For each phase change, determine the sign of δpch and δpcs

General
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for each phase change, determine the sign of δpch and δpcs .

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Question: What is the sign of δpch and δpcs for each phase change?

Answer:
The signs of δpch and δpcs depend on the direction of the phase change and the system undergoing the change. Let’s consider the different phase changes and determine the signs of the enthalpy and entropy changes.

  1. Melting (solid to liquid):

    • δpch: Positive (+)
      • The enthalpy change (δpch) is positive because energy is being absorbed to break the intermolecular forces holding the solid together and convert it into a liquid.
    • δpcs: Positive (+)
      • The entropy change (δpcs) is positive because the liquid phase has higher molecular disorder compared to the solid phase.

  2. Freezing (liquid to solid):

    • δpch: Negative (-)
      • The enthalpy change (δpch) is negative because energy is released as the intermolecular forces form and the liquid transforms into a solid.
    • δpcs: Negative (-)
      • The entropy change (δpcs) is negative because the molecular disorder decreases when the liquid transforms into a solid.

  3. Vaporization (liquid to gas):

    • δpch: Positive (+)
      • The enthalpy change (δpch) is positive because energy is absorbed to break the intermolecular forces holding the liquid together and convert it into a gas.
    • δpcs: Positive (+)
      • The entropy change (δpcs) is positive because the gaseous phase has higher molecular disorder compared to the liquid phase.

  4. Condensation (gas to liquid):

    • δpch: Negative (-)
      • The enthalpy change (δpch) is negative because energy is released as the intermolecular forces form and the gas transforms into a liquid.
    • δpcs: Negative (-)
      • The entropy change (δpcs) is negative because the molecular disorder decreases when the gas transforms into a liquid.

  5. Sublimation (solid to gas):

    • δpch: Positive (+)
      • The enthalpy change (δpch) is positive because energy is absorbed to bypass the liquid phase and directly convert the solid into a gas.
    • δpcs: Positive (+)
      • The entropy change (δpcs) is positive because the gaseous phase has higher molecular disorder compared to the solid phase.

  6. Deposition (gas to solid):

    • δpch: Negative (-)
      • The enthalpy change (δpch) is negative because energy is released as the intermolecular forces form and the gas transforms into a solid.
    • δpcs: Negative (-)
      • The entropy change (δpcs) is negative because the molecular disorder decreases when the gas transforms into a solid.

It’s important to note that the signs of δpch and δpcs can vary depending on the specific substance and conditions of the phase change.