Does aluminum burn in an open fire? How to get an even flame color? Combustion of aluminum-magnesium alloys in air

It is not difficult to guess that the hue of a flame is determined by the chemicals burning in it, if exposure to high temperature releases individual atoms of the combustible substances, coloring the fire. To determine the effect of substances on the color of fire, various experiments were carried out, which we will discuss below.

Since ancient times, alchemists and scientists have tried to find out what substances burn, depending on the color that the flame acquires.

The flames of gas water heaters and stoves, available in all houses and apartments, have a blue tint. When burned, this shade is produced by carbon, carbon monoxide. The yellow-orange color of the flame of a fire that is lit in the forest, or of household matches, is due to the high content of sodium salts in natural wood. Largely thanks to this - red. The flame of a gas stove burner will acquire the same color if you sprinkle it with ordinary table salt. When copper burns, the flame will be green. I think you have noticed that when you wear a ring or chain made of ordinary copper that is not coated with a protective compound for a long time, the skin becomes green. The same thing happens during the combustion process. If the copper content is high, a very bright green light occurs, almost identical to white. This can be seen if you sprinkle copper shavings on a gas burner.

Many experiments have been carried out using an ordinary gas burner and various minerals. In this way their composition was determined. You need to take the mineral with tweezers and place it in the flame. The color that fire takes on can indicate the various impurities present in the element. A green flame and its shades indicate the presence of copper, barium, molybdenum, antimony, and phosphorus. Boron produces a blue-green color. Selenium gives the flame a blue tint. The flame is colored red in the presence of strontium, lithium and calcium, and violet - potassium. The yellow-orange color is produced when sodium burns.

Studies of minerals to determine their composition are carried out using a Bunsen burner. The color of its flame is even and colorless; it does not interfere with the course of the experiment. Bunsen invented the burner in the mid-19th century.

He came up with a method that allows one to determine the composition of a substance by the shade of the flame. Scientists had tried to conduct similar experiments before him, but they did not have a Bunsen burner, the colorless flame of which did not interfere with the progress of the experiment. He placed various elements on a platinum wire into the burner fire, since when this metal is added, the flame does not become colored. At first glance, the method seems good; labor-intensive chemical analysis can be dispensed with. You just need to bring the element to the fire and see what it consists of. But substances in their pure form can be found extremely rarely in nature. They usually contain large quantities of various impurities that change the color of the flame.

Bunsen tried to highlight colors and shades using various methods. For example, using colored glass. Let’s say that if you look through blue glass, you won’t see the yellow color that fire turns when burning the most common sodium salts. Then the lilac or crimson shade of the desired element becomes distinguishable. But even such tricks led to the correct determination of the composition of a complex mineral in very rare cases. This technology could not achieve more.

Nowadays, such a torch is used only for soldering.

Flames come in different colors. Look into the fireplace. Yellow, orange, red, white and blue flames dance on the logs. Its color depends on the combustion temperature and the combustible material. To visualize this, imagine the spiral of an electric stove. If the tile is turned off, the spiral turns are cold and black. Let's say you decide to heat up the soup and turn on the stove. At first the spiral turns dark red. The higher the temperature rises, the brighter the red color of the spiral. When the tile reaches its maximum temperature, the coil turns orange-red.

Naturally, the spiral does not burn. You don't see the flame. She's just really hot. If you heat it further, the color will change. First, the color of the spiral will turn yellow, then white, and when it heats up even more, a blue glow will emanate from it.

Something similar happens with fire. Let's take a candle as an example. Different areas of a candle flame have different temperatures. Fire needs oxygen. If you cover a candle with a glass jar, the fire will go out. The central area of ​​the candle flame adjacent to the wick consumes little oxygen and appears dark. The top and side areas of the flame receive more oxygen, so these areas are brighter. As the flame moves through the wick, the wax melts and crackles, breaking into tiny carbon particles. (Coal also consists of carbon.) These particles are carried upward by the flame and burn. They are very hot and glow like the spiral of your tile. But the carbon particles are much hotter than the coil of the hottest tile (carbon combustion temperature is approximately 1,400 degrees Celsius). Therefore, their glow is yellow. Near the burning wick, the flame is even hotter and glows blue.

