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Enthalpy in changes of state

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  • Why does diethyl ether (C2H5OC2H5) evaporate faster than water?
    It has weaker intermolecular forces
  • Why does evaporation occur only at the surface of a liquid?
    Only surface molecules with enough energy can escape into the gas phase.
  • How much heat is needed to melt 50.0g of ice at 0°C?
    16.7 kJ
  • What is the enthalpy of fusion (ΔH_fus)?
    The heat required to melt one mole of a solid at constant temperature.
  • What is the boiling point of water in Celsius?
    100°C.
  • What happens to energy when ice melts? (energy absorbed or released)
    Energy is absorbed to break the hydrogen bonds between water molecules.
  • What happens to temperature during a phase change?
    remains constant
  • What happens to kinetic energy when water is boiling?
    It remains constant because all the added energy goes into breaking intermolecular forces.
  • Why does the temperature remain constant during phase changes?
    All the absorbed energy is used to change the phase, not increase kinetic energy.
  • What is the unit for measuring enthalpy change?
    Kilojoules per mole (kJ/mol).
  • Which phase change absorbs more energy: melting or vaporization?
    Vaporization absorbs more energy.
  • What is the mathematical relationship between ΔH_fus and ΔH_solid?
    ΔH_fus = -ΔH_solid
  • What phase changes occur when a substance loses energy but stays the same chemically?
    Condensation or freezing.
  • What type of intermolecular force affects the phase changes of water?
    Hydrogen bonding.
  • What is the enthalpy of vaporization (ΔH_vap)?
    The heat required to convert one mole of a liquid into a gas at constant temperature.
  • What happens to the temperature of water at 100°C when it is boiling?
    It remains at 100°C until all the liquid has vaporized.
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