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 ) sign of equilibrium reaction  CH3COOC2H5 ( l ) + H2O ( l )

Equilibrium constant for reaction between ethanol and acetic acid

NH3 ( g ) + 5O2 ( g ) sign of equilibrium reaction 4NO ( g ) + 6H2O ( g )

Equilibrium constant for formation of ammonia

Equilibrium constant for formation of sulphuric acid

Homogeneous equilibrium

For Heterogeneous Equilibrium

Heterogeneous equilibrium

Decomposition of sodium bicarbonate

Equilibrium constant for decomposition of sodium bicarbonate

Reaction between water and carbon

Equilibrium constant for reaction between water and carbon

Units Of Equilibrium Constant

general reaction

equilibrium constant for general reaction

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

Keq =( mol L-1 ) (c+d ) – (a + b )

Keq = (mol L-1 ) Δn

Equilibrium constant in terms of partial pressure

Kp = (atm) (c+d)  / (atm)a+b

Kp = (bar) (c+d)  / (bar)a+b

Kp = ( 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 , Keq or Kp will have no units.

For Ex:

1) H2 ( g ) + I2 ( g ) sign of equilibrium reaction 2HI ( g )

Δn =0 , Kc or Kp will have no units.

2) N2 ( g ) + 3H2 ( g ) sign of equilibrium reaction 2NH3 ( g )

Δn = 2 – ( 1+ 3 ) = -2

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

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