The flames of a fireplace or fire are mostly motley in appearance. Wood burns at a lower temperature than a candle wick, so the base color of the fire is orange rather than yellow. Some carbon particles in a fire flame have a fairly high temperature. There are few of them, but they add a yellowish tint to the flame. Cooled particles of hot carbon are soot that settles on the chimneys. The burning temperature of wood is lower than the burning temperature of a candle. Calcium, sodium and copper, when heated to high temperatures, glow in different colors. They are added to rocket powder to color the lights of holiday fireworks.

Flame color and chemical composition

The color of the flame may vary depending on the chemical impurities contained in the logs or other flammable substance. The flame may contain, for example, sodium impurities.

Even in ancient times, scientists and alchemists tried to understand what kind of substances burned in fire, depending on the color of the fire.

• Sodium is a component of table salt. When sodium is heated, it turns bright yellow.
• Calcium may be released into the fire. We all know that milk contains a lot of calcium. It's metal. Hot calcium turns bright red.
• If phosphorus burns in a fire, the flame will turn greenish. All these elements are either contained in wood or enter the fire with other substances.
• Almost everyone at home has gas stoves or water heaters, the flames of which are colored blue. This is due to combustible carbon, carbon monoxide, which gives this shade.

Mixing the colors of a flame, like mixing the colors of a rainbow, can produce white, which is why white areas are visible in the flames of a fire or fireplace.

Flame temperature when burning certain substances:

How to get an even flame color?

To study minerals and determine their composition, it is used Bunsen burner, giving an even, colorless flame color that does not interfere with the course of the experiment, invented by Bunsen in the middle of the 19th century.

Bunsen was an ardent fan of the fire element and often tinkered with flames. His hobby was glassblowing. By blowing various cunning designs and mechanisms out of glass, Bunsen could not notice the pain. There were times when his calloused fingers began to smoke from the hot, still soft glass, but he did not pay attention to it. If the pain had already gone beyond the threshold of sensitivity, then he saved himself using his own method - he pressed his earlobe tightly with his fingers, interrupting one pain with another.

It was he who was the founder of the method of determining the composition of a substance by the color of the flame. Of course, before him, scientists tried to carry out such experiments, but they did not have a Bunsen burner with a colorless flame that did not interfere with the experiment. He introduced various elements on platinum wire into the burner flame, since platinum does not affect the color of the flame and does not color it.

It would seem that the method is good, there is no need for complex chemical analysis; bring the element to the flame and its composition is immediately visible. But it was not there. Very rarely substances are found in nature in their pure form; they usually contain a large range of various impurities that change color.

Bunsen tried various methods of isolating colors and their shades. For example, I tried to look through colored glass. Say, blue glass extinguishes the yellow color produced by the most common sodium salts, and one could discern a crimson or purple tint of the native element. But even with the help of these tricks, it was possible to determine the composition of a complex mineral only once in a hundred.

This is interesting! Due to the property of atoms and molecules to emit light of a certain color, a method was developed for determining the composition of substances, which is called spectral analysis. Scientists study the spectrum that a substance emits, for example, when it burns, compare it with the spectra of known elements, and thus determine its composition.

Aluminum burning

Aluminum burning in air

Unlike magnesium, single aluminum particles do not ignite when heated in air or water vapor to 2100 K. Burning magnesium particles were used to ignite aluminum. The latter were placed on the surface of the heating element, and the aluminum particles were placed on the tip of the needle at a distance of 10-4 m above the former.

