Endothermic vs. exothermic reactions
Endothermic reactions: Heat is absorbed.
Exothermic reactions: Heat is released.
Why is heat released or absorbed in a chemical reaction?
What is enthalpy of a reaction?
Depiction of an energy diagram
Energy diagrams for endothermic and exothermic reactions
- In the section entitled, "Why is heat released or absorbed in a chemical reaction," I am confused as to why it says that when chemical bonds are formed, heat is released and when chemical bonds are broken, heat is absorbed. In my science class, I was taught that when heat is absorbed, something gets hotter. Yet, in an endothermic reaction, I was taught the substance gets colder. Please help my confusion!
- You need energy to break up bonds (heat absorbed)....where does this heat come from? It comes from the surroundings.....thus during this process the surrounding close to the substance "are deprived of energy"....As a consequence of that the surroundings get colder "not" the substance that is actually getting hotter....you perceive the hotness or coldness of the surroundings not the hotness or coldness of the substances involved in the reaction.........
- The only part that confused me was the section involving the enthalpy equation. In the video labeled "Hess's law and reaction enthalpy change", the equation states H(sum of products) - H(sum of reactants). However, this is backwards in this article. Could someone further explain this difference?
This website also states the same equation as what's in the video http://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Thermodynamics/State_Functions/Enthalpy/Standard_Enthalpy_Of_Formation
Thank you in advanced!
- The equation you gave (Hproducts - Hreactants) is also a valid equation, but the interpretation of delta H would just be the opposite of what was described above. Using your equation, a negative delta H would indicate an endothermic reaction and a positive delta H would indicate an exothermic reaction. Same concept, different interpretation.
- Not really an expert in thermochemistry (or really anything yet, for that matter), but drawing from what I know about biophysics and its analysis of the world, your interpretation would be correct. The change would be representative of a process; a scalar energy value converted into a vectorial physical change. But when looking at energy diagrams, the terms reaction and process are ambiguous until you describe the alteration of the physical matter, where they are not ambiuous terms.
- Since in an exothermic reaction heat is released, the products will be more stable than the reactants. It's like saying that the products have less heat than the reactants. Since heat is needed to break bonds, the reactants are going to need less heat that the products-their bonds are weaker. :)
- I'm not sure the changing states part of this article is correct. It mentions the breaking of bonds when water changes physical state (eg. solid ice to liquid). I understood that when a substance changes state that no bonds are formed or broken., generally speaking when a substance changes state there's no chemical reaction. Quite happy to stand corrected!
- Think about it like this: When iron rusts, it is oxidizing. The surface of a piece of iron will begin to corrode first in the presence of oxygen and water. The process of rusting is a combustion reaction, similar to fire. Left in contact with oxygen, iron will react with the oxygen to form rust. In short, the bonds in the iron are breaking to form something new and in this case, it's rust.