2.8
Calculation of First Law Quantities I
Thermodynamic systems undergoing phase transitions or temperature changes experience energy transfer in the form of heat (q) and work (w). For a rever…
A reversible phase change at constant temperature and pressure involves the transfer of energy between phases without any chemical reaction.
The heat exchanged during this process is the latent heat of transition, and the work done equals the pressure multiplied by the volume change.
Under constant pressure, the enthalpy change equals the heat exchanged during the phase change, while the internal energy change accounts for both heat and work.
In constant-pressure heating without a phase change, the system expands or contracts as temperature changes, so it performs pressure–volume work.
The heat supplied equals the enthalpy change, which depends on the heat capacity at constant pressure and the temperature interval. This holds even for irreversible processes if initial and final pressures are equal.
In constant-volume heating without a phase change, the volume is fixed, so no work is done. All the heat supplied changes the internal energy, which depends on the heat capacity at constant volume and the temperature change, and applies to both reversible and irreversible processes.
The corresponding enthalpy change follows from the internal energy change combined with the change in the pressure–volume term.
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Q1: What is latent heat of transition in a reversible phase change?
Latent heat of transition is the energy absorbed or released when a substance changes between physical states at constant temperature and pressure without chemical reaction. At constant pressure, this latent heat equals the enthalpy change. The work done during the phase change is calculated as pressure multiplied by the volume change between phases.
Q2: How does internal energy change during a constant-pressure heating process?
During constant-pressure heating without phase change, internal energy change is derived from the first law as ΔU = ΔH − pΔV, where the enthalpy change depends on heat capacity at constant pressure and temperature interval. This relationship holds for both reversible and irreversible processes when initial and final pressures are equal, since enthalpy is a state function.
Q3: Why is no work done during constant-volume heating?
In constant-volume heating, the volume remains fixed, so pressure-volume work (w = 0) cannot occur. All heat supplied directly increases internal energy, which depends on heat capacity at constant volume and temperature change. This principle applies to both reversible and irreversible processes.
Q4: What is the relationship between enthalpy change and heat at constant pressure?
At constant pressure, enthalpy change equals the heat exchanged during the process. For heating without phase change, the heat supplied is determined by integrating heat capacity at constant pressure over the temperature range. This relationship holds regardless of whether the process is reversible or irreversible.
Q5: How is enthalpy change calculated from internal energy change at constant volume?
At constant volume, all heat increases internal energy through ΔU = ∫Cv dT. The enthalpy change is then obtained from ΔH = ΔU + Δ(pV), accounting for changes in the pressure-volume term. This calculation applies to both reversible and irreversible constant-volume processes.
Q6: What does the first law tell us about energy transfer in phase transitions?
The first law states that internal energy change equals heat plus work: ΔU = q + w. During reversible phase transitions at constant temperature and pressure, the heat exchanged is the latent heat, and work equals pressure times volume change. This framework applies to all thermodynamic systems undergoing energy transfer.
Q7: Why does enthalpy remain constant during reversible phase changes at constant pressure?
During reversible phase changes at constant temperature and pressure, enthalpy change equals the latent heat of transition. Since enthalpy is a state function depending only on initial and final states, the enthalpy change remains constant regardless of the path taken, making it a reliable measure of energy transfer between phases.