Lechatelier Principle

 Lechatelier's Principle :

Imagine there are 2 room (A and B) connected with each other and 100 students sitting in each room . Since both rooms are connected equal number of students (say 10) move from one room to another in a given time interval without affecting total number of students in each room . We say it as equilibrium . Why? Because movement (reaction ) occurs in both direction without affecting the strength (Concentration) of each room .

 If any external stress is induced in any room the net flow of number student from one room to another will change , and we say equilibrium will get disturbed . This is natural , system fights back to counter external force . 

Similar situation arise if any reversible reaction which is at equilibrium faces any external force . Rate of forward and backward reaction does not remain equal on application of external force (means equilibrium gets disturbed). We can relate it with concept of Reaction Quotient(Q) . 

At equilibrium any of the measurable parameter of reactant or product is forcefully changed, reaction starts from scratch to counter it. Reaction again comes to most stable state (Equilibrium state) after some time. However, changed parameter need not come back to its initial value when equilibrium is reestablished.  

 what can be external force in chemistry ? I will discuss one by one here. 

1.  CONCENTRATION(Moles)/Partial pressure :

Lets imagine our room analogy .  If we increase number of students to 110 in Room A or decrease number to 90 in room B in both case flow students from room A to B will be more and equilibrium will get disturbed . Its opposite is also true . If we decrease number to 90 in room A and incrase number to 110 in room B , flow of students from room B to A will be more . same thing takes place in reactions also . 

Since both concentration and partial pressure are measure of active mass, they have same effect in gaseous reaction. 

* By increasing concentration/moles/partial pressure of reactant or decreasing concentration/moles/partial pressure of product equilibrium shifts into forward direction (Qc < Kc) or (Qp<Kp)

* By increasing concentration/moles/partial pressure of product or decreasing concentration/moles/partial pressure of reactant equilibrium shifts into backward direction (Qc>Kc) or (Qp>Kp)

Note: that changing moles/concentration of solid reactant or product will have no effect on equilibrium , provided some amount of solid remains present. This is because activity of pure solid is defined as unity(constant). Also, pure solid is not present in the expression of reaction quotient(Qc). If you completely remove the solid reactant the reaction cant proceed and equilibrium collapses.  

Note: that Both concentration and partial pressure are measure of active mass of gaseous species. Just like concentration ,increasing partial pressure of Reactant favors forward reaction (Qp <Kp) and increasing partial pressure of product will favors Backward reaction (Qp > Kp) . Do not confuse Partial pressure with total pressure.

Please remember that in above points I mentioned partial pressure not Total pressure . Effect of total pressure is explained in later part of this article separately. 

2.  PRESSURE/VOLUME  :

a) Reactions involving gas/gases (With or without solid/liquid):

We know, pressure and volume of gases are inversely related . Pressure can be increased by decreasing volume of container and vice-e-versa.  

if pressure is increased/decreased volume of container, reaction proceeds into net direction where fewer moles of gas are present and if pressure decreased/increase volume of container reaction proceeds into net direction where more moles gases are present. 

A reaction having equal number of moles in reactant and product will have no effect of pressure/Volume change 

b) Reactions or phase change involving only solid and/or liquids:

If pressure is increased then equilibrium shifts in the direction which produces more dense phase . In general density of solid is more than liquid (except water where density of liquid is more than solid) .

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3. TEMPERATURE:

A  + Q ⇌ B   (forward endothermic and backward exothermic ) 

Increasing temperature favors endothermic direction ,because system consumes heat .

A⇌B + Q   (forward exothermic and backward endothermic ) 

Decreasing temperature favors exothermic direction , because system don't need heat. 

4.CATALYST:

Catalyst only changes activation energy of elementary reaction (both forward and backwards ). Kc = Kf/Kb remains unaffected by catalyst . Since catalyst alters rate of both forward and backwards reaction in same ratio .It has no effect on equilibrium composition. It only changes time of attainment of equilibrium . 

5.Addition of inert gas:

In this case there are 2 scenarios:

a) at constant Volume and temperature:

Increases the total pressure, but partial pressure of both reactant and product remains unchanged (because mole fraction of gases decreased by addition of inert gas : Dalton's law of partial pressure)  . Thus, no shift in equilibrium 

b) at constant pressure and temperature :

Increases the volume . It will have same effect as discussed in increasing volume . Means equilibrium shifts into direction where more number of moles of gases are produced .   

Note: Equilibrium constant doesn't change by changing any of these parameter except Temperature . 

QUESTIONS: