Expression and Units Of Equilibrium Constant

Expression for Equilibrium constant ( K )

The active mass of a pure solid is constant irrespective of its amount and if a pure liquid is present in excess, its active mass is also constant.In both the cases we put active mass equal to 1.

Molar concentration of a substance means mol L^-1  of the substance which is obtained by dividing the amount of the substance in moles by the volume of the substance in litres.

Molar Conc. = Moles of the substance / Volume of the substance

Molar Conc. =(  Mass of the substance / Molecular mass ) / Volume of the substance

Molar Conc. = Density of the substance/ Molecular mass of the substance

As density of a particular pure substance at a particular temperature is constant and molecular mass of the substance is also constant, therefore, molar concentration is constant.

For Homogeneous Equilibrium

CH₃COOH ( l ) + C₂H₅OH ( l )   CH₃COOC₂H₅ ( l ) + H₂O ( l )

NH₃ ( g ) + 5O₂ ( g )  4NO ( g ) + 6H₂O ( g )

For Heterogeneous Equilibrium

Units Of Equilibrium Constant

K_eq = ( mol L^-1 ) ^c+d / ( mol L^-1 ) ^a+b

K_eq =( mol L^-1 ) ^{(c+d ) – (a + b )}

K_eq = (mol L^-1 ) ^Δn

K_p = (atm) ^(c+d)  / (atm)^a+b

K_p = (bar) ^(c+d)  / (bar)^a+b

K_p = ( atm or bar )^{(c+d) – (a+b)}

Kp = (atm)^Δn  or (bar)^Δn

If Δn =0 i.e. number of moles of products = number of moles of reactants , K_eq or K_p will have no units.

For Ex:

1) H₂ ( g ) + I₂ ( g )  2HI ( g )

Δn =0 , K_c or K_p will have no units.

2) N₂ ( g ) + 3H₂ ( g )  2NH₃ ( g )

Δn = 2 – ( 1+ 3 ) = -2

The units will be ( mol L^-1 )^-2 or atm^-2 or bar^-2