When aluminum particles are ignited, ignition occurs in the vapor phase, and the intensity of the glow zone that appears around the particle increases slowly. Stationary combustion is characterized by the existence of a glow zone, which does not change its size until the metal is almost completely burned out. The ratio of the sizes of the glow zone and the particle is 1.6-1.9. In the glow zone, small oxide droplets are formed, which merge upon collision.

The residue after combustion of the particle is a hollow shell containing no metal inside. The dependence of the burning time of a particle on its size is expressed by the formula (symmetrical combustion).

Combustion of aluminum in water vapor

Ignition of aluminum in water vapor occurs heterogeneously. The hydrogen released during the reaction contributes to the destruction of the oxide film; in this case, liquid aluminum oxide (or hydroxide) is sprayed in the form of droplets with a diameter of up to 10-15 microns. Such destruction of the oxide shell is periodically repeated. This suggests that a significant fraction of the metal burns on the surface of the particle.

At the beginning of combustion, the ratio rsv /r 0 equals 1.6-1.7. During the combustion process, the particle size decreases, and the gs/?o ratio increases to 2.0-3.0. The burning rate of an aluminum particle in water vapor is almost 5 times greater than in air.

Combustion of aluminum-magnesium alloys

Combustion of aluminum-magnesium alloys in air

The ignition of particles of aluminum-magnesium alloys of variable composition in air, oxygen-argon mixtures, water vapor and carbon dioxide proceeds, as a rule, similar to the ignition of magnesium particles. The onset of ignition is preceded by oxidative reactions occurring on the surface.

The combustion of aluminum-magnesium alloys differs significantly from the combustion of both aluminum and magnesium and strongly depends on the ratio of components in the alloy and on the parameters of the oxidizing environment. The most important feature of the combustion of alloy particles is the two-stage process (Fig. 2.6). At the first stage, the particle is surrounded by a set of torches, forming a non-uniform zone of luminescence of the reaction products. Comparing the nature and size of the luminous zone surrounding the alloy particle during the first stage of combustion with the nature and size of the luminous zone around the burning magnesium particle (see Fig. 2.4), we can conclude that at this stage, mainly magnesium burns out of the particle.

Rice. 2.6. Combustion of an alloy particle of 30% Al + 70% Mg at normal atmospheric pressure in a mixture containing 15% O by volume 2and 85% Ar:

1, 2 – magnesium burnout; 3-6 – aluminum burnout

A feature of the first stage of alloy combustion is the constancy of particle size and flame zone. This means that the liquid drop of the alloy is contained within a solid oxide shell. The oxide film is dominated by magnesium oxide. Through film defects, magnesium flows out, burning in a vapor-phase diffusion flame.

At the end of the first stage, the occurrence of heterogeneous reactions increases, as evidenced by the appearance of areas of bright luminescence on the surface of the particle. The heat released during heterogeneous reactions contributes to heating the particle to the melting point of the oxide and the beginning of the second stage of combustion.

At the second stage of combustion, the particle is surrounded by a uniform, brighter glow zone, which decreases as the metal burns out. The homogeneity and sphericity of the flame zone indicates that the oxide film on the surface of the particle is molten. Diffusion of the metal through the film is ensured by the low diffusion resistance of the liquid oxide. The size of the flame zone significantly exceeds the particle size, which indicates combustion of the metal in the vapor phase. Comparison of the nature of the second stage of combustion with the known pattern of aluminum combustion indicates a great similarity; it is likely that aluminum burns at this stage of the process. As it burns out, the size of the flame and, consequently, the burning drop decrease. The burnt particle glows for a long time.

Changing the size of the glow zone of a particle burning in accordance with the described mechanism is complex (Fig. 2.7). After ignition the value r St. /r 0 quickly (in -0.1 ms) reaches the maximum value (section ab). Further, during the main time of the first stage of combustion, the ratio r St/ r 0 remains constant (section bv). When the magnesium burnout ends, r cv/ r 0 is reduced to a minimum (point G), and then, with the beginning of aluminum combustion, it increases (section gd). Finally, but as aluminum burns out r St. /r 0 decreases monotonically (section de) to a final value corresponding to the size of the formed oxide.

