11.3 Phase Change and Latent Heat
3 min read•june 24, 2024
Phase changes are crucial in understanding matter's behavior. They involve energy transfer without temperature change, known as . This concept is key to grasping how substances transition between , , and states.
Latent heat calculations, temperature stability during transitions, and phase diagrams are essential tools. These help us predict and analyze how materials behave under different conditions, from everyday occurrences like ice to complex industrial processes.
Phase Change and Latent Heat
Energy calculations for phase changes
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- Latent heat represents energy required to change a substance's phase without altering its temperature
- () is energy needed to convert a substance between solid and liquid states (melting or )
- () is energy needed to convert a substance between liquid and gas states ( or )
- Calculate the energy required for a using the formula
- represents the energy required in (J)
- represents the substance's mass in (kg)
- represents the specific latent heat for the phase change in joules per kilogram (J/kg)
- Examples:
- Energy required to melt 2 kg of ice at 0°C ( of water is 334 kJ/kg):
- Energy required to vaporize 0.5 kg of water at 100°C ( of water is 2,260 kJ/kg):
Temperature and energy in transitions
- During a , a substance's temperature remains constant while energy is added or removed
- The energy is used to break or form instead of increasing the particles'
- When a substance changes from solid to liquid or liquid to gas, energy is added to the system
- This energy overcomes the attractive forces between particles, allowing them to move more freely
- Examples: melting ice, boiling water
- When a substance changes from gas to liquid or liquid to solid, energy is removed from the system
- This allows the attractive forces between particles to dominate, causing them to move closer together and form a more ordered structure
- Examples: condensing steam, freezing water
- The energy involved in phase changes is also known as the
Analysis of phase diagrams
- A graphically represents a substance's states of matter at different temperatures and pressures
- The phase diagram has three main regions: solid, liquid, and gas
- Lines separating these regions represent conditions where two phases can coexist in equilibrium (solid-liquid, liquid-gas, solid-gas)
- The is where all three phases (solid, liquid, gas) can coexist in equilibrium
- The is where the distinction between liquid and gas phases disappears
- To determine a substance's state of matter at a given temperature and pressure:
- Locate the point on the phase diagram corresponding to the given conditions
- If the point lies within one of the three main regions, the substance exists in that state of matter
- If the point lies on a line separating two regions, the substance exists in both states of matter simultaneously
- Examples:
- Water at 1 atm and 25°C: liquid state
- Carbon dioxide at 1 atm and -80°C: solid state ()
- Water at 0.006 atm and 0.01°C: triple point (solid, liquid, and gas coexist)
- The direct transition from solid to gas state is called
Thermodynamic concepts in phase changes
- is a measure of the total heat content of a system, which changes during phase transitions
- is the amount of heat required to raise the temperature of a unit mass of a substance by one degree
- is stored or released during phase changes without changing the temperature
- occurs when two phases of a substance coexist at a specific temperature and pressure
Key Terms to Review (27)
Condensation: Condensation is the process by which water vapor in the air is converted into liquid water. It occurs when warm air comes into contact with a cooler surface, causing the water vapor to lose energy and transform into tiny water droplets.
Critical Point: The critical point is the point in a phase diagram where the distinction between the liquid and gas phases disappears, and the substance can no longer be liquefied by increasing pressure alone. At the critical point, the properties of the liquid and gas phases become identical, marking the end of the phase transition between the two states.
Dry Ice: Dry ice is the solid form of carbon dioxide (CO2) that is commonly used for refrigeration and cooling purposes. It is called 'dry' because it does not melt into a liquid state, but rather transitions directly from a solid to a gas through the process of sublimation.
Enthalpy: Enthalpy is a thermodynamic property that represents the total energy of a system, including its internal energy and the work done by or on the system due to changes in pressure and volume. It is a crucial concept in understanding phase changes and energy transformations in physical and chemical processes.
Evaporation: Evaporation is the process by which a liquid transitions into a gaseous state due to the absorption of energy, typically in the form of heat. This transformation occurs at the surface of the liquid, where molecules with sufficient kinetic energy escape the liquid phase and enter the surrounding atmosphere as a gas.
Freezing: Freezing is the physical process by which a liquid transitions into a solid state due to a decrease in temperature. It is a phase change that occurs when the kinetic energy of the molecules in a liquid is reduced, allowing the attractive intermolecular forces to dominate and form a crystalline solid structure.
Gas: A gas is one of the four fundamental states of matter, characterized by its ability to expand and fill any container, having no fixed shape or volume. Gases are highly compressible, have low density, and exhibit high rates of molecular motion compared to solids and liquids.
