Name: free energy and reaction direction
Uploaded: 2023-04-30T10:09:12.000Z
Duration: 55 s
Channel: JoVE

The free energy change, or &#916;G, of a particular reaction is the sum of the standard free energy and RT times the natural log of the ratio of the concentrations of products to reactants.
If the &#916;G of a reaction is negative, then the reaction progresses forward. The &#916;G becomes less negative as the concentrations of the reactants decrease, and that of the products increase.
Eventually, the &#916;G equals zero when the reaction reaches equilibrium, where the forward and reverse reaction rates are equal.
At equilibrium, the ratio of the concentration of products and reactants equals the equilibrium constant, K.
The equation can be rearranged to show the mathematical relationship between &#916;G0&#160;and K.

free energy and equilibrium

Free Energy and Equilibrium

The free energy change for a process may be viewed as a measure of its driving force. A negative value for &#916;G represents a driving force for the process in the forward direction, while a positive value represents a driving force for the process in the reverse direction. When &#916;G is zero, the forward and reverse driving forces are equal, and the process occurs in both directions at the same rate (the system is at equilibrium).
The reaction quotient, Q, is a convenient measure of the status of an equilibrium system. Q is the numerical value of the mass action expression for the system, and it can be used to identify the direction in which a reaction will proceed in order to achieve equilibrium. When Q is lesser than the equilibrium constant, K, the reaction will proceed in the forward direction until equilibrium is reached and Q = K. Conversely, if Q &#62; K, the process will proceed in the reverse direction until equilibrium is achieved.
The free energy change for a process taking place with reactants and products present under nonstandard conditions (pressures other than 1 bar; concentrations other than 1 M) is related to the standard free energy change according to this equation:
&#916;G = &#916;G&#176; + RT lnQ
R is the gas constant (8.314 J/K mol), T is the kelvin or absolute temperature, and Q is the reaction quotient. For gas-phase equilibria, the pressure-based reaction quotient, QP, is used. The concentration-based reaction quotient, QC, is used for condensed phase equilibria.
For a system at equilibrium, Q = K and &#916;G = 0, and the previous equation may be written as
0 = &#916;G&#176; + RT lnK (at equilibrium)
&#916;G&#176; = &#8722;RT lnK
This form of the equation provides a useful link between these two essential thermodynamic properties, and it can be used to derive equilibrium constants from standard free energy changes and vice versa.
This text is adapted from Openstax, Chemistry 2e, <a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energy">Section 16.4: </a><a href="https://openstax.org/books/chemistry-2e/pages/16-4-free-energyy">Free Energy</a>.

The free energy change for a process may be viewed as a measure of its driving force. A negative value for &#916;G represents a driving force for the ...

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Cell Biology

Unit 1

Energy and Catalysis

KEY TERMS AND CONCEPTS

free energy and reaction direction

Free energy and reaction direction are linked in chemical equilibrium. A free energy change, ΔG, shows the driving force for a process. When ΔG is negative, the process is driven forward. When ΔG is positive, the reverse direction is favored. When ΔG is zero, the forward and reverse driving forces are equal, and the system is at equilibrium.

The reaction quotient, Q, helps show how close a system is to equilibrium. Q is the numerical value of the mass action expression for the system. It can be used to predict which way a reaction will shift to reach equilibrium. If Q is less than the equilibrium constant, K, the reaction proceeds forward until Q = K. If Q is greater than K, the process shifts in reverse until equilibrium is reached.

For reactions under nonstandard conditions, ΔG depends on both the standard free energy change and the reaction quotient. The relationship is ΔG = ΔG° + RT ln Q, where R is the gas constant, T is temperature in kelvin, and Q is the reaction quotient. For gas-phase equilibria, the pressure-based quotient QP is used. For condensed-phase equilibria, the concentration-based quotient QC is used.

At equilibrium, Q = K and ΔG = 0. Substituting that condition into the equation gives 0 = ΔG° + RT ln K. This can be rearranged to ΔG° = −RT ln K. That link lets chemists find an equilibrium constant from a standard free energy change, or find a standard free energy change from K.

Free Energy and Reaction Direction

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Free energy and reaction direction are linked in chemical equilibrium. A free energy change, ΔG, shows the driving force for a process. When ΔG is negative, the process is driven forward. When ΔG is positive, the reverse direction is favored. When ΔG is zero, the forward and reverse driving forces are equal, and the system is at equilibrium. The reaction quotient, Q, helps show how close a system is to equilibrium. Q is the numerical value of the mass action expression for the system. It can...

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key terms and concepts

Learn how free energy change, reaction quotient, and equilibrium constant work together to show reaction direction and equilibrium in chemistry.

Free Energy and Reaction Direction | JoVE K12

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JoVE Core Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

Free Energy and Reaction Direction

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