Glossary

4.3 Latent heat and enthalpy of phase transitions

3 min read•july 23, 2024

Phase transitions are key in thermodynamics. They involve , which is energy absorbed or released when a substance changes state without changing temperature. This energy breaks or forms bonds between molecules.

Enthalpy, the total heat content of a system, changes during phase transitions. These changes equal the latent heat involved. Phase diagrams show how substances behave under different conditions, helping us analyze enthalpy changes during transitions.

Latent Heat and Phase Transitions

Role of latent heat in transitions

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  • Latent heat energy required to change phase of substance without changing temperature
    • Absorbed or released during phase transition (, , )
  • Temperature remains constant during phase transition until all substance has changed phase
    • Energy used to break or form intermolecular bonds rather than increasing kinetic energy of particles

Fusion vs vaporization latent heat

  • Latent heat of fusion energy required to change substance from solid to liquid (melting) or liquid to solid (freezing) at constant temperature
    • Energy needed to overcome intermolecular forces in solid state
  • Latent heat of vaporization energy required to change substance from liquid to gas (vaporization) or gas to liquid (condensation) at constant temperature
    • Energy needed to overcome intermolecular forces in liquid state
  • Latent heat of vaporization typically higher than latent heat of fusion for given substance
    • More energy required to completely separate molecules in liquid state to form gas compared to energy needed to break intermolecular forces in solid state

Calculations with latent heat equation

  • Latent heat equation calculates energy required for phase change
    • [Q = mL](https://www.fiveableKeyTerm:q_=_ml), where QQ is energy (J), mm is mass of substance (kg), and LL is latent heat (J/kg)
  • To calculate energy required for phase change, multiply mass of substance by appropriate latent heat value
    • Example: calculate energy required to melt 2 kg of ice with latent heat of fusion of 334 kJ/kg
      • Q=2 kg×334 kJ/kg=668 kJQ = 2 \text{ kg} \times 334 \text{ kJ/kg} = 668 \text{ kJ}

Enthalpy and Phase Transitions

Enthalpy in phase transitions

  • Enthalpy thermodynamic property representing total heat content of system
    • Defined as [H = U + PV](https://www.fiveableKeyTerm:h_=_u_+_pv), where UU is internal energy, PP is pressure, and VV is volume
  • Changes in enthalpy (ΔH\Delta H) associated with phase transitions
    • When substance undergoes phase transition, change in enthalpy due to absorption or release of latent heat
  • Change in enthalpy during phase transition equal to latent heat of transition
    • Example: change in enthalpy during melting equal to latent heat of fusion, change in enthalpy during vaporization equal to latent heat of vaporization

Enthalpy analysis using phase diagrams

  • Phase diagrams graphical representations of different phases of substance as function of pressure and temperature
    • Show conditions under which substance exists as solid, liquid, or gas, as well as phase transition lines
  • Phase transition lines on represent conditions at which two phases coexist in equilibrium
    • Solid-liquid line represents melting and freezing points, liquid-gas line represents vaporization and condensation points, solid-gas line represents sublimation and deposition points
  • Enthalpy changes associated with phase transitions analyzed using Clapeyron equation
    • Clapeyron equation relates slope of phase transition line to change in enthalpy and volume during transition: dPdT=ΔHΔV\frac{dP}{dT} = \frac{\Delta H}{\Delta V}
  • By analyzing slope of phase transition lines on phase diagram, relative magnitudes of enthalpy changes associated with different phase transitions can be compared
    • Example: steeper slope of liquid-gas line compared to solid-liquid line indicates enthalpy change during vaporization greater than enthalpy change during melting

Key Terms to Review (19)

