Reverse reactions always the opposite?

[Toadie]

Hi all-

 

Sorry for asking such an elementary question... I feel like I'm getting progressively worse at chemistry as I go through the AP course.

 

Anyway, I'm studying for the midterm (which is tomorrow), and I'm reading over some stuff about Le Châtelier's principle, specifically the effect of temperature changes. I understand it all and don't really have a problem, per se, just a bit of a curiosity.

 

The book explains the effect of temperature changes very well in terms of endo- and exothermic reactions:

"In an endothermic reaction, we can consider heat as a reactant, whereas in an exothermic reaction we can consider heat as a product."

 

So my question is: Is the reverse reaction always the opposite? I.e., if the forward reaction is exothermic, is the reverse reaction always endothermic? Are there any cases where both the forward and reverse reactions are exothermic or both endothermic?

 

 

Thanks

[Kaeroll]

In brief: yes.

 

The energy change of a reaction depends only on the start and end points, not the route taken. Going from A to B (under the same conditions) has the same energy change regardless of the route taken. Going from B to A has the equal and opposite energy change regardless of the route taken.

 

Think of it this way: if A -> B is exothermic, B must be in a lower energy state than A. For B -> A to be exothermic too, B would have to be higher energy than A. All else being equal, this is just not possible.

[Mad Scientist]

If the reactants react to give an exothermic reaction the the enthalpy change will be a minus ie heat is given out. For the reverse reaction the enthalpy change will have to be positive meaning that energy is taken in making the reverse reaction endothermic. I have never seen a case of both the forward and the reverse reactions eing both exothermic or endothermic.

Edited February 17, 2010 by Mad Scientist
Typo

[JGK]

If the "Reverse" reaction did not produce the starting materials it would not be a "reverse" reaction, you would have

 

A + B ------> C + D -----> E + F

 

Instead of

 

A + B ------> C + D -----> A + B

 

[John Cuthber]

If the reaction were exothermic or endothermic in both directions it would violate the principle of conservation of energy.

As Homer Simpson once said "In this house we obey the laws of thermodynamics!"

[farmboy]

If the reaction were exothermic or endothermic in both directions it would violate the principle of conservation of energy.

As Homer Simpson once said "In this house we obey the laws of thermodynamics!"

 

Lol, was going to post something similar. Such a reaction could solve all the worlds energy needs.

[John Cuthber]

So, apparently, could Lisa Simpson.