Relationship between Kc and Kp
In a reaction,aA + bB = cC + dD
we can write
Assuming the gaseous components to behave ideally,
PiVi = niRTor
where (i) is the molar concentration of the species 'i'.
n = (Number of moles of gaseous products)-(Number of moles of gaseous reactants).Thus, n is equal to the difference in the number of gaseous moles of products and the number of gaseous moles of reactants. The above equation can be rewritten as
Kp = Kc (RT)n
2. Write the relationship between Kp and Kc for the following reactions.
(a) Here, n = 1+ 1 - 1 = 1Kp = Kc (RT)1= KcRT
(b) Here, n = 2- (3+1) = -2Kp = Kc (RT)-2
(c) Here, n = 2- (1+1) = 0Kp = Kc (RT)0 = Kc
(d) Here, n = 2+1-2 =1Kp = Kc (RT)1= KcRT
(d) Here, n = 2- (2+1)= -1Kp = Kc (RT)-1= Kc / RT
3. At 700 K, the equilibrium constant, Kp for the reaction
is 1.80 x 10-3 kPa. What is the numerical value in moles per litre of Kc for this reaction at the same temperature?
For the reaction,
Kp = 1.80 x 10-3 kPa, Kc = ?We know that, Kp = Kc (RT)n
where n = ng(products) - ng(reactants)For the given reaction,
n = 2 + 1 - 2 = 1So, Kp = Kc (RT)1= Kc x RT
R = 8.31 L kPa K-1 mol-1
4. Two litres of a solution of acetic acid contains 15 g of acetic acid. What is the active mass?
Molecular weight of acetic acid = 60Weight of acetic acid = 15 gms.
Volume of the solution = 2 litres.
Active mass = number of moles/litre
Here the products are X and Y and reactants are A and B. Thepowers of the concentrations of A, B, X and Y correspond to the
respective number of moles in the equation.
6. Write down the expression for the equilibrium constants
What is the relation between Kp and Kc for this reaction?
n = number of moles of products - number of moles of reactants
= 3 - 5 = -2Kp = Kc (RT)n = Kc (RT)-2
Kp = Kc(RT) -2 for this reaction.