Equilibrium Constant in Gaseous Systems (Kp)
Formulating and calculating the equilibrium constant in terms of partial pressures for gaseous reactions.
What is Equilibrium Constant in Gaseous Systems (Kp)?
The equilibrium constant expressed in terms of the partial pressures of the gaseous components involved in a reversible reaction.
Key formula / rule: Kp Expression
Key points
- Define and write the expression for Kp for a given gaseous reaction.
- Calculate Kp using partial pressures at equilibrium.
- Relate Kp to Kc.
- Interpret the significance of Kp value.
Common exam trap
Forgetting to use partial pressures instead of total pressures.
Definitions
- Term
Kp
- Meaning
The equilibrium constant expressed in terms of the partial pressures of the gaseous components involved in a reversible reaction.
- Term
Partial Pressure
- Meaning
The pressure exerted by a single component gas in a mixture of gases, as if it were the only gas present in the container.
- Term
Δn
- Meaning
The change in the number of moles of gaseous products minus the number of moles of gaseous reactants in a balanced chemical equation.
Learning objectives
Define and write the expression for Kp for a given gaseous reaction.
Calculate Kp using partial pressures at equilibrium.
Relate Kp to Kc.
Interpret the significance of Kp value.
Formulae
- Name
Kp Expression
- Note
For aA(g) + bB(g) ⇌ cC(g) + dD(g)
- Expression
Kp = (pCc * pDd) / (pAa * pBb)
- Name
Partial Pressure
- Note
where pi is partial pressure, xi is mole fraction, Ptotal is total pressure.
- Expression
pi = xi * Ptotal
- Name
Relationship between Kp and Kc
- Note
R is the universal gas constant, T is absolute temperature in Kelvin, Δn = (sum of stoichiometric coefficients of gaseous products) - (sum of stoichiometric coefficients of gaseous reactants).
- Expression
Kp = Kc(RT)^Δn
Prerequisites
Understanding of chemical equilibrium.
Concept of reversible reactions.
Partial pressures and mole fractions.
Stoichiometry.
Common mistakes
Forgetting to use partial pressures instead of total pressures.
Incorrectly calculating the stoichiometric coefficients (Δn).
Including solids or liquids in the Kp expression (they are omitted).
Confusing Kp with Kc without considering the units or Δn.
Keywords
Kp
Equilibrium Constant
Gaseous Systems
Partial Pressure
Stoichiometric Coefficients
Δn
Kc
Chemical Equilibrium
Practice preview
In a 10 L vessel, 2 moles of SO2 and 1 mole of O2 are introduced. At equilibrium, 1 mole of SO3 is formed. If the total pressure at equilibrium is 4 atm and the temperature is 300 K, what is the value of Kp for the react…
hard
What is the correct expression for Kp for the reaction 2NO(g) + O2(g) <=> 2NO2(g)?…
easy
For the heterogeneous equilibrium CaCO3(s) <=> CaO(s) + CO2(g), the expression for Kp is:…
medium