Laboratory Experiments for General, Organic, and Biochemistry.pdf

Experiment 1

Laboratory techniques: use of the laboratory

gas burner; basic glassworking

Background

The Laboratory Gas Burner

Tirrill or Bunsen burners provide a ready source of heat in the chemistry laboratory. In

general, since chemical reactions proceed faster at elevated temperatures, the use of heat

enables the experimenter to accomplish many experiments more quickly than would be

possible at room temperature. The burner illustrated in Fig. 1.1 is typical of the burners

used in most general chemistry laboratories.

Figure 1.1

The Bunsen burner.

Violet outer cone

Pale-blue middle cone

Hottest part of the

flame (800°C)

Dark-blue inner cone

Barrel

Air vents

Gas inlet

Gas control valve

Base

Main gas valve

A burner is designed to allow gas and air to mix in a controlled manner. The gas often

used is “natural gas,” mostly the highly ﬂammable and odorless hydrocarbon methane,

CH 4 . When ignited, the ﬂame’s temperature can be adjusted by altering the various

proportions of gas and air. The gas ﬂow can be controlled either at the main gas valve or at

the gas control valve at the base of the burner. Manipulation of the air vents at the bottom

of the barrel allows air to enter and mix with the gas. The hottest ﬂame has a violet outer

cone, a pale-blue middle cone, and a dark-blue inner cone; the air vents, in this case, are

opened sufﬁciently to assure complete combustion of the gas. Lack of air produces a cooler,

luminous yellow ﬂame. This ﬂame lacks the inner cone and most likely is smoky, and often

deposits soot on objects it contacts. Too much air blows out the ﬂame.

Harcourt, Inc.

Experiment 1 1

Basic Glassworking

In the chemistry laboratory, it is often necessary to modify apparatus made from glass or

to connect pieces of equipment with glass tubing. Following correct procedures for working

with glass, especially glass tubing, is important.

Glass is a super-cooled liquid. Unlike crystalline solids which have sharp melting

points, glass softens when heated, ﬂows, and thus can be worked. Bending, molding, and

blowing are standard operations in glassworking.

Not all glass is the same; there are different grades and compositions. Most

laboratory glassware is made from borosilicate glass (containing silica and borax

compounds). Commercially, this type of glass is known as Pyrex (made by Corning Glass)

or Kimax (made by Kimble glass). This glass does not soften very much below 800 C and,

therefore, requires a very hot ﬂame in order to work it. A Bunsen burner ﬂame provides a

hot enough temperature for general glassworking. In addition, borosilicate glass has a low

thermal coefﬁcient of expansion. This refers to the material’s change in volume per degree

change in temperature. Borosilicate glass expands or contracts slowly when heated or

cooled. Thus, glassware composed of this material can withstand rapid changes in

temperature and can resist cracking.

Soft glass consists primarily of silica sand, SiO 2 . Glass of this type softens in the

region of 300–400 C, and because of this low softening temperature is not suitable for most

laboratory work. It has another unfortunate property that makes it a poor material for

laboratory glassware. Soft glass has a high thermal coefﬁcient of expansion. This means

that soft glass expands or contracts very rapidly when heated or cooled; sudden, rapid

changes in temperature introduce too much stress into the material, and the glass cracks.

While soft glass can be worked easily using a Bunsen burner, care must be taken to

prevent breakage; with annealing, by ﬁrst mildly reheating and then uniformly, gradually

cooling, stresses and strains can be controlled.

Objectives

1. To learn how to use a Bunsen burner.

2. To learn basic glassworking by bending and ﬁre-polishing glass tubing.

Procedure

The Laboratory Gas Burner; Use of the Bunsen Burner

1. Before connecting the Bunsen burner to the gas source, examine the burner and

compare it to Fig. 1.1. Be sure to locate the gas control valve and the air vents and see

how they work.

