OCL 3

How can boiling point be measured?

Boiling point can be measured during miniscale distillation. The temperature at which most of the materials distills is recorded as the boiling point. The barometric pressure at the time of distillation should be recorded, so that a correction can be applied to determine the normal boiling point.

What is it important to find a substances boiling point?

Boiling point is a useful physical property for identifying pure liquids as well as for evaluating the purity of a substance.

How does boiling point vary?

Boiling point varies with atmospheric pressure and with elevation. For example, in Denver, at 5,280 feet, boiling points of common solvents are 4-5 C lower than at sea level.

How do you go about using the nomograph in figure 1D-1?

FIrst a straight line is drawn between the reported normal boiling point (78.3 C) and the pressure is (650 torr). The extension of this line cross the delta T correction axis at 4*C. At 650 torr this liquid is predicted to boil at 74.3 C (78.3 C - 4 C).

How do I go about doing a microscale boiling point determination?

First use a syringe to inject 3-5 microliters of liquid sample into a capillary tube. Centrifuge if necessary to get the entire sample to the bottom of the capillary.

Give me an example of when to use a nomograph.

For example, the reported normal boiling point of an organic liquid is 78.3 C. The nomograph can be used to determine the boiling point of this liquid in a lab where the atmospheric pressure is 650 torr.

If molecules have similar molecular weights, how can we differentiate them via boiling points?

For molecules with similar weights, boiling points increase with increasing polarity of the molecules. More polar molecules have stronger intermolecular forces of attraction. Molecules of similar molecular weight that can hydrogen bond have even stronger intermolecular interactions.

After setting up your apparatus for the miniscale boiling point determination what should be done?

Gently heat the liquid to boiling. Continue to heat slowly until the refluxing vapor forms a ring of condensate about 1-2 cm above the tip of the bulb. When the temperature stabilizes for at least one minute, record the temperature. This procedure works well for liquids that boil higher than 50 C.

How do I prepare a bell?

Prepare a bell by drawing a melting point capillary tube over a microburner to make a very final capillary. Cut a small length of this and carefully seal at one end to make a bell top. Place this bell into the melting point capillary containing the liquid.

What else can the nomograph be used for other than to estimate the boiling point of a substance at various pressures?

The nomograph may also be used to correct an observed (or measured) boiling point to the boiling point at 760 torr(the reported or normal boiling point).

What is considered the normal boiling point?

The normal boiling point is the boiling point when atmospheric pressure is 1 atmosphere (760 torr) at sea level. Reference books list normal boiling points unless indicated otherwise.

How does one go about correcting a measured boiling point using a nomograph?

To do this draw a straight line between the pressure and the temperature corresponding to the observed boiling point. The extension of this line intersects the correction factor that must be added to the observed boiling point. For example, the boiling point of a liquid that boils at 122 C in a lab where the ambient pressure is 685 torr would be reported as 125 C.

What is a nomograph?

Using a pressure-temperature nomograph one can estimate the boiling point of a substance at various pressures.

How do I begin the protocol for a miniscale boiling point determination?

Add 0.3 to 0.5 mL of the liquid to a small diameter test tube or Craig tube that contains a boiling chip. Clamp the tube in a heat block or sand bath. Suspend the thermometer so that the bottom of the thermometer bulb is 0.5 cm above the level of the liquid. Position the thermometer so that it does not contact the sides of the tube.

What is another trend in boiling point?

Another trend is the increase in boiling point with decreasing branching of isomeric molecules, if the functional groups are the same. This is due to the decreased surface area and intermolecular attractions for more highly branched molecules.

After bring your bell and melting point capillary containing liquid to a Mel-Temp, what should you do?

Apply heat until a rapid and steady stream of bubbles flows from the inverted capillary. Stop heating and watch the flow of bubbles. Bubbling will stop and the liquid will flow back up into the inverted capillary. The temperature at which the liquid reenters the bell is recorded as the boiling point.

Do liquids with high vapor pressures have high or low boiling points?

Liquids with high vapor pressures have lower boiling points than liquids with lower vapor pressure.

How can a correction to the boiling point be made?

In general, a correction to the boiling point can be made by allowing approximately 0.35 C per 10 torr deviation from 760 torr (the standard pressure). in the vicinity of 760 torr.

What does the boiling point of an organic liquid depend on?

It depends upon molecular properties. Boiling points increase with increasing molecular weight in a homologous series of molecules, for example pentane has a smaller boiling point than heptane because it has a smaller molecular weight. This phenomenon is explained in part by the fact that larger molecules have greater intermolecular attractions and require more heat to vaporize.

What must be taken into account when comparing boiling points measured in different labs and measured at different times?

The atmospheric pressure may vary by a few degrees from day to day (or hour to hour) because of weather changes and passage of weather fronts. These are the differences that must be taken into account when comparing boiling points in different labs and measured at different times.

What is the boiling point of a liquid defined as?

The boiling point of a liquid is defined as the temperature at which its vapor pressure equals atmospheric pressure. Vapor pressure is a measure of the tendency of molecules to escape from the surface of a liquid.