Understanding Keq: Effects of Concentration Changes on Equilibrium

[Fusilli_Jerry89]

I have read in many places that if you change the concentration of reactants or products in an equilibrium equation, that it re-shifts, and the keq doesn't change. However, doesn't it change, but just ever so slightly? Because when you remove a reactant, yes that reactants concentration does go back up, but no completely back to its original concentration, so wouldn't the Keq have to change ever so slightly?

 

[chemisttree]

By definition the equilibrium constant is... constant.

 

[Gokul43201]

I have read in many places that if you change the concentration of reactants or products in an equilibrium equation, that it re-shifts, and the keq doesn't change. However, doesn't it change, but just ever so slightly? Because when you remove a reactant, yes that reactants concentration does go back up, but no completely back to its original concentration, so wouldn't the Keq have to change ever so slightly?

No, you've misunderstood. For K to be constant, the reactant concentration can not go back up to its original value (unless there's an infinite supply of the product). The reactant concentration goes back up some (but not all the way, so it is less than before), and the product is also decreased (in order to remake some of the reactant). The appropriate ratio is a constant.

 

[Fusilli_Jerry89]

so the Keq never changes? not even by 0.0000000001?

 

[chaoseverlasting]

It depends on temperature. If the temperature is constant, then the keq doesn't change. Its just that. A constant.

 

[GCT]

I have read in many places that if you change the concentration of reactants or products in an equilibrium equation, that it re-shifts, and the keq doesn't change. However, doesn't it change, but just ever so slightly? Because when you remove a reactant, yes that reactants concentration does go back up, but no completely back to its original concentration, so wouldn't the Keq have to change ever so slightly?

The equilibrium constant can be derived from the ratio of the rate constants that are involved in the particular reaction, it is also linked to the thermodynamic parameters, such as the standard Gibb's, in association with the reaction.

Although I'm not aware of any direct connections between thermodynamic parameters and quantum mechanical descriptions, the equilibrium constant may be predictable and is deemed constant with respect to a specified temperature.