Free Energy & Equilibrium (College Board AP® Chemistry)

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Alexandra Brennan

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Chemistry

Free Energy & Equilibrium

  • When Δ < 0 for a reaction at constant temperature and pressure, the reaction is thermodynamically favored

    • This means that the products are favored when the reaction reaches equilibrium 

    • We can say that the equilibrium constant K, is above 1

  • When Δ is above 0, the reaction is not thermodynamically favored

    • The reaction will proceed in the reverse direction 

    • The equilibrium constant, K, is below 1

  • When ΔG° is equal to zero the reaction is at equilibrium and no further reaction occurs 

    • The equilibrium constant will be close to 1 

  • The qualitative relationship between Δ, the equilibrium constant, and temperature can be represented as:

    Δ = -RT In K 

    • R is the ideal gas constant

    • T represents the absolute temperature in Kelvin.

    • ln is the natural logarithm.

  • To find the equilibrium constant we use:

K = e-ΔG°/RT

Worked Example

At 298 K, the value of the equilibrium constant is 45.

Calculate a value for Δfor the reaction at 298 K. 

R = 8.314 J mol-1 K-1

Identify whether the reaction will move in the forward or reverse direction. 

Answer:

  • ΔG° = -RT In K 

    • ΔG° = -(8.314) (298) [In(45)]

    • ΔG° = -9431 J mol-1

    • ΔG° = -9.43 kJ mol-1

  • ΔG° is negative so the reaction will proceed in the forward direction

Free energy diagrams

  • Remember: ΔG is an indication of whether the forward or backward reaction is favoured

  • For a thermodynamically favored reaction:

    • Most of the reactants are converted into products as the minimum is closer to the product side 

    • The slope of the curved line is to the left-hand side of the equilibrium point so Δis negative and the reaction moves towards the products (and the minimum

Free energy diagram for a thermodynamically favored reaction

free-energy-diagrams

The reaction favors the products 

  • For a not thermodynamically favored reaction:

    • A small amount of reactant is converted into products at equilibrium (where the minimum is)

    • The slope and Δare positive so the reaction proceeds towards the reactants (and towards the minimum)

Free energy diagram for a not thermodynamically favored reaction

free-energy-diagrams-2

The reaction favors the reactants

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.