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Calculate The Kp for The Following Reaction at 25c

Reviewed by Calculator Editorial Team

Calculating the equilibrium constant (Kp) for a gas-phase reaction at 25°C is essential for understanding reaction behavior. This guide explains the process, provides a calculator, and offers practical insights.

What is Kp?

The equilibrium constant (Kp) is a measure of the ratio of product concentrations to reactant concentrations at equilibrium for a gas-phase reaction. It quantifies the extent to which a reaction proceeds to completion.

For a general reaction:

aA + bB ⇌ cC + dD

The equilibrium expression is:

Kp = [C]c[D]d / [A]a[B]b

Where [ ] represents partial pressures of the gases.

How to calculate Kp

To calculate Kp, you need:

  • The balanced chemical equation
  • Partial pressures of all gases at equilibrium
  • Temperature (25°C in this case)

The calculation involves:

  1. Writing the equilibrium expression
  2. Substituting the equilibrium partial pressures
  3. Calculating the numerical value of Kp

Note: Kp is temperature-dependent. This calculator assumes standard conditions (25°C).

Example calculation

Consider the reaction:

2NO(g) + O2(g) ⇌ 2NO2(g)

At equilibrium, the partial pressures are:

  • NO: 0.10 atm
  • O2: 0.20 atm
  • NO2: 0.30 atm

The equilibrium expression is:

Kp = [NO2]2 / ([NO]2 × [O2])

Substituting the values:

Kp = (0.30)2 / ((0.10)2 × 0.20) = 0.09 / 0.002 = 45

The equilibrium constant Kp for this reaction is 45.

Interpreting Kp values

Kp values indicate reaction favorability:

  • Kp > 1: Products favored
  • Kp = 1: Equal concentrations at equilibrium
  • Kp < 1: Reactants favored

In the example, Kp = 45 indicates the reaction strongly favors products.

Kp value interpretation
Kp range Reaction favorability
Kp > 100 Strongly favors products
10 < Kp < 100 Moderately favors products
1 < Kp < 10 Slightly favors products
Kp ≈ 1 Approximately equal concentrations
0.1 < Kp < 1 Slightly favors reactants
0.01 < Kp < 0.1 Moderately favors reactants
Kp < 0.01 Strongly favors reactants

FAQ

What units are used for Kp?

Kp has no units because it's a ratio of pressures raised to stoichiometric coefficients.

Can Kp be negative?

No, Kp is always positive because concentrations and pressures are squared in the expression.

How does temperature affect Kp?

Kp changes with temperature. This calculator assumes standard conditions (25°C).