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Energetics II

Gibbs free energy and thermodynamic feasibility

  • 3 things to know
  • Papers 1 and 3

Overview

Recall questions on the Gibbs equation, on what makes a reaction feasible, on finding the temperature at which it becomes so, and on why a feasible reaction may still be slow.

What you need to know

Answer each one before you reveal it.

Your recallNot started
Process

What decides whether a reaction is thermodynamically feasible?

Numbers

State the equation for Gibbs free energy change.

Key term

What two factors together determine whether a reaction is thermodynamically feasible?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What two factors together determine whether a reaction is thermodynamically feasible?

    Model answer

    Whether the enthalpy term or the entropy term wins out.

  2. 2

    State the equation for Gibbs free energy change.

    Model answer

    ΔG = ΔH − TΔS_system

  3. 3

    In the Gibbs free energy equation, what does T represent?

    Model answer

    The absolute temperature in kelvin.

  4. 4

    Calculate the Gibbs free energy change when enthalpy change = 421 kJ mol⁻¹, temperature = 209 K, entropy change of system = 196 J K⁻¹ mol⁻¹.

    Model answer

    Gibbs free energy change = 421 - (209 × 196 ÷ 1000) = 380 kJ mol⁻¹.

  5. 5

    What condition for Gibbs free energy indicates that a reaction is thermodynamically feasible?

    Model answer

    The Gibbs free energy change must be negative (ΔG < 0).

  6. 6

    Given ΔH and ΔS, how do you find the temperature above which a reaction becomes feasible?

    Model answer

    By setting ΔG = 0 and rearranging to give T = ΔH / ΔS_system.

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