latent heat of vaporization

Examples of latent heat of vaporization in the following topics:

• Latent Heat

  • The latent heat is the energy associated with a phase change of a substance.
  • where the latent heat of fusion, Lf, and latent heat of vaporization, Lv, are material constants that are determined experimentally.
  • Latent heat is an intensive property measured in units of J/kg.
  • Lf and Lv are collectively called latent heat coefficients.
  • The long stretches of constant temperature values at 0ºC and 100ºC reflect the large latent heat of melting and vaporization, respectively.

• Water’s Heat of Vaporization

  • Evaporation of water requires a substantial amount of energy due to the high heat of vaporization of water.
  • As a result of the network of hydrogen bonding present between water molecules, a high input of energy is required to transform one gram of liquid water into water vapor, an energy requirement called the heat of vaporization.
  • Water has a heat of vaporization value of 40.65 kJ/mol.
  • They depend only on the vapor pressure of water.
  • Explain how heat of vaporization is related to the boiling point of water

• Convection

  • Example: Calculating Heat Transfer by Convection: Convection of Air Through the Walls of a House.
  • The specific heat of air is a weighted average of the specific heats of nitrogen and oxygen, which is c=cp≅1000 J/kg⋅C (note that the specific heat at constant pressure must be used for this process).
  • If the water vapor condenses in liquid droplets as clouds form, heat is released in the atmosphere (this heat release is latent heat)  .
  • Water vapor carried in by convection condenses, releasing tremendous amounts of energy, and this energy allows air to become more buoyant (warmer than its surroundings) and rise.
  • Cumulus clouds are caused by water vapor that rises because of convection.

• Liquid to Gas Phase Transition

  • Vaporization of a sample of liquid is a phase transition from the liquid phase to the gas phase.
  • Vaporization of a sample of liquid is a phase transition from the liquid phase to the gas phase.
  • This is because the liquid can be heated less in order for its vapor pressure to equal the atmospheric pressure.
  • If the vapor pressure of the drop is greater than the partial pressure of vapor in the gas phase, the drop will evaporate.
  • Run the model, then heat the liquids.

• Specific Heat and Heat Capacity

  • Heat capacity is a measure of the amount of heat energy required to change the temperature of a pure substance by a given amount.
  • the molar heat capacity, which is the heat capacity per mole of a pure substance.
  • Specific heat capacity is the measure of the heat energy required to raise the temperature of a given quantity of a substance by one kelvin.
  • Latent heat of melting describes tœhe amount of heat required to melt a solid.
  • The above simulation demonstrates the specific heat and the latent heat.

• Humidity, Evaporation, and Boiling

  • The amount of water vapor in air is a result of evaporation or boiling, until an equilibrium is reached.
  • The expression "it's not the heat, it's the humidity" makes a valid point.
  • The amount of water vapor the air can hold depends on its temperature.
  • The capacity of air to hold water vapor is based on vapor pressure of water.
  • The vapor pressure of water at 100ºC is 1.01×105 Pa, or 1.00 atm.

• Evaporation

  • Evaporation is a type of vaporization of a liquid that only occurs on the liquid's surface.
  • Usually, the molecules in a glass of water do not have enough heat energy to escape from the liquid.
  • With sufficient heat, however, the liquid would quickly turn into vapor.
  • Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to escape and form water vapor; this vapor can then rise and form clouds.
  • They depend only on the vapor pressure of water.

• Heating Curve for Water

  • Water transitions from ice to liquid to water vapor as heat is added to it.
  • A constant rate of heating is assumed, so that one can also think of the x-axis as the amount of time that goes by as a substance is heated.
  • Liquid water becomes water vapor or steam when it enters the gaseous phase.
  • Use the heat of vaporization ($\Delta H_{vap}$ ) to calculate how much heat was absorbed in this process: $q=m\cdot C_{H_2O(g)}\cdot \Delta T$, where m is the mass of the sample of water.
  • After breaking the bonds, heat is then absorbed and converted to increased kinetic energy of the molecules in order to vaporize them.

• Solid to Gas Phase Transition

  • The solid has such high vapor pressures that heating leads to a substantial amount of direct vaporization even before the melting point is reached.
  • The enthalpy of sublimation (also called heat of sublimation) can be calculated as the sum of the enthalpy of fusion and the enthalpy of vaporization.
  • Even ice has a measurable vapor pressure near its freezing point, as evidenced by the tendency of snow to evaporate in cold dry weather.
  • There are other solids whose vapor pressure overtakes that of the liquid before melting can occur.
  • Notice the triple point of the substance.

• The Evaporating Atmosphere

  • Most of the water is in the liquid phase, where it is held by the mutual attraction of water molecules.
  • Likewise, every once in a while a vapor molecule collides with the liquid surface and condenses into the liquid.
  • If the liquid is heated a little over 100 °C, the transition from liquid to gas will occur not only at the surface, but throughout the liquid volume: the water boils.
  • The water vapor in it changes phases.
  • Collisions between water molecules in the atmosphere allows some to condense and some to remain in vapor.