2. Connect the gas inlet of your burner to the main gas valve by means of a short piece of

thin-walled rubber tubing. Be sure the tubing is long enough to provide some slack for

movement on the bench top. Close the gas control valve. If your burner has a screw-needle

valve, turn the knob clockwise. Close the air vents. This can be done by rotating the barrel

of the burner (or sliding the ring over the air vents if your burner is built this way).

2 Experiment 1

Harcourt, Inc.

3. Turn the main gas valve to the open position. Slowly open the gas control valve

counterclockwise until you hear the hiss of gas. Quickly strike a match or use a gas

striker to light the burner. With a lighted match, hold the ﬂame to the top of the barrel.

The gas should light. How would you describe the color of the ﬂame? Hold a Pyrex test

tube in this ﬂame. What do you observe?

4. Carefully turn the gas control valve, ﬁrst clockwise and then counterclockwise. What

happens to the ﬂame size? (If the ﬂame should go out, or if the ﬂame did not light

initially, shut off the main gas valve and start over, as described above.)

5. With the ﬂame on, adjust the air vents by rotating the barrel (or sliding the ring). What

happens to the ﬂame as the air vents open? Adjust the gas control valve and the air

vents until you obtain a ﬂame about 3 or 4 in. high, with an inner cone of blue (Fig.

1.1). The tip of the pale blue inner cone is the hottest part of the ﬂame.

6. Too much air will blow out the ﬂame. Should this occur, close the main gas valve

immediately. Relight following the procedure in step 3.

7. Too much gas pressure will cause the ﬂame to rise away from the burner and “roar”

(Fig. 1.2). If this happens, reduce the gas ﬂow by closing the gas control valve until a

proper ﬂame results.

Figure 1.2

The ﬂame rises away

from the burner.

8. “Flashback” sometimes occurs. If so, the burner will have a ﬂame at the bottom of the

barrel. Quickly close the main gas valve. Allow the barrel to cool. Relight following the

procedures in step no. 3.

Basic Glassworking; Working with Glass Tubing

Cutting glass tubing

1. Obtain a length of glass tubing (5–6 mm in diameter). Place the tubing ﬂat on the

bench top, and with a grease pencil mark off a length of 30 cm. Grasp a triangular ﬁle

with one hand, placing your index ﬁnger on a ﬂat side of the ﬁle. With your other hand,

hold the tubing ﬁrmly in place against the bench top. At the mark, press the edge of the

ﬁle down ﬁrmly on the glass, and in one continuous motion scratch the glass (Fig. 1.3).

Figure 1.3

Cutting glass tubing with

a triangular ﬁle.

Harcourt, Inc.

Experiment 1 3

2. Place a drop of water on the scratch (this seems to help the glass break). Wrap the

tubing with cloth or paper towels and grasp with both hands, as shown in Fig. 1.4.

Place your thumbs on the unscratched side of the tubing, one thumb on either side of

the scratch. Position the scratch away from your body and face. Snap the tubing by

simultaneously pushing with both thumbs and pulling with both hands toward your

body. The tubing should break cleanly where the glass was scratched. Should the

tubing not break, repeat the procedure described above.

Figure 1.4

Breaking glass tubing.

Glass bends

1. Turn off the Bunsen burner and place a wing top on the barrel. The wing top will

spread out the ﬂame so that a longer section of glass will be heated to softness. Relight

the burner and adjust the ﬂame until the blue inner cone appears along the width of

the wing top (Fig. 1.5).

Figure 1.5

Wing top on the Bunsen burner.

2. Hold the midsection of the newly cut glass tubing in the ﬂame. Keep the tubing in the

hottest part of the ﬂame, just above the spread-out blue cone (Fig. 1.6). Rotate the

tubing continuously to obtain uniform heating. As the glass gets hot, the ﬂame should

become yellow; this color is due to sodium ions, which are present in the glass.

Figure 1.6

Holding the glass tubing

in the ﬂame.

When the glass gets soft and feels as if it is going to sag, remove the glass from the

ﬂame. Hold it steady without twisting or pulling (Fig. 1.7), and quickly, but gently, bend

it to the desired angle (Fig. 1.8).

4 Experiment 1

Harcourt, Inc.

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: