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Zhu Shiming 1 Chen Kai 2
1 Daiyao Senior High School, Xinghua City, Jiangsu Province 2 School of Biochemical and Environmental Engineering, Nanjing Xiaozhuang University
The "High School Chemistry Curriculum Standards" in the "Chemical Reaction Principles" elective module requires "to explain the causes of electrochemical corrosion of metals, to understand the hazards of metal corrosion, and to explore measures to prevent metal corrosion through experiments." "Most of the teaching relies on multimedia animation demonstrations to cooperate with theoretical teaching, and the relevant experiments are limited to" preventing corrosion ", not paying attention to" how to corrode "-why the corrosion of steel is mainly represented by oxygen participation? What galvanic reaction occurred during corrosion? Why does steel corrosion eventually generate rust? This series of problems is always difficult for middle school students who are accustomed to macro-representation thinking. The students' knowledge of steel corrosion still stays in the rote learning mode of the hard-back electrode.
In view of this, we have borrowed the special research literature on steel corrosion, combined with personal exploration, designed a series of classroom demonstration experiments, the total time of the experiment only takes about 10 minutes. If the following demonstration experiments can be added to the classroom during the new lecture, it will not only help students have a deep understanding of the corrosion of steel, but also have greater benefits for the protection of metal learning in the future.
1. Reproduce an accidental discovery (leading to interesting steel corrosion problems)
When I was in high school, I especially liked to make some handmade. When fiddling with his "earth" "multimeter" once, I discovered a phenomenon that the author at the time could not explain: When I inserted two self-made iron rods of the same kind into the damp soil, I found that The pointer of the multimeter is slightly deflected! Can the same iron electrode also constitute a primary battery?
Figure 1 An accidental discovery
Let us reproduce the facts at the time:
[Experiment 1] Add sodium chloride solution to the water tank with cotton cloth diaphragm, connect two iron plate electrodes with wire and a demonstration current meter for physics teaching (range 100μA), and observe whether there is current.
As shown in Figure 2, a rectangular plastic sink is used in this experiment, and a cotton diaphragm is added in the middle to connect the two sides, but it only slows the mutual diffusion of oxygen in the two poles. First add an appropriate amount of saline solution to it, and then prepare the two same iron electrode (previously polished with iron sandpaper to remove surface rust), and a physical 100μA demonstration galvanometer, and connect the wires. Special attention should be paid: the iron piece with the negative electrode of the ammeter should be inserted into the solution for about 10 to 20 seconds, and then the iron piece with the positive electrode of the ammeter should be inserted into the solution (this is the secret to the success of this experiment! Part of the oxygen is consumed due to local oxygen absorption corrosion, so the concentration of oxygen must be less than the solution on the other side of the cotton cloth, and the later inserted iron sheet must become the positive electrode of the original battery because the solution contains more oxygen. Therefore, the experimenter It is expected that the meter will deflect in the positive direction!). A miracle happened and the meter was obviously deflected (the current can be greater than 100μA, as shown in Figure 3, and then the current gradually decreased. According to the secondary school electrochemical theory, the primary battery should have 2 electrodes with different activities, but this What does the phenomenon mean? Since it turns out that the current is generated, it shows that the galvanic reaction has occurred, and the negative electrode must be iron lost electrons, and the oxidation reaction will be corroded.
Figure 2 Experimental device to verify that oxygen participated in steel corrosion (improvement 1)
Figure 3 The same two iron sheet electrodes can actually constitute a primary battery
The conclusion of this experiment: the same iron electrode inserted in sodium chloride solution can produce current, which proves the universality of steel corrosion.
At that time, the author tried it by the river, the pond, and the wet land, and the phenomenon was the same. Who is the cause of such steel corrosion?
2. What corroded the iron (design fun experiment to verify that oxygen participated in the corrosion of steel)
[Experiment 2] Use a fish tank aeration pump to blow air into one iron plate electrode, observe whether the galvanometer is deflected, and confirm the positive and negative poles of the primary battery; blow air into the other iron plate electrode, and observe whether the ammeter is deflected in reverse? What happens when the oxygen concentration is increased (as shown in Figure 4)?
