Thermal Reactions Query

[rktpro]

Are all exothermic reactions combination reactions?

Also, are all endothermic reactions decomposition reactions?

[mississippichem]

Are all exothermic reactions combination reactions?

Also, are all endothermic reactions decomposition reactions?

 

A quality question

 

Not always, the decomposition of some very unstable species can be exothermic overall. Bond breaking always requires energy input, so is always an endothermic step (a chemical bond by definition is a sort of potential well). However, another bond could immediately form with another atom; an exothermic step. So it really depends on if you're talking about individual mechanistic steps, or entire chemical processes.

Edited March 16, 2011 by mississippichem

[Horza2002]

There are some isomerisation reactions that are exothermic. For example, a carbonyl is more stable than the corrosponding enol form....so when a compound isomerises from the enol to the carbonyl compound, heat will be released and therefore be exothermic.

[rktpro]

Thanks a ton!

[rktpro]

A quality question

 

Not always, the decomposition of some very unstable species can be exothermic overall. Bond breaking always requires energy input, so is always an endothermic step (a chemical bond by definition is a sort of potential well). However, another bond could immediately form with another atom; an exothermic step. So it really depends on if you're talking about individual mechanistic steps, or entire chemical processes.

 

So, I should know the energy profile of a reaction rather than just estimating that decomposition is always an endothermic process.

H2 + I2 = 2HI. Such a reaction can only be judged when given an energy profile.

So energy profile of all reactions should be learnt. Isn't this a difficult thing?

[mississippichem]

So, I should know the energy profile of a reaction rather than just estimating that decomposition is always an endothermic process.

H2 + I2 = 2HI. Such a reaction can only be judged when given an energy profile.

So energy profile of all reactions should be learnt. Isn't this a difficult thing?

 

It is somewhat of a difficult thing. We can run thermochemistry in a bomb and observe heat changes, this can get complicated when intermediates are involved though as the heat capacity of the mixture varies as the reaction progresses. This is only done if you want the gory detail.

 

Here is a simpler approach. For a general reaction:

 

[math] \sum \Delta H^{0}_{rxn} = \sum \Delta H^{0}_{products}- \sum \Delta H^{0}_{reactants} [/math]

 

We can derive any of these summation terms by adding up all the elementary steps involved. For example, the ionization and solvation of sodium metal would be as follows:

 

[math] \sum \Delta H^{0} = \Delta H^{0}_{atomization}+\Delta H^{0}_{ionization}+\Delta H^{0}_{solvation} [/math]

 

Atomization, is the energy input required to break a sodium atom from the crystal lattice. Ionization is the energy input required to remove an electron from the outer orbital. Solvation is the final step where the ion finds a solvent coordination shell and is exothermic. We could go into more detail than that. Technically, energy input is required to break the interactions between solvent molecules.

 

Finding a detailed reaction/energy profile for a reaction can be quite difficult and requires a lot of kinetics. After a while at university, you can get a feel for what the reaction progress/energy diagram would look like for certain reactions. Intermediates usually causes "valley's" in the profile and transition states cause "humps". It's possible to develop a somewhat intuitive feel for some of these things.

 

EDIT: All the little [math] ^{0} [/math] super scripts just denote standard conditions.

Edited May 27, 2011 by mississippichem