AP Chemistry 6.5 Energy of Phase Changes Study Notes - New Syllabus Effective fall 2024
AP Chemistry 6.5 Energy of Phase Changes Study Notes.- New syllabus
AP Chemistry 6.5 Energy of Phase Changes Study Notes – AP Chemistry – per latest AP Chemistry Syllabus.
LEARNING OBJECTIVE
Explain changes in the heat q absorbed or released by a system undergoing a phase transition based on the amount of the substance in moles and the molar enthalpy of the phase transition.
Key Concepts:
- Energy of Phase Changes
- Exothermic & Endothermic Reactions
- Energy Diagrams
- Thermal Energy & Molecular Collisions
6.5.A.1 Energy Transfer During Phase Changes:
1. Thermal Energy and Heat:
i. Difference Between Temperature and Thermal Energy:
– Temperature: Measures the mean kinetic energy of particles in an object; indicates how hot or cold something is.
– Thermal Energy: Internal energy of a substance, which is the total kinetic and potential energy of all of its particles. It increases with temperature and mass.
ii. Heat Transfer and Phase Changes
– Heat Transfer: The passage of thermal energy from a body to another through conduction, convection, or radiation.
– Phase Changes: During phase changes (vaporization, melting), heat is either absorbed or released but without a change in temperature. Latent heat energy is used to alter the molecular structure and not the temperature.
2. Phase Changes:
i. Melting: Solid to liquid (heat is absorbed).
ii. Freezing: Liquid to solid (heat is released).
iii. Boiling: Liquid to gas (heat is absorbed).
iv. Condensation: Gas to liquid (heat is released).
Energy for Phase Changes:
– Latent Heat of Fusion: Energy for solid to liquid change.
– Latent Heat of Vaporization: Energy for liquid to gas change.
3. Latent Heat: The energy required for a substance to undergo a phase transition (e.g., melting, boiling) without a change in temperature. It is absorbed or released during the process.
4. Temperature Plateau: During a phase change, the temperature remains constant because the energy added (or removed) is used to change the phase (e.g., melting, boiling) rather than increasing the temperature. This energy goes into breaking or forming molecular bonds, not increasing the kinetic energy of particles.
5. Energy Conservation: During phase changes, energy is conserved. The energy absorbed (e.g., during melting or boiling) or released (e.g., during freezing or condensation) is used to alter the substance’s phase, not to change its temperature. The total energy before and after the phase transition remains the same.
6.5.A.2 Energy Absorption and Release During Phase Changes:
1. Enthalpy: Change in energy per mole of substance in a change of state, e.g., fusion (solid to liquid) or vaporization (liquid to gas). It is the heat per mole transferred in the change of state.
2. Phase Changes and Energy Transfer:
– Energy Absorbed: During boiling/melting (vaporization/fusion), heat is required to overcome intermolecular forces.
– Energy Released: Heat is emitted during freezing/condensation when molecules aggregate.
Molar Enthalpy Relationship:
– The molar enthalpy of condensation is equal to the negative of the molar enthalpy of vaporization.
– The molar enthalpy of freezing is the opposite of the molar enthalpy of fusion.
3. Latent Heat:
Latent Heat: The amount of energy involved in a change of state without a temperature change.
– Latent Heat of Fusion: Energy associated with a substance changing from solid to liquid (or vice versa).
– Latent Heat of Vaporization: Amount of energy required for a substance to change from liquid to gas (or from gas to liquid).
4. Energy Conservation: The energy absorbed during one phase change (e.g., melting or boiling) equals the energy released during the reverse phase change (e.g., freezing or condensation). This ensures that energy is conserved during the entire process.
OLD Content
Energy of Phase Changes
- → endothermic (atractive forces are being broken); ← exothermic (attractive forces are forming)
- Heat of fusion (
) will be positive for melting and negative for freezing; heat of vaporization will be positive for vaporization and negative for freezing
- Heat of fusion (
- Can’t use q = mcΔT during phase changes bcuz temp remains constant
- No change in KE, only change in PE
- During HoV, all the intermolecular forces/attraction are breaking but during HoF are only breaking/separating some
- Energy required for phase changes (ex: required to melt a substance)
- Heat to melting point: q = mcΔT
- Melting: q = (moles)(
) - Add up values
- Value of c will be different in each phase