Figure 4 Air flow control device modified from fish tank aerator and medical infusion set (improvement 2)
Figure 5 When the air is fed to the other side, the pointer deflects in the reverse direction
In order to facilitate the continuous air flow, we use the aquarium aeration pump as the air supply device, and use the medical infusion device as the flow rate control device. When the power supply of the aquarium pump is turned on, air is fed to one of the iron plate electrodes, and the pointer will be deflected; and then the air is fed to the other iron plate electrode, and the pointer will be deflected in the opposite direction. (As shown in Figure 5). Then through the air flow control device to increase the air intake, we found that the greater the air intake, the greater the pointer deflection.
The conclusion of this experiment: according to the direction of the current, we found that one pole of oxygen is the positive electrode of the primary battery, and the other iron electrode is the negative electrode of the primary battery; the deflection of the galvanometer pointer is affected by the oxygen concentration, which proves that oxygen participates The primary battery reaction means that oxygen does participate in the corrosion of steel, and the concentration of oxygen in the positive electrode solution is increased, and the corrosion rate of steel is accelerated.
This kind of iron corrosion with oxygen participating in steel that is close to neutral humid air is called oxygen absorption corrosion of steel. Therefore, the interesting question that was discovered by accident in the past also has an answer: the original concentration of oxygen in the same solution is not necessarily the same everywhere. The pole with a large oxygen concentration is used as the positive electrode of the original battery. In more professional information, this This kind of oxygen absorption corrosion caused by uneven distribution of oxygen is also called differential inflation corrosion. Although the corrosion problem of steel is very common, the reaction process is relatively complicated, but it is mainly manifested as oxygen absorption corrosion with oxygen participation. So, how to write the electrode reaction of such oxygen absorption corrosion of steel?
3. How steel is corroded (doubts about writing electrode equations for oxygen absorption corrosion of steel)
Oxygen participates in the galvanic cell reaction. What happens after the reaction? Is it hydroxide? Do the following experiment with doubts:
[Experiment 3] Add a few drops of phenolphthalein test solution to the solution that has been exposed to air for a period of time.
Phenomenon of this experiment: the solution on the side where oxygen is added does not turn red after adding phenolphthalein!
Oxygen is involved in the electrode reaction, is the ion generated after the reaction really hydroxide? Could it be that the diffusion rate of hydroxide is too fast, and the concentration of hydroxide is not enough to change the color of phenolphthalein?
4. Ingeniously design micro experiments for steel corrosion (verify the ions generated by the two poles during steel corrosion)
[Experiment 4] A micro experiment to verify the principle of steel corrosion in a U-shaped tube, as shown in Figure 6: In the U-shaped tube with gauze, add sodium chloride solution, and add a few drops of potassium ferricyanide solution to the two electrodes , Phenolphthalein test solution, then insert the wire electrode, carbon rod electrode, connect the physics teaching demonstration current meter (range 2.5mA), observe the deflection of the pointer and the color change of the two electrodes, and analyze the electrode reaction formula.
Figure 6 Schematic and physical picture of micro experiments to verify the principle of steel corrosion (improvement 3)
In this experiment, a cylindrical medical cotton gauze should be added to the lower part of the U-shaped tube in advance to slow down the diffusion rate of ions generated by the reaction of the primary battery. Because the cotton gauze layer is thick, we add sodium chloride solution from both sides (note: the amount of reagent is very particular, otherwise the red color of the positive electrode cannot be observed. According to the OH generated in the unit time when the current intensity of the original battery is certain, -It must be, OH- is still diffusing to the other electrode, plus the alkaline of the positive electrode must reach pH> 8 before red may appear, so the amount of carbon rod positive electrode solution should be as small as possible, U-shaped tube The caliber should be as close as possible to the diameter of the carbon rod, so as to ensure that the contact area of ​​the electrode and the solution is large, the internal resistance of the primary battery is small, and the generated OH-concentration is enough to discolor the phenolphthalein). Then add potassium ferricyanide on the iron electrode side and add phenolphthalein solution on the carbon rod electrode side (the purpose is to test Fe2 + and OH- separately, and explain to students that Fe2 + and potassium ferricyanide are blue and can be used to test Fe2 +) , Add the wire electrode and carbon rod electrode, change the ammeter to 5mA range, connect the circuit, and find that the pointer deflects and the current reaches 2mA (as shown in Figure 7).
