Notes on Thermochemistry and the First Law of Thermodynamics

Thermochemistry

  • Energy Transformation Examples:

    • Electrical energy in a flat iron converts to thermal energy for ironing.
    • A windmill converts mechanical energy from the turbine to electrical energy.

  • First Law of Thermodynamics:

    • States: "The energy of an isolated system is constant."
    • Also known as the Law of Conservation of Energy: Energy cannot be created or destroyed.

  • Internal Energy (U):

    • Total energy a molecule possesses, including kinetic and potential energy.
    • Change in internal energy () can be expressed as:
      ΔU=U<em>products−U</em>reactants\Delta U = U<em>{products} - U</em>{reactants}

  • Heat and Work:

    • The First Law can be mathematically stated as:
      ΔU=q+w\Delta U = q + w
    • Where:
      • q = heat absorbed (+) or released (−)
      • w = work done on the system (+) or by the system (−)

  • Units:

    • SI unit for heat, work, and internal energy is joule (J):
      1extcal=4.184extJ1 ext{ cal} = 4.184 ext{ J}
    • 1 Joule = 1 kg·m²/s².

  • Examples of Calculation:

    • If a gas expands and exerts 22.4 kJ work with 14.6 kJ heat absorbed, then:
      ΔU=14.6extkJ−22.4extkJ=−7.8extkJ\Delta U = 14.6 ext{ kJ} - 22.4 ext{ kJ} = -7.8 ext{ kJ}
    • Given internal energy of -1420 J and 250 J heat added, work done:
      w=−1420extJ+250extJ=−1170extJw = -1420 ext{ J} + 250 ext{ J} = -1170 ext{ J}

  • Summary:

    • The First Law emphasizes that energy in an isolated system remains constant and supports the foundational principle of thermodynamics regarding energy transformation.