Glossary
Practice Quizzes

16.4 Free Energy

3 min read•june 25, 2024

predicts whether a chemical reaction will happen on its own. It combines heat changes and disorder changes to tell us if a process is spontaneous. This helps chemists understand which reactions will occur naturally and which need a push.

Temperature plays a big role in . Some reactions only happen when it's hot, while others prefer the cold. Understanding these relationships helps us control chemical processes in labs and industries.

Gibbs Free Energy and Spontaneity

Gibbs free energy and spontaneity

Top images from around the web for Gibbs free energy and spontaneity

1 of 3

Top images from around the web for Gibbs free energy and spontaneity

1 of 3
  • free energy (GG) thermodynamic quantity predicts spontaneity of process at constant temperature and pressure
    • Spontaneous processes occur without external intervention release free energy (rusting of iron)
    • processes require external energy input to proceed (electrolysis of water)
  • Change in Gibbs free energy (ΔG\Delta G) determines spontaneity of reaction
    • ΔG<0\Delta G < 0: Reaction is spontaneous proceeds in forward direction (combustion of fuel)
    • ΔG>0\Delta G > 0: Reaction is non-spontaneous proceeds in reverse direction (photosynthesis)
    • ΔG=0\Delta G = 0: Reaction is at no net change occurs (saturated solution)

Free energy calculations from formation data

  • (ΔGf∘\Delta G_f^\circ) change in free energy when one mole of compound formed from constituent elements in standard states at standard conditions (25℃ and 1 atm)
  • Calculate standard change in Gibbs free energy (ΔG∘\Delta G^\circ) using equation:
    • ΔG∘=∑ΔGf∘(products)−∑ΔGf∘(reactants)\Delta G^\circ = \sum \Delta G_f^\circ(\text{products}) - \sum \Delta G_f^\circ(\text{reactants})
      • ∑ΔGf∘(products)\sum \Delta G_f^\circ(\text{products}): Sum of standard Gibbs free energies of formation for products multiplied by stoichiometric coefficients (2 moles of H2O)
      • ∑ΔGf∘(reactants)\sum \Delta G_f^\circ(\text{reactants}): Sum of standard Gibbs free energies of formation for reactants multiplied by stoichiometric coefficients (1 mole of CH4 and 2 moles of O2)

Free energy from enthalpy and entropy

  • Gibbs free energy change (ΔG\Delta G) related to change in (ΔH\Delta H) and change in entropy (ΔS\Delta S) by equation:
    • ΔG=ΔH−TΔS\Delta G = \Delta H - T\Delta S
      • ΔH\Delta H: Change in enthalpy (heat released or absorbed)
      • TT: Absolute temperature in Kelvin (0℃ = 273.15 K)
      • ΔS\Delta S: Change in entropy (measure of disorder)
  • To calculate ΔG\Delta G using enthalpy and entropy values:
    1. Determine ΔH\Delta H and ΔS\Delta S for reaction
    2. Multiply absolute temperature (TT) by change in entropy (ΔS\Delta S)
    3. Subtract product TΔST\Delta S from change in enthalpy (ΔH\Delta H)

Temperature effects on spontaneity

  • Temperature affects spontaneity of reaction by influencing relative contributions of enthalpy and entropy to Gibbs free energy change
  • For reactions (ΔH<0\Delta H < 0):
    • Low temperatures favor spontaneity ΔH\Delta H term dominates (formation of ice)
    • High temperatures may cause reaction to become non-spontaneous if TΔST\Delta S term becomes larger than ΔH\Delta H term (melting of ice)
  • For reactions (ΔH>0\Delta H > 0):
    • High temperatures favor spontaneity TΔST\Delta S term can overcome positive ΔH\Delta H term (evaporation of water)
    • Low temperatures typically result in non-spontaneous reactions ΔH\Delta H term dominates (condensation of water vapor)

