boiling point elevation

Examples of boiling point elevation in the following topics:

• Boiling Point Elevation

  • The boiling point of a solvent is elevated in the presence of solutes.
  • This is referred to as boiling point elevation.
  • Boiling point elevation can be explained in terms of vapor pressure.
  • The extent of the boiling point elevation can be calculated.
  • In this equation, $\Delta T_b$ is the boiling point elevation, $K_b$ is the boiling point elevation constant, and m is the molality of the solution.

• Boiling Point and Water Solubility

  • It is instructive to compare the boiling points and water solubility of amines with those of corresponding alcohols and ethers.
  • Corresponding -N-H---N- hydrogen bonding is weaker, as the lower boiling points of similarly sized amines (light green columns) demonstrate.
  • Since 1º-amines have two hydrogens available for hydrogen bonding, we expect them to have higher boiling points than isomeric 2º-amines, which in turn should boil higher than isomeric 3º-amines (no hydrogen bonding).
  • Indeed, 3º-amines have boiling points similar to equivalent sized ethers; and in all but the smallest compounds, corresponding ethers, 3º-amines and alkanes have similar boiling points.
  • In the examples shown here, it is further demonstrated that chain branching reduces boiling points by 10 to 15 ºC.

• Boiling & Melting Points

  • The formula of each entry is followed by its formula weight in parentheses and the boiling point in degrees Celsius.
  • The melting points of crystalline solids cannot be categorized in as simple a fashion as boiling points.
  • Spherically shaped molecules generally have relatively high melting points, which in some cases approach the boiling point.
  • Notice that the boiling points of the unbranched alkanes (pentane through decane) increase rather smoothly with molecular weight, but the melting points of the even-carbon chains increase more than those of the odd-carbon chains.
  • The last compound, an isomer of octane, is nearly spherical and has an exceptionally high melting point (only 6º below the boiling point).

• Hydrogen Bonding

  • Most of the simple hydrides of group IV, V, VI & VII elements display the expected rise in boiling point with molecular mass, but the hydrides of the most electronegative elements (nitrogen, oxygen and fluorine) have abnormally high boiling points for their mass.
  • Once you are able to recognize compounds that can exhibit intermolecular hydrogen bonding, the relatively high boiling points they exhibit become understandable.
  • Alcohols boil considerably higher than comparably sized ethers (first two entries), and isomeric 1º, 2º & 3º-amines, respectively, show decreasing boiling points, with the two hydrogen bonding isomers being substantially higher boiling than the 3º-amine (entries 5 to 7).
  • As expected, the presence of two hydrogen bonding functions in a compound raises the boiling point even further.
  • Comparison of Boiling Points of Methane, Ammonia, Water, and Hydrogen Fluoride

• Physical Properties of Carboxylic Acids

  • The table at the beginning of this page gave the melting and boiling points for a homologous group of carboxylic acids having from one to ten carbon atoms.
  • The boiling points increased with size in a regular manner, but the melting points did not.
  • The factors that influence the relative boiling points and water solubilities of various types of compounds were discussed earlier.
  • The first five entries all have oxygen functional groups, and the relatively high boiling points of the first two is clearly due to hydrogen bonding.
  • Carboxylic acids have exceptionally high boiling points, due in large part to dimeric associations involving two hydrogen bonds.

• Liquid to Gas Phase Transition

  • Evaporation occurs at temperatures below the boiling point, and occurs on the liquid's surface.
  • The boiling point is the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environment (air).
  • The curve suggests that when the atmospheric pressure is lower than 1 atm (for instance, at higher altitudes), then the boiling point will occur at lower temperatures.
  • For both of these reasons, a liquid will not boil until the temperature is raised slightly above the boiling point, a phenomenon known as superheating.
  • Once the boiling begins, it will continue to do so at the liquid's proper boiling point.

• Background and Properties

  • The following table lists some representative derivatives and their boiling points.
  • Boiling points are given for 760 torr (atmospheric pressure), and those listed as a range are estimated from values obtained at lower pressures.
  • As noted earlier, the relatively high boiling point of carboxylic acids is due to extensive hydrogen bonded dimerization.
  • The relatively high boiling points of equivalent 3º-amides and nitriles are probably due to the high polarity of these functions.
  • Indeed, if hydrogen bonding is not present, the boiling points of comparable sized compounds correlate reasonably well with their dipole moments.

• Variation of Physical Properties Within a Group

  • The physical properties (notably, melting and boiling points) of the elements in a given group vary as you move down the table.
  • Different groups exhibit different trends in boiling and melting points.
  • For Groups 1 and 2, the boiling and melting points decrease as you move down the group.
  • For the transition metals, boiling and melting points mostly increase as you move down the group, but they decrease for the zinc family.
  • The noble gases (Group 18) decrease in their boiling and melting points down the group.

• Amines

  • As a result, the boiling points of these compounds are higher than those of the corresponding phosphines, but lower than those of the corresponding alcohols, which hydrogen bond to a stronger extent.
  • This results in a decrease in their solubility in water and high boiling points.
  • In general, the effect of alkyl groups raises the energy of the lone pair of electrons, thus elevating the basicity.

• Freezing Point Depression

  • Freezing point depression is the phenomena that describes why adding a solute to a solvent results in the lowering of the freezing point of the solvent.
  • In this equation, $\Delta T_f$ is the freezing point depression, Kf is the freezing point depression constant, and i is the van 't Hoff factor.
  • The value of 0.93 oC is the change in the freezing point.
  • A triple phase diagram which shows the pressure and temperature of the normal boiling and freezing points of a solvent (green lines) and the boiling and freezing points of a solution (purple lines).
  • Discuss the effects of a solute on the freezing point of a solvent