Enduring Understanding 5.B: Energy Transfer and Phase Transitions

  • Energy can be transferred between two systems either
  • As heat
  • Through one system doing work on the other system.
  • Work is all forms of energy transfer that is not heat transfer. For example, it can be mechanical (e.g. lifting a weight), electrical (causing current to flow), pressure-volume (changes in volume of a gas)...
  • In AP Chemistry, work calculations are limited to pressure-volume changes.

  • When energy is transferred from one system to another:
  • The energy transferred from system 1 is equal in magnitude to that absorbed by system 2.
  • In other words, energy cannot be created or destroyed, only transferred
  • This is called 'conservation of energy'.
  • Example: What is the energy change in a piston system that has 25 joules of work done on it, and loses 15 joules of heat to its surroundings?
  • The piston has gained 25 J energy and lost 15 J, so the net energy change in the system is +10 J.
  • Chemical systems under go three types of processes that change their energy.
  • Heating/cooling (like liquid water warming from 10 °C to 50 °C)
  • Phase changes (ice melting to water at 0 °C, water boiling to steam at 100 °C)
  • Chemical reactions.
  • Phase transitions involve the absorption or release of energy by the system, with no change in temperature.
  • When a liquid boils, energy is absorbed for the liquid to gas phase transition. The amount of energy required to vaporize one mole of a substance is the molar enthalpy of vaporization. The amount of energy required to vaporize a given mass m of a substance is:
    ΔH = (m)(Δ Hvaporization)
  • When a solid melts ('fusion'), energy is absorbed for the solid to liquid phase transition. The amount of energy required to melt one mole of a substance is the molar enthalpy of fusion. The amount of energy required to melt a given mass m of a substance is:
    ΔH = (m)(Δ Hfusion)
  • The amount of energy absorbed when a substance boils, and released when the same amount of substance condenses, is the same. Similarly, the amount of energy absorbed when a substance melts, and released when the same amount of substance freezes, is the same.
  • Sublimation, a substance going straight from solid to gas phase, also involves the absorbing of energy.
  • Sample Problem 1: Consider the phase transition illustrated by the particle diagram below. Would this transition involve the system absorbing or releasing energy?
  • The transition illustrated is a solid A going to a gas C, or sublimation. This would involve the absorption of energy from the surroundings.
  • Sample Problem 2: How much energy would be absorbed or released if 100 g of ice at 0 °C were converted to steam at 100 °C? Use the following values.
  • molar enthalpy of fusion, ΔHfusion = 334 J/g
  • molar enthalpy of vaporization, ΔHvaporization = 2200 J/g
  • molar heat capacity of water, Cp = 4.2 J/g.°C
  • The phase transition is solid to gas, so energy will be absorbed.
  • The process involves the ice melting to water, the water heating from 0 °C to 100 °C, then the water boiling to steam.
  • The energy absorbed will be the heat of fusion + the change in temperature of the liquid + the heat of vaporization, qfusion + qheating + qvaporization
  • qfusion = 100 g x 334 J/g = 33400 J
  • qheating = 100 g x 4.2 J/g•°C x 100 °C = 42000 J
  • qvaporization = 100 g x 2200 J/g = 220000 J
  • qfusion + qheating + qvaporization = 33400 + 42000 + 220000 = 295400 J, or 295 kJ absorbed.



Related Links:
Chemistry
Chemistry Quizzes
AP Chemistry Notes
Kinetic Energy Transfer

AP Chemistry Quizzes
Atoms, Molecules, and Chemical Analysis Quiz
Electronic Structure of Atoms Quiz
Periodicity, Atomic & Ionic Radii, Atomic Models Quiz
Conservation of Matter and Balanced Equations Quiz
Gases and Solutions Quiz
Intermolecular Forces Quiz
Molecular Geometry and VSEPR Quiz
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Balanced and Ionic Equations Quiz
Chemical Reactions Quiz
Redox Reactions and Electrochemistry Quiz
Chemical Kinetics and Rates Quiz
Reaction Rates and Collisions Quiz
Multi-Step Reaction Rates and Catalysis Quiz
Energy and Heat Quiz
Conservation of Energy and Calorimetry Quiz
Bond Energies Quiz
Entropy and Gibbs Free Energy Quiz
Chemical Equilibrium Quiz
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