How to Determine the Equilibrium Constant of a Reaction – 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.

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)

Question:

When S₈(g) is heated at 900 K, the initial pressure of 1 atm falls by 29% at equilibrium:

S₈(g) → 4S₂(g)

Find the value of the equilibrium constant for this reaction.

Solution:

Click to enlarge the image

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

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References:

(1)    Wikipedia Contributors. Dalton’s law. Wikipedia. https://en.wikipedia.org/wiki/Dalton%27s_law.

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

(3)    Wikipedia Contributors. Equilibrium constant. Wikipedia. https://en.wikipedia.org/wiki/Equilibrium_constant.

‌(4)    Ncert. Chemistry : Textbook for Class XI - Part.I; National Council Of Educational Research And Training: New Delhi.