Experimeni 1
The Laboratory Burner
Objectives 1. To become familiar with a laboratory burner and its efficient use. 2. To gain experience using the burner in the manipulation of glass tubing and glass rods. 3. To be introduced to a chemical reaction. 4. To observe and optimize the combustion reaction produced by various burner settings.
Discussion Many experiments require the application of heat. If an open flame is not to be hazardous, a gas burner is needed to control the heat production. A common source of this heat is the heat energy released when a fuel such as natural gas is burned in air.
Almost all laboratory burners used today are modifications of the original Bunsen bwner invented by the German chemist Robert Bunsen. One of the more commonly used modifications is called a Tirri! burner. Bunsen~s fundamental design is also widely used in domestic and indust.."i.al gas burners. To operate the burner a combustible gas is allo·wed to flow rapidly from a jet into a mixing chamber (barrel); air containing oxygen is drawn into the barrel through inlet holes due w a vacuum effect created by the rapidly moving gas. At the top of the burner the gas · air mixture is combusted. The temperature may be regulated by adjusting the ratio of gas to oxygen. With low amounts of oxygen, the flame is relatively cool and is referred to as a luminous yellow flame. As more oxygen is mixed with the gas, the temperature of the flame increases. A very hot flame is referred to as nonluminous and is characterized by a blue cone surrounded by a larger yellowish envelope.
In this experiment the laboratory burner will be used to heat and soften glass tubing in order to manipulate and bend the glass. For some manipulations it is desirable to heat a short section of the tubing rather than a single point. This is accomplished by attaching a wing top {flame spreader) to the top of the barrel. Other types of glassworking to be carried out are glass cutting and fire-polishing (smoothing out sharp edges on glass by heating and softening).
The primary use of the lab bu..-ner in chemistry courses is to heat water, and if necessary keep the water hot or warm.
The combustion of the gaseous mixture in the burner is a chemical reaction. Natw·al gas is principally the chemical methane, a C-"Olorless, odorless gas. Gas companies reduce the potential danger of asphyxiation or explosion, which could result from gas leakage, by adding, in small amounts, another chemical with a pungent odor. Thus the presence of natural gas in the air from leaks or inadvertent open valves can usually be detected before dangerous amounts escape. The chemical change which occurs when natural gas is burned results in the formation of two other common chemical substances, carbon dioxide and water. The overall reaction may be stated as follows:
When methane and oxygen gases are mixed and ignited, they react to form carbon dioxide gas, water vapor, heat and light. The chemical reaction may also be described in the form of a word equation.
methane heat and light l-2 Procedure A. Tirri! Burner Examine the construction of vour burner and become familiar with the name and function of each part. The burner may be disassembled into three parts, (see Figure 1). B..ru:rcl_: The top part of the burner which screws onto the base. Air, hence oxygen, enters the barrel through air inlet holes in the bottom of the barrel. The amount of oxygen is regulated by screwing the barrel up or down. Base: The part that rests on the laboratory bench. It has a gas inlet which is connected to a gas jet with a rubber hose. Needle Valve: A specially designed screw that threads into the base from the bottom. When screwed all the way in, the tip (needle) can be felt protruding through a small hole (spud) in the top of the base. This valve regulates the hole opening thereby adjusting the amount of methane flowing through the base into the barrel. Barrel (mixmg chamberl Air is drawn into ---1111u... the barrel.. Gas is sup plied to the base via a rubber hose. \ .. ~V-----:::-1..~'-~"'- Gas emerges from this hole, ~~...... iitr--1-~~ Base~-==~~~:!::::!:=:t:ir-~~.J+· (spud) at a rate regulated by the needle valve. Needle Valve More gas Less gas Tirril Burner Burner Components Figure 1: Tirril Burner and Components B. Lighting the Burner 1. Close the air inlet holes by screwing the barrel down on the base as far as it will go. 2. Screw the needle valve all the way into the base, then reverse direction about one revolution. 3. Light a match and hold it near the burner just below the top of the barrel. 4. Keep face, hair, clothing, etc. well away from the burner. 5. Tum on the gas at the jet (main valve) so that the handle is parallel with the jet. 6. Raise the match until the gaseous mixture flowing from the burner lights. 7. Put out the match, dowse it in water, and dispose of it in the trash (do not put the used match in the sink). C. Characterizing the Luminous Flame The above flame is called a luminous flame because it contains small particles of carbon (soot) which glow at the elevated temperature in the flame. This is not the best adjustment for obtaining maximum temperature from the fuel. Actually it is the coolest flame obtainable from the burner; note its color. Under these conditions the methane is not completely combusted (or oxidized) resulting in the formation of some soot. Hold a crucible in the luminous flame for several seconds using crucible tongs. Note the color and character of the deposit on the crucible. D. Adi us ting the Flame Slowly open the air holes by screwing the barrel upward. As more air is introduced the flame color changes from luminous yellow to blue and the flame volume decreases slightly. When sufficient air has been admitted to cause the flame to separate from the rim of the barrel and a roaring sound is heard, screw the barrel down until the flame returns to the ii.m and the roaring sound ceases. It