Free energy changes vs equilibrium constants

  • Standard Gibbs free energy change (ΔG∘\Delta G^\circ) related to (KK) by equation:
    • ΔG∘=−RTln⁡K\Delta G^\circ = -RT \ln K
      • RR: Gas constant (8.314 J/mol·K)
      • TT: Absolute temperature in Kelvin
      • ln⁡K\ln K: Natural logarithm of equilibrium constant
  • Rearranging equation to solve for KK:
    • K=e−ΔG∘/RTK = e^{-\Delta G^\circ / RT}
  • Relationship between ΔG∘\Delta G^\circ and KK:
    • ΔG∘<0\Delta G^\circ < 0 corresponds to K>1K > 1 indicating spontaneous forward reaction (product-favored)
    • ΔG∘>0\Delta G^\circ > 0 corresponds to K<1K < 1 indicating non-spontaneous forward reaction (reactant-favored)
    • ΔG∘=0\Delta G^\circ = 0 corresponds to K=1K = 1 indicating system at equilibrium (no net change)

Thermodynamics and Free Energy

  • is the study of energy transformations in physical and chemical processes
  • Gibbs free energy is a fundamental concept in thermodynamics, developed by
  • is the change in Gibbs free energy when the amount of a substance in a system changes
  • in thermodynamics refers to processes that can be reversed without any net change in the system or surroundings

Key Terms to Review (29)

