How to Determine the Standard Gibbs Free Energy Change in a Reaction at Equilibrium – 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.

The Gibbs free energy (or Gibbs energy) is a thermodynamic potential which may be used to calculate the maximum amount of work, other than pressure–volume work, which may be done by a thermodynamically closed system at constant temperature and pressure, and provides a required condition for processes, viz., chemical reactions that may occur under these conditions.

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

Question:

At 60°C, N2O4 is 50% dissociated. Calculate the standard free energy change at this temperature & 1 atm.

Solution:



Click/tap to enlarge the image

The negative value indicates that the process is thermodynamically favored.

α = Degree of dissociation, Kp = Equilibrium constant in terms of partial pressure, ΔrGѲ = Standard Gibbs free energy change in the reaction, R = Gas constant, T = Absolute temperature

NOTE: Post updated since published

Got some free energy now ❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

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)    Wikipedia Contributors. Gibbs free energy. Wikipedia. https://en.wikipedia.org/wiki/Gibbs_free_energy.

‌(5)    Ncert. Chemistry : Textbook for Class XI - Part.I; National Council Of Educational Research And Training: New Delhi.

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