Adiabatic process: An adiabatic process is a thermodynamic process in which no heat is exchanged between the system and its surroundings. This means that any change in the internal energy of the system is entirely due to work done on or by the system, making it a critical concept in understanding various thermodynamic cycles and processes.
Calories: Calories are a unit of energy commonly used to quantify the amount of energy obtained from food or expended through physical activity. This term is particularly significant in thermodynamics, as it relates to the heat energy involved in phase transitions, calorimetry processes, and biological systems' energy requirements and transformations. Understanding calories helps illustrate how energy is transferred and transformed in various physical and biological contexts.
Enthalpy of Fusion: The enthalpy of fusion is the amount of heat energy required to change a substance from a solid to a liquid at constant pressure, without changing its temperature. This phase transition occurs at the melting point, where the solid absorbs heat energy to break the bonds holding its molecules in a rigid structure, allowing them to move freely in the liquid state. The enthalpy of fusion is a key concept in understanding latent heat and the thermodynamic behavior of materials during phase transitions.
Enthalpy of Vaporization: Enthalpy of vaporization is the amount of energy required to convert a unit mass of a liquid into vapor at constant temperature and pressure. This energy input overcomes the intermolecular forces holding the liquid together, allowing the molecules to escape into the gas phase. It plays a crucial role in understanding phase transitions and thermal processes, particularly during boiling and condensation.
H = u + pv: The equation h = u + pv defines enthalpy (h) as the sum of internal energy (u) and the product of pressure (p) and specific volume (v). This relationship is crucial for understanding how energy is stored in a system and how it changes during phase transitions, especially when heat is added or removed from a substance during changes in state like melting or boiling.
Heating curve: A heating curve is a graphical representation that shows the change in temperature of a substance as it is heated over time, highlighting the phase transitions that occur at specific temperature points. This curve effectively illustrates how temperature remains constant during phase changes, such as melting and boiling, due to the absorption of latent heat, which is crucial for understanding latent heat and enthalpy in phase transitions.
Isothermal process: An isothermal process is a thermodynamic process in which the temperature of the system remains constant while heat is exchanged with the surroundings. This constant temperature implies that any internal energy changes in the system are fully compensated by heat transfer, making it an essential concept in understanding how systems behave under thermal equilibrium and the laws governing energy conservation.
Joules: Joules are a unit of energy in the International System of Units (SI) that measure work done or energy transferred. It quantifies the amount of energy required to move an object one meter against a force of one newton. Understanding joules is essential when studying concepts like heat transfer during phase changes, the energy involved in calorimetry, and biological systems' energy transformations.
Kj/mol: The unit 'kj/mol' (kilojoules per mole) is a measurement of energy per mole of a substance, commonly used in thermodynamics to quantify energy changes during chemical reactions and phase transitions. This unit provides a clear understanding of the amount of energy involved when a mole of a substance undergoes a transformation, such as melting, boiling, or reacting with another substance. It plays a vital role in calculating the latent heat and enthalpy changes associated with various processes.
Latent heat: Latent heat is the amount of heat energy absorbed or released by a substance during a phase change without a change in temperature. This concept is crucial in understanding how substances transition between solid, liquid, and gas phases, as well as in various thermodynamic processes that involve energy transfer.
Melting: Melting is the phase transition where a solid turns into a liquid as it absorbs heat. This process involves breaking the bonds that hold the solid's structure together, requiring energy input, and is crucial for understanding heat transfer, latent heat, calorimetry, phase equilibria, and alloy systems.
Phase Diagram: A phase diagram is a graphical representation that shows the relationship between the physical state of a substance and the conditions of temperature and pressure. It helps visualize where different phases, like solid, liquid, and gas, exist and how they transition from one to another under varying conditions. Understanding phase diagrams is crucial for comprehending latent heat, enthalpy changes during phase transitions, and the behaviors of ideal gases and solutions.
Phase Equilibrium: Phase equilibrium refers to a condition in which distinct phases of a substance coexist in a stable manner, with no net change in their respective quantities over time. This balance occurs when the rates of transition between phases, such as solid, liquid, and gas, are equal, leading to an overall stability in the system. Understanding phase equilibrium is essential for analyzing latent heat during phase transitions, chemical potential in thermodynamic systems, the construction of phase diagrams, and the behavior of gases under varying conditions.
Q = ml: The equation q = ml represents the relationship between heat energy, phase change, and mass. In this equation, 'q' stands for the heat energy absorbed or released, 'm' is the mass of the substance undergoing a phase change, and 'l' is the latent heat of the substance, which varies depending on the specific phase transition. This equation is crucial in understanding how energy is transferred during processes like melting, freezing, vaporization, and condensation, without changing the temperature of the substance.
Specific heat during phase change: Specific heat during phase change refers to the amount of heat energy required to change the phase of a substance without changing its temperature. This concept is crucial for understanding how materials behave when they undergo transitions, such as melting or boiling, where energy is absorbed or released without altering the temperature of the substance itself.
Specific Heat of Water: The specific heat of water is the amount of heat energy required to raise the temperature of one gram of water by one degree Celsius. This property is crucial in understanding how water interacts with heat during phase transitions, such as melting and boiling, where it absorbs or releases significant amounts of energy without changing temperature.
Sublimation: Sublimation is the phase transition in which a substance changes directly from a solid to a gas without passing through the liquid state. This process involves the absorption of heat, which corresponds to the latent heat of sublimation, and is essential for understanding enthalpy changes during phase transitions as well as how different phases coexist in equilibrium under varying conditions.
Thermodynamic Potentials: Thermodynamic potentials are quantities that help describe the energy available for doing work in a thermodynamic system. They provide insights into the equilibrium and stability of the system and are crucial in understanding phase transitions and statistical mechanics. The main types of thermodynamic potentials include internal energy, enthalpy, Helmholtz free energy, and Gibbs free energy, each serving specific functions in different contexts, like phase transitions or systems at constant temperature and volume.
Vaporization: Vaporization is the process by which a liquid transforms into a gas, either through boiling or evaporation. This transition involves energy exchange, specifically the absorption of heat, which breaks intermolecular forces, allowing the molecules to escape into the gas phase. Understanding vaporization is essential as it connects to concepts like latent heat, calorimetry during phase changes, and the thermodynamic behavior of solutions and mixtures.
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