Key Difference – Kc vs Kp
Kc and Kp are equilibrium constants. The equilibrium constant of a reaction mixture is a number that expresses the ratio between the concentrations or pressure of products and reactants in that reaction mixture. The key difference between Kc and Kp is that Kc is the equilibrium constant given by the terms of concentration whereas Kp is the equilibrium constant given by the terms of pressure.
This equilibrium constant is given for reversible reactions. Kc is the equilibrium constant given as a ratio between concentrations of products and reactants whereas Kp is the equilibrium constant given as a ratio between the pressure of products and reactants.
CONTENTS
1. Overview and Key Difference
2. What is Kc
3. What is Kp
4. Relationship Between Kc and Kp
5. Side by Side Comparison – Kc vs Kp in Tabular Form
6. Summary
What is Kc?
Kc is the equilibrium constant given as a ratio between concentrations of products and reactants. The molar concentrations of the components are used for the expression of Kc.
aA + bB ↔ cC + dD
The equilibrium constant for the above reaction can be written as:
Kc = [C]c[D]d / [A]a[B]b
[A], [B], [C] and [D] are concentrations of A, B reactants and C, D products. The exponents “a’, “b”, “c” and “d” are stoichiometric coefficients of each reactant and product in the chemical equation. In the expression of Kc, the concentrations of reactants and products are raised to the powers equal to their stoichiometric coefficients.
What is Kp?
Kp is the equilibrium constant given as a ratio between the pressure of products and reactants. This equilibrium constant is applicable for gaseous reaction mixtures. Kp depends on partial pressures of gaseous components in the reaction mixture.
Figure 1: Partial pressures of gas components in a mixture.
pP + qQ ↔ rR + sS
The equilibrium constant for the above reaction can be written as:
Kp = pRr.pSs / pPp.pQq
“p” indicates the partial pressure. Therefore, pP, pQ, pR and pS are partial pressures of P, Q, R and S gas components. The exponents “p’, “q”, “r” and “s” are stoichiometric coefficients of each reactant and product in the chemical equation.
What is the Relationship Between Kc and Kp?
Kp = Kc(RT)Δn
Where Kp is the equilibrium constant of pressure, Kc is the equilibrium constant of concentration, R is the universal gas constant (8.314 Jmol-1K-1), T is the temperature and Δn is the difference between total moles of gas products and the total moles of gas reactants.
What is the Difference Between Kc and Kp?
Kc vs Kp |
|
| Kc is the equilibrium constant given as a ratio between concentrations of products and reactants. | Kp is the equilibrium constant given as a ratio between the pressure of products and reactants. |
| Reactants | |
| Kc can be used for gaseous or liquid reaction mixtures. | Kp is used only for gaseous reaction mixtures. |
| Units | |
| Kc is given by units of concentration. | Kp is given by units of pressure. |
Summary – Kc vs Kp
Equilibrium constant of a reaction mixture explains the ratio between products and reactants present in that reaction mixture in terms of either concentrations (given as Kc) or partial pressure (given as Kp). The key difference between Kc and Kp is that Kc is the equilibrium constant which is given by the terms of concentration whereas Kp is the equilibrium constant which is given by the terms of pressure.
Reference
1. “Equilibrium Constant.” Merriam-Webster, Available here.
2. “Gas Equilibrium Constants: Kc And Kp.” Chemistry LibreTexts, Libretexts, 21 July 2016, Available here.
Image Courtesy
1. “2318 Partial and Total Pressure of a Gas” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013., CC BY 3.0) via Commons Wikimedia