Rice. 2.7.:

1 – alloy 30% Al + 70% Mg, air; 2 – alloy 30% A1 + 70% Mg, mixture 15% O2 + 85% Ar; 3 – alloy 50% A1 + 50% Mg, air

The mechanism and parameters of the combustion process of aluminum-magnesium alloys significantly depend on the composition of the alloy. With a decrease in magnesium content in the alloy, the size of the glow zone during the first stage of combustion and the duration of this stage decreases. When the magnesium content in the alloy is less than 30%, the process remains a two-stage process, but becomes intermittent. At the end of the first stage, the glow zone is reduced to the size of the particle itself, the combustion process stops, and aluminum burns out only after the particle is re-ignited. The particles that do not ignite again are hollow, porous oxide shells containing droplets of unburned aluminum inside.

The dependence of the burning time of particles on their initial diameter is expressed by the following empirical formulas:

Combustion of aluminum-magnesium alloys in mixtures of oxygen with argon, in water vapor and in carbon dioxide.

The nature of combustion of particles of aluminum-magnesium alloys in oxygen-argon mixtures is the same as in air. With a decrease in oxygen content, the size of the glow zone during magnesium burnout noticeably decreases. The dependence of the combustion time of particles of the 50% Al + 50% Mg alloy on the particle size and oxygen content in the mixture in volume percent is expressed by the formula

The combustion of alloys in water vapor is significantly different (Fig. 2.8). The oxide film formed during the first stage is destroyed by hydrogen, and the particle takes on the appearance of coral. The aluminum remaining in the coral ignites only 1-10 ms after the end of the first stage. Such intermittency of the process is typical for alloys of any composition.

Rice. 2.8. Combustion of aluminum-magnesium alloy particles (50:50) spherical(A) and wrong(b) forms in water vapor at normal atmospheric pressure:

1 – initial particle; 2 – particle before ignition; 3 – magnesium burnout; 4 – aluminum burnout; 5 – coral formed after the particle

When aluminum-magnesium alloys burn in carbon dioxide, only magnesium burns out of the particle, after which the combustion process stops.

Combustion of aluminum-magnesium alloys in a high-temperature flame

To study the combustion process of metal particles at high temperatures, a pressed tablet of mixtures of ammonium perchlorate and hexamine, having calculated combustion temperatures of 2500, 2700 and 3100 K, was burned under a particle mounted on the tip of a needle.

Combustion of particles of aluminum-magnesium alloys under these conditions occurs, as a rule, with an explosion. The presence of an explosion is typical for particles of all compositions. As a result of the explosion, a significant luminescence zone is formed, which is a sign of the predominance of vapor-phase combustion. Photographs of a burning particle at the beginning of combustion (Fig. 2.9, A) show that heterogeneous reactions occur over the entire surface of the oxide shell. Due to the heat of heterogeneous reactions, rapid evaporation of the metal occurs (Fig. 2.9, b), promoting rupture of the oxide shell and splashing of the unevaporated drop (Fig. 2.9, V).

Rice. 2.9. Combustion of 95% Al alloy particle with 5% Mg in oxidizing flame (temperature 2700 K):

A– initial stage of combustion; b– stationary combustion; V- splitting up

According to B. G. Lrabey, S. E. Salibekov and Yu. V. Leninsky, crushing of particles of aluminum-magnesium alloys is caused by a very large difference in the boiling temperatures of magnesium and aluminum, as a result of which the boiling of magnesium when the particle is in a high-temperature zone is explosive. and leads to crushing of the remaining aluminum. A temperature of 2500 K is already sufficient for explosive combustion to occur, which is quite natural, since this temperature exceeds the boiling point of both components.