Chemical Potential: Chemical potential is a measure of the tendency of a chemical species to escape from a given phase and participate in a chemical reaction. It represents the amount of energy that a substance has due to its chemical composition and physical state, which determines its reactivity and ability to undergo spontaneous changes.
Chemical thermodynamics: Chemical thermodynamics studies the interrelation of heat and work with chemical reactions or physical changes. It applies principles of thermodynamics to predict the direction and extent of chemical processes.
Endothermic: Endothermic refers to a process or reaction that absorbs heat from the surrounding environment. This means that the system undergoing the endothermic process requires an input of energy in the form of heat in order to proceed. Endothermic processes are central to understanding various topics in chemistry, including energy basics, enthalpy, dissolution, equilibrium, and free energy.
Endothermic process: An endothermic process is a chemical reaction or physical change that absorbs heat energy from its surroundings. These processes result in a decrease in the temperature of the surrounding environment.
Enthalpy: Enthalpy is a measure of the total energy of a thermodynamic system, including both the internal energy of the system and the work done by or on the system due to changes in pressure and volume. It is a key concept in understanding the energy changes that occur during chemical reactions and phase changes.
Enthalpy (H): Enthalpy (H) is the total heat content of a system at constant pressure. It is a thermodynamic property that includes internal energy and the product of pressure and volume.
Equilibrium: Equilibrium is a state of balance or stability in a system, where the opposing forces or processes are in a state of dynamic balance. It is a fundamental concept that underpins various aspects of chemistry, including phase changes, chemical reactions, and thermodynamic processes.
Equilibrium Constant: The equilibrium constant is a quantitative measure of the extent of a chemical reaction at equilibrium. It represents the ratio of the concentrations of the products to the reactants, raised to their respective stoichiometric coefficients, and is a fundamental concept in understanding the behavior of chemical systems at equilibrium.
Equilibrium constant, K: The equilibrium constant, $K$, is a ratio that quantifies the concentrations of reactants and products in a chemical reaction at equilibrium. It provides insight into the position of the equilibrium and the extent to which reactants are converted into products.
Exothermic: Exothermic refers to a chemical reaction or process that releases energy in the form of heat to the surrounding environment. These reactions produce more energy than they consume, resulting in a net release of heat.
Exothermic process: An exothermic process is a chemical reaction or physical change that releases heat to its surroundings. This release of energy usually results in an increase in the temperature of the surroundings.
Gibbs: Gibbs free energy is a thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure. It indicates the spontaneity of a process; negative values denote spontaneous reactions.
Gibbs Free Energy: Gibbs free energy is a thermodynamic property that combines the concepts of enthalpy and entropy to determine the spontaneity and feasibility of a chemical process. It is a crucial factor in understanding the driving forces behind chemical reactions and phase changes.
Gibbs free energy (G): Gibbs free energy (G) is a thermodynamic potential that measures the maximum reversible work obtainable from a system at constant temperature and pressure. It is used to predict the direction of chemical reactions.
Josiah Willard Gibbs: Josiah Willard Gibbs was an American scientist who made significant contributions to the field of thermodynamics. He is best known for his work on the concept of free energy, which is a crucial factor in determining the spontaneity and feasibility of chemical reactions.
Non-spontaneous: A non-spontaneous process is one that does not occur naturally under a given set of conditions and requires an input of energy to proceed. These processes are often associated with a positive change in free energy, indicating that the system needs an external source of energy to drive the reaction forward. Non-spontaneous reactions can be contrasted with spontaneous reactions, which occur without any added energy.
Reversibility: Reversibility refers to the ability of a process or reaction to be reversed, meaning it can proceed in both the forward and backward directions. This concept is particularly important in the context of spontaneity and free energy, as it determines the direction and feasibility of a given process.
Spontaneity: Spontaneity refers to the inherent tendency of a system or process to occur naturally, without the need for external intervention or energy input. It is a fundamental concept in chemistry that describes the natural, unforced progression of a reaction or physical change towards a more stable or favorable state.
Standard free energy of formation ΔGf°: The standard free energy of formation, $\Delta G_f^\circ$, is the change in Gibbs free energy when 1 mole of a compound is formed from its elements in their standard states at 1 atm pressure and 298.15 K (25°C). It indicates the thermodynamic stability of the compound relative to its elements.
Standard Gibbs Free Energy of Formation: The standard Gibbs free energy of formation is a thermodynamic property that quantifies the spontaneity and feasibility of a chemical reaction under standard conditions. It represents the change in Gibbs free energy that occurs when a substance is formed from its constituent elements in their standard states.
Thermodynamics: Thermodynamics is the branch of physics that deals with the relationships between heat, work, temperature, and energy. It describes the fundamental physical laws governing the transformation of energy and the flow of heat, which are essential to understanding the behavior of chemical systems and processes.
ΔG: ΔG, or Gibbs free energy change, is a thermodynamic quantity that represents the maximum amount of non-expansion work that can be extracted from a closed system under constant temperature and pressure conditions. It is a crucial concept in understanding chemical equilibria and the spontaneity of chemical reactions.
ΔG < 0: The change in Gibbs free energy (ΔG) being less than 0 indicates that a chemical reaction or process is thermodynamically favorable and will occur spontaneously. This term is particularly important in the context of understanding the driving forces behind chemical reactions and the feasibility of various processes.
ΔG = 0: ΔG = 0 is a condition that describes a state of equilibrium in a thermodynamic system, where the change in Gibbs free energy (ΔG) is equal to zero. This signifies that the system has reached a point where there is no net change in the spontaneity of the process, and the forward and reverse reactions are occurring at equal rates.
ΔG = ΔH - TΔS: ΔG, or the change in Gibbs free energy, is a fundamental concept in thermodynamics that describes the spontaneity and feasibility of a chemical process. It is defined by the equation ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the absolute temperature, and ΔS is the change in entropy. This equation provides a comprehensive understanding of the driving forces behind chemical reactions and physical transformations.
ΔG > 0: The term ΔG > 0 refers to a situation where the change in Gibbs free energy of a system is greater than zero. Gibbs free energy is a thermodynamic quantity that combines the concepts of energy, entropy, and temperature to determine the spontaneity and feasibility of a chemical process or reaction.
ΔG_f°: ΔG_f° is the standard Gibbs free energy of formation, which is a measure of the spontaneity and feasibility of a chemical reaction. It represents the change in Gibbs free energy that occurs when a substance is formed from its constituent elements in their standard states.
ΔG° = -RT ln K: ΔG° is the standard Gibbs free energy change, which represents the maximum amount of useful work that can be obtained from a spontaneous chemical reaction at constant temperature and pressure. This term is directly related to the equilibrium constant (K) of the reaction, as described by the equation ΔG° = -RT ln K, where R is the universal gas constant, and T is the absolute temperature.
ΔG° = ∑ΔG_f°(products) - ∑ΔG_f°(reactants): ΔG° represents the standard Gibbs free energy change of a reaction, which is the maximum amount of useful work that can be obtained from a reaction under standard conditions. It is calculated by subtracting the sum of the standard Gibbs free energies of formation of the reactants from the sum of the standard Gibbs free energies of formation of the products.
© 2024 Fiveable Inc. All rights reserved.
AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
Back
Glossary
Glossary
Next