Energetically favourable / Exothermic? Energetically unfavourable / Endothermic?

[Enhancement]

First off, allow me to introduce myself. I'm a student at the University of Waterloo in Waterloo, Ontario, Canada. I was in the International Development program for a couple years but left because it was a very poorly designed program. In the fall I will be starting the Biotechnology/Economics Co-Op program.

 

Anyway, I've been reading my Cell Biology textbook to get a head start. Just finished the third chapter, which kept talking about "energetically favourable" and "energetically unfavourable" reactions. Is energetically favourable synonymous with exothermic? Is energetically unfavourable synonymous with endothermic?

 

If not, how are they different? If so, why are they throwing new terms at me?

[studiot]

 

which kept talking about "energetically favourable" and "energetically unfavourable" reactions. Is energetically favourable synonymous with exothermic? Is energetically unfavourable synonymous with endothermic?

 

Thermodynamically that is correct.

 

 

However many reactions are also subject to 'activation energy', which has to be supplied before the reaction will progress, even if you finally get out more than you put in.

 

Many reactions are spontaneous ie they will proceed as soon as the reactants contact. These are all exothermic. You do not need to add anything else to cause the reaction, unlike endothermic reactions, which are non spontaneous since you need to add energy for them to proceed.

Edited July 4, 2013 by studiot

[daniton]

In other way you could understand exothermic and endothermic chemical rxns as formation of bonds which really is. In endothermic rxns chemical bonds get broken so as to proceed and that is why they need energy input but exothermic rxns don't need energy because they are forming bonds so they release energy.

This may be the reason why they're called energetically favourable and unfavorable.

Edited July 5, 2013 by daniton

[studiot]

I have had a rethink about my first answer, which was too hasty. My apologies.

 

The terms exothermic and endothermic refer specifically to the generation or absorption of energy as heat by the process. Other forms of energy change (eg work) are not included.

Nor are entropy changes included.

 

To account for this further thermodynamic functions called free energy have been defined.

 

The free energy may be negative because heat is evolved in an exothermic reaction (eg combustion) or it may be negative because of a large entropy change which outweighs the positive heat input in an endothermic process (eg vapourisation)

 

This, I think, is what was meant by an energetically favourable reaction.

 

We can expand on this further if you wish. Have you heard of Gibbs or Helmholtz free energy?

 

Please note that just because a reaction is energetically favourable it does not mean it will necessarily occur, because of what I said before about activation energy. For instance you can have a container of hydrogen and oxygen without them reacting. But apply a spark!

[Enhancement]

Gibbs or Helmholtz free energy? Nope, haven't heard of it.

[daniton]

which one?both ?

we usually use Gibbs free energy for chemical reaction.

Edited July 7, 2013 by daniton

[BabcockHall]

Exothermic and endothermic refer to enthalpy. But spontaneous and nonspontaneous refer to Gibbs' free energy. It is important not to confuse the two.

Edited July 9, 2013 by BabcockHall