• Arabey B. G., Salibekov S. E., Levinsky Yu. V. Some characteristics of ignition and combustion of metal dust // Powder metallurgy. 1964. No. 3. P. 109-118.

To formulate the laws of the ongoing processes, we can limit ourselves to considering cations and exclude anions, since they themselves do not participate in the reaction. (However, the rate of deposition is affected by the type of anions.) If, for simplicity, we assume that both the released and dissolved metals are divalent, then we can write:

Me 1 + Me 2 2+ => Me 1 2+ + Me 2

Moreover, for the first experiment Me 1 = Fe, Me 2 = Cu. So, the process consists of the exchange of charges (electrons) between atoms and ions of both metals. If we separately consider (as intermediate reactions) the dissolution of iron or the precipitation of copper, we obtain:

Fe => Fe 2+ + 2е -
Сu 2+ + 2е - => Сu

Now consider the case when a metal is immersed in water or in a salt solution, with a cation of which exchange is impossible due to its position in the stress series. Despite this, the metal tends to go into solution in the form of an ion. In this case, the metal atom gives up two electrons (if the metal is divalent), the surface of the metal immersed in the solution becomes negatively charged relative to the solution, and a double electric layer is formed at the interface. This potential difference prevents further dissolution of the metal, so that the process soon stops. If two different metals are immersed in a solution, they will both charge, but the less active one will be somewhat weaker, due to the fact that its atoms are less prone to losing electrons. Let's connect both metals with a conductor. Due to the potential difference, a flow of electrons will flow from the more active metal to the less active one, which forms the positive pole of the element. A process occurs in which the more active metal goes into solution, and cations from the solution are released on the more noble metal.

The essence of a galvanic cell

Let us now illustrate with several experiments the somewhat abstract reasoning given above (which, moreover, represents a gross simplification).

First, fill a 250 ml beaker to the middle with a 10% solution of sulfuric acid and immerse not too small pieces of zinc and copper in it. We solder or rivet copper wire to both electrodes, the ends of which should not touch the solution.

As long as the ends of the wire are not connected to each other, we will observe the dissolution of zinc, which is accompanied by the release of hydrogen. Zinc, as follows from the voltage series, is more active than hydrogen, so the metal can displace hydrogen from the ionic state. An electrical double layer is formed on both metals. The easiest way to detect the potential difference between the electrodes is with a voltmeter. Immediately after connecting the device to the circuit, the arrow will indicate approximately 1 V, but then the voltage will quickly drop. If you connect a small light bulb that consumes 1 V to the element, it will light up - at first quite strongly, and then the glow will become weak.

Based on the polarity of the device terminals, we can conclude that the copper electrode is the positive pole. This can be proven without a device by considering the electrochemistry of the process. Let's prepare a saturated solution of table salt in a small beaker or test tube, add about 0.5 ml of an alcohol solution of the phenolphthalein indicator and immerse both electrodes closed with wire into the solution. A faint reddish color will be observed near the negative pole, which is caused by the formation of sodium hydroxide at the cathode.

In other experiments, one can place various pairs of metals in a cell and determine the resulting voltage. For example, magnesium and silver will give a particularly large potential difference due to the significant distance between them and a series of voltages, while zinc and iron, on the contrary, will give a very small one, less than a tenth of a volt. By using aluminum, we will not receive practically any current due to passivation.

All these elements, or, as electrochemists say, circuits, have the disadvantage that when measuring current, the voltage across them drops very quickly. Therefore, electrochemists always measure the true value of voltage in a de-energized state using the method voltage compensation, that is, comparing it with the voltage of another current source.

