Why do reversible reactions occur in the first place?

[TheExibo]

It is generally said that reactants react so that they can achieve a lower energy state. Then why does a reversible reaction occur in the first place?

If catalysts lower activation energy both ways in a reversible reactions by making the transition state less thermodynamically unfavourable, then why is it that the transition state sometimes decides to go backward to form the reactants?

 

[oz93666]

A catalyst provides an alternative route for the reaction with a lower activation energy." It does not "lower the activation energy of the reaction"..

The concentrations of the reactants is important ... if one of the reactants exits the system ...Ca CO3 + 2HCl → CaCl2 + H2O + CO2↑... here the CO2 escapes and the reaction is called 'irreversible' ... If the CO2 was retained in the reaction environment then the reverse reaction would also be occurring and equilibrium would be reached ,
I believe it's true that all reactions could be considered reversible under some conditions.

To put it another way ...in the reaction above , the solution will contain all the ions , bumping into one another and reacting , going in both directions ... the reaction which requires most energy input will occur less frequently.

 

[TheExibo]

A catalyst provides an alternative route for the reaction with a lower activation energy." It does not "lower the activation energy of the reaction"..

The concentrations of the reactants is important ... if one of the reactants exits the system ...Ca CO3 + 2HCl → CaCl2 + H2O + CO2↑... here the CO2 escapes and the reaction is called 'irreversible' ... If the CO2 was retained in the reaction environment then the reverse reaction would also be occurring and equilibrium would be reached ,
I believe it's true that all reactions could be considered reversible under some conditions.

To put it another way ...in the reaction above , the solution will contain all the ions , bumping into one another and reacting , going in both directions ... the reaction which requires most energy input will occur less frequently.

Right but how can a transition state decide to go backwards? Or is a reverse reaction a completely different mechanism with a different transition state?

 

[oz93666]

... but how can a transition state decide to go backwards?

I don't see why not ...it just requires a large input of energy from random molecular/ionic motion , this back reaction will be much more less frequent than the reaction going the other way.,,, effectively undetectable .

 

[Ygggdrasil]

Right but how can a transition state decide to go backwards? Or is a reverse reaction a completely different mechanism with a different transition state?

The reverse reaction proceeds with the same transition state. Remember that chemcial reactions are thermally driven, and thermal energy provides the energy to overcome the activation energy required. Molecules that at the transition state can either go forward or backward (in fact the transition state can be defined as the point in the energy landscape where 50% of the molecules will become product and 50% of the molecules will go back to being reactants. All of this comes from the principle of microscopic reversibility. (see also http://www.ilpi.com/organomet/microscopic.html)