Boiling points issues............?

Question

Hey guys do you happen to know a couple of methods to correct boling points of organic compounds?? If so please explain them briefly....

Answer 1

The boiling point can vary due to several factors. As previously mentioned, contamination is one, but let's consider a pure substance.

Another factor is atmospheric pressure. At high altitudes some foods must be cooked longer as a lower atmospheric pressure will yield a drop in the boiling point. To correct this, the compound must be boiled in an enclosed container where one can measure and regulte the pressure.

Another factor to consider is the calibration of the thermometer. I have used the exact same model of three different thermometers and found a variance of +/- 4 degrees Celsius. This should actually be the first step.

The rate at which a compound is heated will also play a role. Most often the compound in question is heated too rapidly and will begin to break down, thus a pure compound can become contaminated even in an enclosed container. The solution of course is to heat the compound more gradually.

The type of apparatus used can play a role. The old bunsen burners with their open flames had a tendency to concentrate the heat in a specific area, and thus the breakdown of a compound could occur. A heating mantle (looks like a bird nest with wires sticking out) is a good source.

Boiling chips are small, insoluble, porous stones made of calcium carbonate or silicon carbide. These stones have pores inside which provide cavities both to trap air and to provide spaces where bubbles of solvent vapor can form. When a boiling chip is heated in a solvent, it releases tiny bubbles. These bubbles ensure even boiling and prevent bumping and boiling over and loss of the solution.

Always use a boiling chip when heating a solvent.

Never add a boiling chip to a hot solvent, because it can cause immediate boiling over of the solution. If you forget to add a boiling chip before you begin, you must cool the solution before adding one to prevent product loss. Boiling chips cannot be re-used since the pores inside these stones become filled with liquid on cooling.

Finally, make absolutely sure that your glassware is not contaminated. Avoid the use of acetone or other organic solvents for cleaning unless absolutely necessary. If they must be used, wash thoroughly three times with warm soapy water, followed by thorough rinsing with distilled (or filtered) deionized water. Glassware ideally should be dried at 120+ degrees Celsius for 2 hours.

Good lab technique, keen observation, and patience are the best tools one can use in the lab.

Hope this helps.

Answer 2

If you want to correct the boiling point (listed for exactly 1 atm pressure) for small deviations from that pressure, you need to use the Clausius-Clapeyron equation. A derivation is given here:

http://www.chem.arizona.edu/~salzmanr/480a/480ants/clapeyro/clapeyro.html

see particularly equation 21. This can be rearranged to give:

ln(p2/p1)*(R/∆Hvap) = 1/T2 - 1/T1

where R is the gas constant, ∆Hvap is the molar enthalpy of vaporization, p2 is the pressure in your lab, p1 is standard atmospheric pressure, T2 is the boiling temperature you've measured, and T1 is the standard boiling point.

If you need to correct boiling point for the presence of impurities, the equation (from http://www.chem.purdue.edu/gchelp/solutions/eboil.html) is ∆T = Kb∆m

where ∆T is the number of degrees the boiling point is elevated, Kb is the boiling point elevation constant for your compound, and m is the molal concentration of the impurity in your compound (molal = moles impurity per kg of your compound)

You can find Kb for well known organic solvents and for water, but if you have to calculate it,

Kb = MRT^2/P∆Hvap
where M is molecular weight of the compound
R is the gas constant
T is the known boiling point at pressure P
∆Hvap is the enthalpy of vaporization.

Be VERY sure you have all the units balanced out in these equation, with all the necessary conversion factors.

Answer 3

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Answer 4

correct the boiling point? for what, unless it's pressure you should just take the bp at face value. for an organic compound you should have a range. if your range doesn't compare to the literature value then you're product isn't pure

Answer 5

The boiling point of a substance is the temperature at which it can change its state from a liquid to a gas throughout the bulk of the liquid at a given pressure. A liquid may change to a gas at temperatures below the boiling point through the process of evaporation. Any change of state from a liquid to a gas at boiling point is considered vaporization. However, evaporation is a surface phenomenon, in which only molecules located near the gas/liquid surface could evaporate. Boiling on the other hand is a bulk process, so at the boiling point molecules anywhere in the liquid may be vaporized, resulting in the formation of vapor bubbles.

A somewhat clearer (and perhaps more useful) definition of boiling point is "the temperature at which the vapor pressure of the liquid equals the pressure of the surroundings

A saturated liquid or saturated vapor contains as much thermal energy as it can without boiling or condensing.

Saturation temperature means boiling point. The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase. The liquid can be said to be saturated with thermal energy. Any addition of thermal energy results in a phase change.

If the pressure in a system remains constant (isobaric), a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy (heat) is removed. Similarly, a liquid at saturation temperature and pressure will boil into its vapor phase as additional thermal energy is applied.

The boiling point corresponds to the temperature at which the vapor pressure of the substance equals the ambient pressure. Thus the boiling point is dependent on the pressure. Usually, boiling points are published with respect to standard pressure (101.325 kilopascals or 1 atm). At higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. The boiling point increases with increased ambient pressure up to the critical point, where the gas and liquid properties become identical. The boiling point cannot be increased beyond the critical point. Likewise, the boiling point decreases with decreasing ambient pressure until the triple point is reached. The boiling point cannot be reduced below the triple point.

Saturation Pressure, or vapor point, is the pressure for a corresponding saturation temperature at which a liquid boils into its vapor phase. Saturation pressure and saturation temperature have a direct relationship: as saturation pressure is increased so is saturation temperature.

If the temperature in a system remains constant (an isothermal system), vapor at saturation pressure and temperature will begin to condense into its liquid phase as the system pressure is increased. Similarly, a liquid at saturation pressure and temperature will tend to flash into its vapor phase as system pressure is decreased


The process of changing from a liquid to a gas requires an amount of heat called the latent heat of vaporization. As heat is added to a liquid at its boiling point, all of this heat goes toward the phase change from liquid to gas, thus the temperature of the substance remains constant even though heat has been added. The word latent, which comes from Latin and means hidden, is used to describe this "disappearing" heat that is added, but doesn't result in an increase in temperature. Since heat is added with no corresponding change in temperature, the heat capacity of the liquid is essentially infinite at the boiling point.

terms of intermolecular interactions, the boiling point represents the point at which the liquid molecules possess enough heat energy to overcome the various intermolecular attractions binding the molecules into the liquid (eg. dipole-dipole attraction, instantaneous-dipole induced-dipole attractions, and hydrogen bonds). Therefore the boiling point is also an indicator of the strength of these attractive forces.

The boiling point of water is 100 °C (212 °F) at standard pressure. On top of Mount Everest the pressure is about 260 mbar (26 kPa) so the boiling point of water is 69 °C.

For purists with a knowledge of thermodynamics, the normal boiling point of water is 99.97 degrees Celsius (at a pressure of 1 atm, i.e. 101.325 kPa). Until 1982 this was also the standard boiling point of water, but the IUPAC now recommends a standard pressure of 1 bar (100 kPa). At this slightly reduced pressure, the standard boiling point of water is 99.61 degrees Celsius.

Answer 6

If the boiling point is incorrect, it means that your reagent is contaminated. I suggest disposing of the reagent and getting a fresh aliquot from the stock bottle.