What determines if a reaction is exotermic or endotermic?

[FG_313]

Using a more physics oriented approach, rather than a chemistry one, and looking to an energy point of view if possible. For example, why when ice turns into water there is a need for energy (heat)? I know in some extent that the energy is used into re-organizing the chemical bonds, but that seems very superficial. And also I don't really understand the more general cases, like things burning and "making fire". Thank you for the responses in advance, anything will be helpfull.

 

[Astronuc]

Endothermic reactions (phase change from solid to liquid, or liquid to gas) require heat input to the system, as opposed to exothermic reactions which generate heat.

In the case of ice (solid) to water (liquid), the molecules of water require more kinetic energy (from an external source) to overcome the intermolecular forces.

In the case of combustion, the oxidizer (oxygen) combines with the fuel (e.g., hydrocarbon) whereby the molecules of oxygen and fuel (reactants) reform to products such as H₂O and CO₂, which releases the stored chemical energy from the bonds (C-C, C-H). Usually there is some initial energy to start the reaction, which then proceeds based on the energy generated from the reformation of fuel and oxygen molecules.

 

[FG_313]

Thank you very much for the fast response. However, when thinking about heat as chaotic motion of molecules, I get confused. How does an exothermic reaction or a combustion causes that kind of motion? And why does a more caotic motion of molecules causes an endothermic reaction to happen?

 

[DrDu]

I don't know what you mean with a more physics related approach. This is a classical topic of chemical thermodynamics, and it is less about the change of energy than about the change of entropy.
When melting water, you have to break hydrogen bonds between the water molecules. This costs energy. At the same time, the entropy increases, as a water molecule in liquid water have more degrees of freedom to move around.
Taking both things together, water will melt if the increase of entropy of the surrounding, which is given as $\Delta H/T$, equals the entropy change of the liquid, which is $\Delta S$. I. e. the melting temperature is given as $T_m=\Delta S_m/\Delta H_m$.

 

[FG_313]

Thank you! But I meant a descripiton on how it happens and not "why"...