How to Calculate Different Parameters in a Mixture at Equilibrium or after – Part 1

Chemical equilibrium is the state in which both the reactants and products in a chemical reaction are present in concentrations that have no more propensity to change with time, s.t. there is no visible alteration in the system properties.

This state results when the forward reaction proceeds at the same rate as the reverse/backward reaction, and is known as dynamic equilibrium.

The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium. For a given set of reaction conditions like temperature, solvent, and ionic strength, the equilibrium constant is independent of the initial concentrations of the reactants and products. The equilibrium constant will be dimensionless only if the number of moles of the reactants is equal to that of the products.

Dalton’s law (aka Dalton’s law of partial pressures) states that the total pressure exerted is equal to the sum of the partial pressures of the individual gases in a mixture of non-reacting ideal gases.

Let’s recall that the partial pressures of the gases in a mixture tell us the relative number of particles or moles of the gases in that mixture.

Mnemonic: ICE: Initial Change Equilibrium (concentration or partial pressure)

The pressure versus time curve can be used to evaluate the kinetics of the reaction from the rising part and the equilibrium constant from the horizontal part.

Question:

A pure sample of N₂O₄(g) is placed into a rigid, evacuated, 0.500 L container. The initial pressure of N₂O₄(g) is 760 torr. The temperature is held constant until N₂O₄(g) reaches equilibrium with its decomposition product NO₂(g). The figure below shows how the pressure of the system changes while reaching equilibrium.

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1. How much will the equilibrium pressure increase by if this reaction goes to completion?
2. What can be said about K_p for this reaction; >, =, or < 1?
3. Which diagram is an approximate description of the system if more NO₂(g) is rapidly injected, at time = 0, into the container after equilibrium is established?

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Solution:

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Based on the data, the reaction does not go to completion because it is not 1520 torr when the reaction stopped.

Α = Degree of dissociation, K_p = Equilibrium constant in terms of partial pressure

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References:

(1)    Wikipedia Contributors. Chemical equilibrium. Wikipedia. https://en.wikipedia.org/wiki/Chemical_equilibrium.

(2)    Ncert. Chemistry : Textbook for Class XI - Part.I; National Council Of Educational Research And Training: New Delhi.

(3)    Brown, C.; Ford, M. Higher Level Chemistry, 2nd ed.; Pearson Education: Harlow, Essex, 2014.

‌(4)    Jespersen, N. D.; Barron's Educational Series, Inc. Barron’s AP Chemistry; Barron’s Educational Series: Hauppauge, N.Y., 2012.