Figure 7 Verification of the ions generated by the two poles when steel is corroded
Phenomenon of this experiment: In the trace experiment of U-shaped tube, the carbon rod electrode (positive electrode) is red; the iron wire electrode (negative electrode) is blue, to prove that during the corrosion of steel, the positive electrode produces OH- and the negative electrode produces Fe2 + .
5. In-depth study of the problem
Thinking 1: Why should we use U-shaped tubes for micro experiments?
The purpose is to generate a certain amount of hydroxide ions in a limited space, so that phenolphthalein quickly turns red. The carbon rod positive electrode solution should be as little as possible, and the diameter of the U-shaped tube should be as small as possible, so that the OH-concentration is enough to discolor the phenolphthalein. Red near the carbon rod electrode (positive electrode); blue near the iron wire electrode (negative electrode) to prove that during the corrosion of steel, the positive electrode produced OH- and the negative electrode produced Fe2 +.
Thinking 2: Must there be 2 electrodes with different activities to constitute the primary battery?
No, if the oxygen absorption corrosion of steel can be the same iron electrode, the same inert electrode can be used for the combustion battery.
Thinking 3: Is there any other way to test the products of oxygen-absorbing corrosion in the classroom?
Yes, but it is not suitable for classroom demonstration experiments. For example, the steel corrosion droplet experiment in "Optimization of the Conditions of Oxygen Oxidation Corrosion Experiment of Steel" designed by Liu Xianhao and Zhu Haiying is only suitable for group experiment of students.
6. Reflection on the design features of this demonstration experiment
1. The device is concise and intuitive, easy to observe, the experimental phenomenon is obvious and persuasive; especially using the current meter used in physics demonstration, the sensitivity is high and the scale is eye-catching, which is convenient for students to observe in the classroom.
2. The fish tank aeration pump used for blowing air is more creative to replace the initial chemical method to produce oxygen. In the "Demonstration Experiment of Oxygen-absorbing Corrosion", Ge Hongtao used the method of making oxygen in the laboratory, and then studied the oxygen-absorbing corrosion of steel through water. The use of a fish tank aerator (the market price is only more than ten yuan), compared with the use of potassium chlorate to produce oxygen or the use of gas to provide air, the air flow is more constant and easier to operate; in addition, the use of air to demonstrate the oxygen absorption of steel Corrosion, consistent with the actual steel corrosion conditions, is more convincing.
3. The micro experiments carried out in the U-shaped tube, the color contrast is bright, the students are particularly impressed.
4. Classroom measurement, students' understanding efficiency of oxygen absorption corrosion electrode reaction is much better than this Xuanke. In particular, students' writing reaction will not appear: 2H + + 2e- = H2 ↑, Fe-3e-= Fe3 + and other easily made mistakes.
According to the actual measurement, when the concentration of NaCl solution is 5mol / L, the partial surface area of ​​the iron sheet in the solution remains about 8cm2, and the distance between the iron sheets is 2cm, the effect of demonstrating that steel corrosion is affected by oxygen is more significant. The greater the concentration of saline solution is, the more obvious the effect is. It is recommended to use refined salt to prepare saturated saline solution, and then take the supernatant for experiment. Thick iron sandpaper should be used for rust removal of iron pieces, and soft non-rust iron wire should be used for iron wire. It is not suitable to use steel wire with greater hardness for experiments.
references
[1] Zou Lan, Li Qin, Wei Zhenshu. Discussion on the mechanism of differential aeration corrosion [J]. Journal of Zhoukou Normal College, 1999, 5: 9
[2] Mark T. Stauffer, Justin P. Fox. Yet Another Variation on the Electrolysis of Water at Iron Nails [J]. Journal of Chemical Education, 2008, 85 (4): 523
[3] Liu Xianhao, Zhu Haiying. Optimization of the conditions of steel oxygen absorption corrosion experiment [J]. Teaching Instruments and Experiments, 2008, 10:37
[4] Ge Hongtao. Demonstration experiment of oxygen absorption corrosion [J]. Teaching Reference for Chemistry in Middle School, 2000, 8 ～ 9: 8
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