat and thermal energy?" The answer given by all was: "Heat IS thermal energy"  and therefore heat would be a form of energy. 

Is there actually an agreement in the scientific community about all this?

[Hushai]

I would like to add what I found in Schaum's Thermodynamics with Chemical Applications:

[Jorriss]

An exothermic reaction where the temperature of the surroundings is higher than the temperature of the system will not occur, at least the equilibrium won't be shifted towards the products.

Levine defines heat as, m2c2(T2-Tf)=m1c1(Tf-T1) is defined as q. He defines heat as you did. When two bodies have different temperatures, energy transfers until they are in thermal equilibrium. The transfer of this energy is heat.

[rabolisk]

Heat & temperature are related but different. According my view (not the book), heat is a parameter which specifies the temperature status of a material. In other sense, heat is the result of internal change in energy.