Heat of Transformation: The heat of transformation refers to the amount of energy released or absorbed during a phase change, such as the transition from a solid to a liquid (melting) or from a liquid to a gas (boiling). This energy is known as the latent heat, and it is a crucial concept in understanding the behavior of substances as they undergo physical transformations.
Intermolecular Bonds: Intermolecular bonds are the attractive forces that exist between molecules, as opposed to the intramolecular bonds that hold atoms together within a single molecule. These intermolecular interactions are critical in understanding the physical properties and phase changes of substances.
Joules: Joules are the standard unit of energy in the International System of Units (SI). They measure the amount of work done or energy transferred in various physical and chemical processes, including those related to phase changes and thermodynamics.
Kilograms: A kilogram is the base unit of mass in the International System of Units (SI). It is a fundamental unit that is used to measure the amount of matter in an object or substance. Kilograms are particularly important in the context of phase changes and latent heat, as they provide a way to quantify the amount of a substance undergoing a phase transition.
Kinetic Energy: Kinetic energy is the energy an object possesses due to its motion. It is the work required to accelerate a body of a given mass from rest to its stated velocity, and it is directly proportional to the mass of the object and to the square of its velocity.
Latent Energy: Latent energy, also known as latent heat, is the energy released or absorbed during a phase change of a substance, such as the transition from a solid to a liquid or from a liquid to a gas. This energy is stored or released without a change in the substance's temperature.
Latent Heat: Latent heat is the energy released or absorbed by a substance during a phase change, such as the transition from solid to liquid or liquid to gas, without a change in temperature. It is a crucial concept in understanding the behavior of matter and energy transfer.
Latent Heat of Fusion: Latent heat of fusion is the amount of energy required to change a substance from a solid state to a liquid state without changing its temperature. It represents the energy needed to overcome the intermolecular forces that hold the solid structure together, allowing the transition to a less ordered liquid phase.
Latent Heat of Vaporization: Latent heat of vaporization is the amount of energy required to change the phase of a substance from a liquid to a gas at a constant temperature, without changing the temperature of the substance. It represents the energy needed to overcome the intermolecular attractive forces and convert the liquid into a gaseous state.
Liquid: A liquid is one of the three fundamental states of matter, characterized by a definite volume but no definite shape. Liquids are able to flow and take the shape of the container they are in, in contrast to solids which maintain a fixed shape and volume.
Melting: Melting is the physical process by which a solid substance is transformed into a liquid state when heated. It is a phase change that occurs as the temperature of a material rises, allowing the molecules to overcome the intermolecular forces that hold them in a fixed, rigid structure.
Phase Change: A phase change, also known as a state change, is the transformation of a substance from one physical state or phase to another. This can involve the transition between the solid, liquid, and gaseous states of matter, driven by changes in temperature or pressure.
Phase Diagram: A phase diagram is a graphical representation that shows the relationships between the different phases of a substance, such as solid, liquid, and gas, as a function of variables like temperature and pressure. It provides a comprehensive understanding of the conditions under which a substance can exist in different states of matter.
Phase Equilibrium: Phase equilibrium refers to the state where different phases of a substance coexist in a stable balance, without any net change in their relative amounts. It is a fundamental concept in thermodynamics that describes the conditions under which multiple phases, such as solid, liquid, and gas, can exist together in a system without undergoing further transitions.
Phase Transition: A phase transition is a physical transformation of a substance from one state of matter (solid, liquid, or gas) to another, often involving a change in the substance's molecular structure and the release or absorption of energy. This term is central to the understanding of 11.3 Phase Change and Latent Heat, as these topics explore the processes and properties associated with phase transitions.
Solid: A solid is one of the fundamental states of matter, characterized by structural rigidity and resistance to changes of shape or volume. Solids possess a definite shape and volume, unlike the other states of matter, liquids and gases.
Specific Heat Capacity: Specific heat capacity is a measure of the amount of energy required to raise the temperature of a substance by one degree. It is a fundamental property that describes how much heat a material can absorb or release as its temperature changes.
Sublimation: Sublimation is the direct transition of a substance from the solid phase to the gas phase, without passing through the liquid phase. It is a phase change that occurs when the vapor pressure of a solid exceeds the pressure of the surrounding environment, allowing the solid to transform directly into a gas.
Thermodynamics: Thermodynamics is the study of the relationships between heat, work, temperature, and energy. It describes the fundamental physical quantities that govern natural processes and the conversion of energy in systems, such as heat engines, refrigerators, and living organisms.
Triple Point: The triple point is a unique point on a substance's phase diagram where the solid, liquid, and gaseous phases of that substance coexist in equilibrium. At this specific temperature and pressure, all three phases can be present simultaneously.