Gibbs free energy example | Thermodynamics | Chemistry | Khan Academy

Khan Academy
28 Sept 200909:57
EducationalLearning
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TLDRThis educational video script explores the spontaneity of a chemical reaction involving methane and oxygen, producing carbon dioxide and water. It explains the use of Gibbs free energy, which is calculated by subtracting the product of temperature and entropy change from the enthalpy change. The script guides through the process of determining the reaction's spontaneity by calculating the change in enthalpy and entropy, concluding that the exothermic reaction is spontaneous at room temperature due to the significant energy release outweighing the entropy decrease.

Takeaways
  • 🔥 The reaction in question involves methane reacting with oxygen to produce carbon dioxide and water.
  • 🔍 To determine if a reaction is spontaneous, Gibbs free energy change (ΔG) is used, which is calculated as ΔH - TΔS.
  • 🌡️ The reaction is exothermic, as indicated by a negative change in enthalpy (ΔH = -890.3 kJ).
  • 🌡️ The standard molar entropies at 298 K (room temperature) are crucial for calculating the change in entropy (ΔS).
  • 📚 The standard molar entropy values for methane, oxygen, carbon dioxide, and water are used in the calculation.
  • 📉 The total entropy change (ΔS) is negative, indicating a decrease in entropy from reactants to products.
  • 🔢 The calculated ΔS is -242.4 J/K, showing a loss of entropy in the reaction.
  • 📉 The Gibbs free energy change (ΔG) is calculated by multiplying the temperature (298 K) by the change in entropy and subtracting it from the enthalpy change.
  • 🌡️ Despite the loss of entropy, the reaction is spontaneous at room temperature due to the large negative ΔH.
  • 🌞 The spontaneity of the reaction could change at very high temperatures where the entropy term becomes more significant.
Q & A

  • What is the reaction discussed in the script?

    -The reaction discussed is the combustion of methane with oxygen to produce carbon dioxide and water: CH4 + 2O2 → CO2 + 2H2O.

  • Why is Gibbs free energy used to determine if a reaction is spontaneous?

    -Gibbs free energy is used because it accounts for both the enthalpy change (energy change) and the entropy change (degree of disorder) of a reaction at a given temperature, providing a measure of whether a reaction will proceed spontaneously.

  • What is the formula for calculating Gibbs free energy change (ΔG)?

    -The formula for calculating Gibbs free energy change is ΔG = ΔH - TΔS, where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy.

  • What does a negative ΔG value indicate about a reaction?

    -A negative ΔG value indicates that a reaction is spontaneous, meaning it will proceed without the input of external energy.

  • What is the change in enthalpy (ΔH) for the given reaction, and what does it signify?

    -The change in enthalpy (ΔH) for the given reaction is -890.3 kilojoules, indicating that the reaction is exothermic, releasing energy.

  • How is the standard molar entropy of a substance defined?

    -The standard molar entropy of a substance is the entropy content per mole of the substance at a standard state, typically at 298 Kelvin (25 degrees Celsius).

  • What is the significance of the entropy change (ΔS) in the context of the reaction?

    -The entropy change (ΔS) indicates the change in disorder or randomness during the reaction. In the given reaction, a negative ΔS suggests a decrease in entropy, meaning the products are more ordered than the reactants.

  • What is the calculated ΔS for the reaction in the script?

    -The calculated ΔS for the reaction is -242.4 joules per Kelvin, indicating a loss of entropy as the reaction proceeds.

  • Why is the reaction considered spontaneous at room temperature according to the script?

    -The reaction is considered spontaneous at room temperature because the exothermic nature (large negative ΔH) overwhelms the loss of entropy (negative ΔS), resulting in a negative ΔG value.

  • How does the temperature affect the spontaneity of a reaction according to Gibbs free energy?

    -The temperature affects the spontaneity of a reaction because it influences the TΔS term in the Gibbs free energy equation. At higher temperatures, the entropy term (TΔS) can become significant enough to outweigh the enthalpy change, potentially making an exothermic reaction non-spontaneous.

  • What could potentially make the given reaction non-spontaneous?

    -The given reaction could potentially become non-spontaneous at extremely high temperatures where the positive contribution from the entropy term (TΔS) is large enough to make ΔG positive.

Outlines
00:00
🔥 Combustion Reaction and Spontaneity Analysis

This paragraph discusses the combustion of methane, a spontaneous exothermic reaction involving one mole of methane and two moles of oxygen producing one mole of carbon dioxide and two moles of water. The spontaneity of the reaction is determined by the change in Gibbs free energy, which is calculated using the formula ΔG = ΔH - TΔS. The change in enthalpy (ΔH) is found to be -890.3 kilojoules, indicating an exothermic process. The standard molar entropies for the involved substances are used to calculate the change in entropy (ΔS), which is found to be negative, suggesting a decrease in disorder. Despite the negative ΔS, the reaction is expected to be spontaneous due to the significant energy release.