is possible that the burner may go out before this final adjustment is made. If so, turn off the gas at the jet until a match is struck and placed in position to light the burner. Turn on the gas at the jet to relight the burner. If any t.1ifficulty is encountered, close the air inlet holes part way, and try lighting the burner again. E. Characterizing the Nonluminous Flame Make fine adjustments by slightly turning the barrel or needle valve as necessary to produce a flame with three zones: an inner dark cone; a central bright blue cone; and an outer larger cone. One zone of this flame is the hottest obtainable from the burner. The flame temperature varie; from region to region. In this relatively hot flame methane undergoes complete combustion (or oxidation) forming only the expected products, carbon dioxide and water; there are no carbon particles so the flame is nonluminous. Hold unlighted matches in various regions of the flame, and note the time it takes for them to ignite in each location. This procedure may also be done using uncharred wooden splints. Hold a wooden splint at both ends and rest its center on top of and across the barrel of the lighted burner until it chars or ignites. Remove it, blow out the flame on the splint and note the charring pattern. F. Characterizing the Combustion Produ~ Hold a large, dry, cool beaker inverted about 15 centimeters above the top of a nonluminous flame. Note the condensation inside the beaker. G. True - False~ Take the quiz on page 1-7 and check your answers with the key posted in the laboratory. l-4 H. Glassworking To cut glass tubing and rod, place it flat on the table and make a single deep scratch with a triangular file. Then place the triangular file on the table and position the scratched glass directly above the pointed edge of the file, with the scratch on top (scratch facing up). Grasp the tube or rod firmly with both hands about two inches each side of the scratch and gently push down with both hands. Prepare a piece of glass tubing and a piece of glass rod about 7-9 inches long using the longer pieces available in the lab. Fire-polishing the ends of glass tubing or rods makes the sharp, jagged end smooth. Hold the glass at an angle with the end in the hottest part of the flame and rotate slowly until the glass softens and becomes smooth (Figure 2) . .Any cut glass or rotate back chipped glassware should be fire-polished before further use. and forth Fire-polish the ends of the 7-9 inch pieces of tubing and rod prepared above. Do not hold the tubing in the flame too long because the glass may become so soft that the opening will start to close. To bend glass tubing, hold the tube lengthwise in a wing top flame (Figure 3) of the burner so that about 2 inches of the glass will be heated (Figure 4). Rotate it slowly so that all sides are heated equally. When the tube softens remove it from the flame and make the desired bend. The bend should be uniformly curved with no flat places or kinks (Figure 5). Figure 2: Fire Polishing Glass Tubing Bend the 7-9 inch piece of glass tubing prepared above until it forms an angle of about 90°. remove from flame before hen.ding wing top rotate with both hands poor poor good Figure 3: Wing Top Flames Figure 4: Heating Glass Tubing in a Wing Top Flame good poor (glass was poor (glass not heated strongly heated in the at one point) middle) Figure 5: B(;mt Glass Tubes of Different Qualities 1-5 Experiment 1 The Laboratory Burner Drawer Number______Questions 1. Why should the match be lighted before turning on the gas? 2. Why must the methane be mixed with the air in the barrel before it is burned? :3. What is the color of the flame when the air inlet holes are closed? A. YELLOW B. BLUE 4. What is observed when a crucible is held in a luminous flame? Why does a luminous flame cause this? 5. What is required to convert the cooler luminous flame to a hotter flame? 6. Draw a diagram of the noniuminous, three-zone flame. and label each region according to its reiative temperature (hottest, medium or coolest). Use your match or splint lighting test data as the basis for your decision. 7. What is the condensate that forms inside the beaker when it is held above the flame? Where does it come from? b. 8. Why should cut glass or chipped glassware always be fire-polished before using for other purposes? Instructor1 s Approval ------(Show fire-polishing and glass bending. Show cool glass only.) 1-6 Intentionally Blank l-7 Experiment 1 Laboratory Burner Drawer Number ______ Feedback Quiz - Laboratory Burners Determine whether each statement is TR1JE or FALSE; circle Tor F. If the answer to a problem is F J..LSK change the underlined word or phrase so that the statement will be TRUE. T F 1. Natural gas is primarily the chemical methane. 'f'I <) J.. F ..... The purpose of the burner is to mix fuel with the nitrogen in air. T F 3. The hottest part of the flame is in the center just above the barrel. T F 4. A yellow flame indicates an excess of air. T F 5. A bluish flame with three distinct zones is the correct flame to use in most laboratory work. T F 6. To increase the amount of air mi.'tlng with the natural gas, screw the barrel of the burner upward. T F 7. If the burner operates with a roaring sound, increase the amount of air. T F 8. If the burner flame is yellow you should screw the barrel downward. T F 9. The needle valve at the bottom of the burner controls the gg_s flow into the barrel. T F 10. The yellow flame will deposit yellow particles on a cold surface above it. T F 11. A match will not ignite if held in the center of the flame just a~:>O"ve the blue cone. T F l'> Methane is a bluish gas. T F 13. When lighting the burner you should always hold your face well away from it. T F 14. The energy released by the burning of methane is greater than that required to ignite the natural gas. T F 15. It is easy to see the difference between hot and cold glass.