Let us consider the processes in the copper-zinc element in a little more detail. At the cathode, zinc goes into solution according to the following equation:

Zn => Zn 2+ + 2е -

Hydrogen ions of sulfuric acid are discharged at the copper anode. They attach electrons coming through the wire from the zinc cathode and as a result, hydrogen bubbles are formed:

2Н + + 2е - => Н 2

After a short period of time, the copper will be covered with a thin layer of hydrogen bubbles. In this case, the copper electrode will turn into a hydrogen one, and the potential difference will decrease. This process is called polarization electrode. The polarization of the copper electrode can be eliminated by adding a little potassium dichromate solution to the cell after the voltage drop. After this, the voltage will increase again, as potassium dichromate will oxidize hydrogen to water. Potassium dichromate acts in this case as depolarizer

In practice, galvanic circuits are used whose electrodes are not polarized, or circuits whose polarization can be eliminated by adding depolarizers.

As an example of a non-polarizable element, consider the Daniel element, which was often used in the past as a current source. This is also a copper-zinc element, but both metals are immersed in different solutions. The zinc electrode is placed in a porous clay cell filled with dilute (about 20%) sulfuric acid. The clay cell is suspended in a large glass containing a concentrated solution of copper sulfate, and at the bottom there is a layer of copper sulfate crystals. The second electrode in this vessel is a cylinder made of copper sheet.

This element can be made from a glass jar, a commercially available clay cell (in extreme cases, we use a flower pot, closing the hole in the bottom) and two electrodes of suitable size.

During operation of the cell, zinc dissolves to form zinc sulfate, and metallic copper is released at the copper electrode. But at the same time, the copper electrode is not polarized and the element produces a voltage of about 1 V. Actually, theoretically, the voltage at the terminals is 1.10 V, but when collecting current we measure a slightly lower value due to the electrical resistance of the cell.

If we do not remove the current from the element, we need to remove the zinc electrode from the sulfuric acid solution, because otherwise it will dissolve to form hydrogen.

A diagram of a simple cell that does not require a porous partition is shown in the figure. The zinc electrode is located at the top of the glass jar, and the copper electrode is located near the bottom. The entire cell is filled with a saturated solution of table salt. Place a handful of copper sulfate crystals at the bottom of the jar. The resulting concentrated copper sulfate solution will mix with the table salt solution very slowly. Therefore, when the cell operates, copper will be released on the copper electrode, and zinc will dissolve in the form of sulfate or chloride in the upper part of the cell.

Nowadays they are used almost exclusively for batteries. dry elements, which are more convenient to use. Their ancestor is the Leclanche element. The electrodes are a zinc cylinder and a carbon rod. The electrolyte is a paste that mainly consists of ammonium chloride. Zinc dissolves in the paste, and hydrogen is released on the coal. To avoid polarization, the carbon rod is dipped into a linen bag containing a mixture of coal powder and pyrolusite. The carbon powder increases the electrode surface, and the pyrolusite acts as a depolarizer, slowly oxidizing the hydrogen. True, the depolarizing ability of pyrolusite is weaker than that of the previously mentioned potassium bichromate. Therefore, when current is received in dry elements, the voltage quickly drops, they “tire” due to polarization. Only after some time does the oxidation of hydrogen occur with pyrolusite. Thus, the elements “rest” if no current is passed for some time. Let's check this on a flashlight battery to which we connect a light bulb. We connect a voltmeter parallel to the lamp, that is, directly to the terminals. At first, the voltage will be about 4.5 V. (Most often, such batteries have three cells connected in series, each with a theoretical voltage of 1.48 V.) After some time, the voltage will drop and the glow of the light bulb will weaken. Based on the voltmeter readings, we can judge how long the battery needs to rest.

A special place is occupied by regenerating elements known as batteries. They undergo reversible reactions and can be recharged after the cell has been discharged by connecting to an external DC source.

Currently, lead-acid batteries are the most common; The electrolyte in them is dilute sulfuric acid, into which two lead plates are immersed. The positive electrode is coated with lead peroxide PbO 2 (modern name is lead dioxide), the negative electrode is metallic lead. The voltage at the terminals is approximately 2.1 V. When discharging, lead sulfate is formed on both plates, which again turns into metallic lead and lead peroxide when charging.