05:07
🌡️ Temperature's Role in Reaction Spontaneity

The second paragraph delves into the impact of temperature on the spontaneity of the combustion reaction. The change in Gibbs free energy (ΔG) is recalculated at room temperature (298 Kelvin), taking into account the change in entropy (-242.4 joules per Kelvin) and the enthalpy change. The calculation shows that even though the reaction results in a loss of entropy, the substantial energy released (exothermic nature) makes the reaction spontaneous. The paragraph also contemplates the effect of extremely high temperatures, where the entropy term could potentially outweigh the enthalpy change, possibly rendering the reaction non-spontaneous. The summary underscores the importance of understanding Gibbs free energy in determining the direction of chemical reactions.

Mindmap
Keywords
💡Methane
Methane is a hydrocarbon with the chemical formula CH4. It is a major component of natural gas and is also produced by the anaerobic decay of organic matter in landfills and marshes. In the context of the video, methane is the reactant in the combustion reaction being discussed, highlighting its role in energy production and environmental impact.
💡Oxygen
Oxygen is a chemical element with the symbol O and is a key component of the Earth's atmosphere. It is essential for aerobic respiration in most living organisms. In the script, oxygen is the other reactant in the combustion reaction, reacting with methane to produce carbon dioxide and water.
💡Carbon Dioxide
Carbon dioxide, or CO2, is a greenhouse gas that is a byproduct of many biological and industrial processes. It is a key factor in climate change discussions. In the video, carbon dioxide is one of the products of the combustion reaction, emphasizing the environmental implications of such reactions.
💡Water
Water is a universal solvent and a vital substance for all known forms of life. In the script, water is listed as a product of the methane combustion reaction, indicating the formation of liquid water from the reaction of methane and oxygen.
💡Gibbs Free Energy
Gibbs free energy is a thermodynamic potential that measures the maximum reversible work that a thermodynamic system can perform at constant temperature and pressure. It is crucial in determining whether a reaction is spontaneous. The video discusses using Gibbs free energy to determine the spontaneity of the methane combustion reaction.
💡Enthalpy Change
Enthalpy change, denoted as ΔH, is the heat absorbed or released by a system at constant pressure during a chemical reaction. It is a key component in calculating Gibbs free energy. In the video, the enthalpy change for the methane combustion reaction is calculated to be negative, indicating an exothermic reaction.
💡Entropy Change
Entropy change, denoted as ΔS, is a measure of the change in the disorder or randomness of a system. It is used in conjunction with enthalpy change to calculate Gibbs free energy. In the script, the entropy change for the reaction is calculated to be negative, suggesting a decrease in disorder as the reaction proceeds.
💡Standard Molar Entropy
Standard molar entropy is the entropy content of one mole of a substance at standard conditions (298 K and 1 atm). It is used to calculate the total entropy change in a reaction. The video script uses standard molar entropies for methane, oxygen, carbon dioxide, and water to determine the entropy change for the combustion reaction.
💡Spontaneous Reaction
A spontaneous reaction is a process that occurs naturally without the need for external energy. The video discusses determining whether the methane combustion reaction is spontaneous by calculating its Gibbs free energy. If the Gibbs free energy change is negative, the reaction is spontaneous.
💡Heats of Formation
Heats of formation are the amounts of heat released or absorbed when one mole of a substance is formed from its constituent elements in their standard states. They are used to calculate the enthalpy change of a reaction. In the video, heats of formation for water, methane, and oxygen are used to determine the enthalpy change for the combustion of methane.
💡Kilojoules
Kilojoules (kJ) are a unit of energy used in the field of chemistry and physics. They are often used in calculations involving enthalpy and Gibbs free energy. The video script converts the entropy change from joules to kilojoules to match the units used for enthalpy change, facilitating the calculation of Gibbs free energy.
Highlights

The video discusses determining if a chemical reaction is spontaneous by using Gibbs free energy.

Gibbs free energy change is calculated as the enthalpy change minus temperature times entropy change.

A spontaneous reaction occurs when the change in Gibbs free energy is less than zero.

The change in enthalpy for the reaction of methane with oxygen is calculated to be -890.3 kilojoules.

The heat of formation for O2 is zero, simplifying the enthalpy calculation.

The standard molar entropies for methane, carbon dioxide, and water are provided to calculate the entropy change.

The reaction results in a decrease in entropy due to the formation of liquid water from gaseous reactants.

The total entropy change (ΔS) for the reaction is calculated to be -242.4 joules per Kelvin.

The units of entropy must be converted to kilojoules for consistency with the enthalpy value.

The calculated Gibbs free energy (ΔG) indicates the reaction is spontaneous at room temperature.

The enthalpy term outweighs the entropy term, leading to a negative ΔG value.

At very high temperatures, the entropy term could potentially make the reaction non-spontaneous.

The video demonstrates the process of solving Gibbs free energy problems using standard values.

The importance of looking up standard molar entropies and heats of formation is emphasized.

The video concludes that the reaction of methane with oxygen is spontaneous at standard temperature.

The impact of temperature on the spontaneity of reactions is briefly discussed.

The video provides a practical guide to understanding and calculating the spontaneity of chemical reactions.

Transcripts

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Gibbs Free EnergyMethane CombustionChemical ReactionThermodynamicsEntropy ChangeExothermic ReactionHeat of FormationStandard Molar EntropySpontaneous ProcessEnergy Release