Select instruments and reagents according to the experimental principle, and determine the arrangement and assembly order of instruments according to the experimental purpose. Generally, it follows the procedures of air extraction → impurity removal (several devices) → drying → main experiment → protection of experimental products and tail gas treatment. Among them, the order of impurity removal and drying, if using solution to remove impurities, is to purify first and then dry. Tail gas treatment generally adopts solution absorption or gas ignition. If the prepared substance is easy to hydrolyze or absorb moisture and CO2 gas, such as the protection of main product in the national college entrance examination question 24 in 2000, a device with a desiccant should be connected after the device for collecting FeCl3 and before the device for removing Cl2 to prevent the main product FeCl3 from being hydrolyzed.
Second, the interface connection
In general, the arrangement order of equipment should be followed. For the absorption device, if it is a gas cylinder, it should be "long" to go in (conducive to full absorption of impurities) and "short" to come out (conducive to gas export); If it is a drying pipe with alkali lime to absorb moisture and CO2, it should be "big" in (fully absorbed by CO2 and steam) and "small" out (beneficial to the output of residual gas); If it is a replacement gas, it should be "short".
Third, the air tightness inspection
All gas generating devices have air tightness inspection problems. The key is when to check the air tightness. How to check the air tightness? Obviously, after the instrument is connected, the air tightness should be checked before dosing. Although there are many ways to check the air tightness, the general principle is to plug one end, insert the other end into the water through a catheter, and then heat the glass container with a large volume slightly (with the palm of your hand or alcohol lamp). If there are bubbles escaping from the water and water column rising in the conduit after stopping heating, the air tightness is good, otherwise it must be reassembled and debugged.
Fourth, prevent sucking.
In the experiment of gas absorption by solution or gas collection by drainage, back absorption should be prevented. Generally speaking, there are two ways to prevent back suction: one is to prevent back suction in the device (such as adding a safety bottle in the device or absorbing gas with an inverted funnel). ), such as 1993, the uncondensed gas in the college entrance examination is absorbed by an inverted funnel filled with NaOH solution to prevent back absorption; 1996 college entrance examination question: install a safety bottle in front of the device for collecting H2 by drainage method to prevent back suction; 1992 NaOH solution was used to absorb SO2 in the college entrance examination, and a safety bottle B was installed before CO2 to prevent backward absorption. Generally speaking, in the experiment of collecting gas by drainage heating or washing gas with solution, at the end of the experiment, the catheter inserted into the solution should be taken out first, and then the alcohol lamp should be extinguished to prevent back suction.
Verb (abbreviation of verb) accident handling
In the process of chemical experiment, accidents will inevitably occur due to improper operation or negligence. The problem is to have a correct attitude and a cool head when an accident happens, so as not to panic, handle it correctly in time, standardize the operation as required, and try to avoid accidents. For example, when diluting concentrated sulfuric acid, the concentrated sulfuric acid should be slowly injected into the water along the inner wall of the container, and the heat should be evenly dissipated while stirring. The experiment of toxic gas should be done in a fume hood as far as possible. When you accidentally touch phenol on your hands, you should immediately scrub it with alcohol and then rinse it with water.
Example 2. (1997 college entrance examination questions) We must pay attention to safety when doing chemical experiments. The following statement is correct ().
A If you accidentally spill acid into your eyes, you should immediately rinse with water and blink while washing.
B. If the concentrated alkali solution touches the skin carelessly, it should be washed with plenty of water immediately and then coated with boric acid solution.
C if the concentrated phenol solution comes into contact with the skin, it should be washed with alcohol immediately.
D. When preparing sulfuric acid solution, a certain volume of water can be added to the measuring cylinder first, and then concentrated sulfuric acid can be slowly added under stirring.
Analysis: Obviously A, B and C are correct and D is incorrect. Because the measuring cylinder is a measuring instrument used to measure a certain volume of solution, not an instrument used to prepare solution, because its thin wall is not easy to stir and is easy to break when stirring, D belongs to wrong operation.
Evaluation of the experimental scheme of intransitive verbs
The evaluation of experimental scheme should follow the following principles: ① Whether the purpose can be achieved; ② Whether the raw materials used are common, easily available and cheap; ③ Utilization rate of raw materials; ④ Whether the process is simple and optimized; ⑤ Whether there is environmental pollution; ⑥ Whether the experimental error is creative, and so on. The experimental scheme that can meet the above six requirements should be said to be the optimal experimental scheme. For example, 1994 college entrance examination question: The experiment of students A and B preparing Mg3N2 belongs to a simple experiment scheme evaluation question. There are two kinds of FeCl3 _ 3 vapor collection device and tail gas treatment device in the college entrance examination in 2000, which also belong to the evaluation of experimental design scheme.
Seven. experiment design
Experimental design belongs to a higher level of ability examination, which is divided into experimental principle design and experimental operation program design. The design of the optimal scheme should follow the above six principles of experimental scheme evaluation. After the scheme is determined, in order to ensure the realization of the experimental purpose, there must be a simple and correct operating procedure.
Eight, the analysis of the experimental results
Experiment is a means. To achieve this goal, it is necessary to scientifically analyze and process experimental phenomena and experimental data, remove the false and retain the true, strip off false images from the outside to the inside, and recognize the true face of Lushan Mountain. Experiment is an effective means and way to cultivate students' scientific quality and various abilities. Generally, the following four aspects are considered: ① Whether the scheme is reasonable is the key to determine the success or failure of the experiment; ② Error caused by improper operation; (3) Inadequate reaction conditions may lead to failure of the reaction or slow reaction speed, resulting in experimental errors; ④ Impure reagents, impurities and even participation in the reaction will lead to experimental errors, and so on.