Wärmemengen

Wärmemengen, or Wärmemenge in singular, refers to the amount of heat transferred between a system and its surroundings during a thermodynamic process. This transfer is caused by a temperature difference and is usually denoted by Q. Wärmemenge is a form of energy in transit, not a property of a system in a given state; it depends on the path taken between the initial and final states.

In ordinary thermodynamics, Q is measured in joules (J) in the SI system, with calories (cal) used in older or specialized contexts. The sign convention commonly used is: Q > 0 when heat enters the system, Q < 0 when heat leaves the system. The Wärmemenge can be involved in changes of internal energy, enthalpy, and phase transitions, and it is related to the work done by or on the system through the first law of thermodynamics: ΔU = Q − W, where ΔU is the change in internal energy and W is the work performed by the system.

Common ways to quantify Wärmemengen include: sensible heat, Q = m c ΔT for a mass m with specific heat capacity c and temperature change ΔT; latent heat, Q = m L for phase changes, where L is the latent heat per unit mass; and, at constant pressure, Qp = ΔH = n Cp ΔT for n moles with molar heat capacity Cp.

Calorimetry is a typical method to determine Wärmemengen experimentally, by measuring temperature changes in a calibrated system. The term Wärmemenge appears in historical German texts; modern usage often favors Q or the concept of heat transfer rather than a standalone property of a state.