Equilibrium (2) The significance of the value of Kc and Kp

 Edexcel A level Chemistry (2017)

Topic 11: Equilibrium II:

Here is the third learning objective:

11/II/5. To be able to understand that the value of the equilibrium constant is not affected by changes in concentration or pressure or by the addition of a catalyst

Within this topic, students can consider how chemists can use the concept of equilibria to predict quantitatively the direction and extent of chemical change.

The equilibrium constant is related to both temperature and the enthalpy change of a chemical reaction:

There is linear relationship between K and 1/T provided ΔH^⦵ remains constant.  This is in fact the case to all intents and purposes, since ΔH^⦵ changes very slowly with temperature.

But concentration and pressure changes have no effect on K for one reason is that changes in pressure on a gaseous equilibrium are only effective if there is a difference in the number of moles of reactants and product as here:

N₂(g)    +     3H₂(g)   ⇌      2NH₃(g)

But if the reaction is this:

                             H₂(g)   +    I₂(g)   ⇌    2HI(g)

Then changes in pressure on this system have no effect on the position of equilibrium or therefore on K_p.

K_p remains constant if the pressure (or the concentration) changes. 

Addition of a catalyst is a different matter.

Catalysts lower the activation energy of a reaction and bring more molecules into a state where they might collide with others and break and make new bonds and form new products.

Catalysts are very inclusive.

But how do they work in equilibria?

Here is the typical energy profile of catalysed reaction:

The catalyst lowers the activation energy of the forward AND the backward reaction.

Therefore it cannot affect the position of equilibrium.

All it can do is reduce the time it takes for the reaction to reach equilibrium since the catalyst speeds up both forward and backward reactions.

Therefore a catalyst or pressure or concentration have no effect on the value of the equilibrium constant.

What the equilibrium constant does tell us is how far a reaction has gone.

From the value of an equilibrium constant we can tell if a reaction has gone to completion or not.

If

N₂O₄(g)    ⇌   2NO₂(g)  ΔH= +57.2kJmol^–1

At 600K, K_p = 13800 atm

This means that the reaction has virtually gone to completion.

The partial pressure of N₂O₄ must be very low and that of NO₂ very high for this value of K_p.

Similarly at 100K, K_p  =  3.61×  10^–21 atm which is incredibly small and means that in the equilibrium mixture there is mainly N₂O₄ and very little NO₂, in fact the decomposition of N₂O₄ has hardly begun.

So K the equilibrium constant is a measure of the extent of chemical change and therefore will be related to ΔG, the free energy change, ΔS_total the total entropy change and E^o the redox potential because all these thermodynamic quantities are also measures of the extent of reaction.