Experimental Topic---Preparation of Trihydrofuran Chromium Chloride, Evaluation

26. (15 points total)

A research group used Cr2O3 (green solid), CCl4, THF (tetrahydrofuran, liquid) and other substances as raw materials to prepare trihydrofuran chromium chloride.

I. Apparatus for the preparation of anhydrous CrCl3 (purple solid, easily sublimated) (clamping device omitted).

1. (1) The name of instrument a is ________。
2. (2) The purpose of this experiment is to continuously access N2 ________。
3. (3) When the temperature of the reaction tube rises to 660 ° C, the reaction occurs to generate CrCl3 and COCl2 (phosgene), and the chemical equation is ________
4. (4) COCl2 is highly toxic and reacts with water to form two acidic gases, and the two salts generated in the b unit are ________ ( chemical formula).
5. (5) The right end of the reaction tube has 15cm outside the tube furnace, and its function is ________

II. Synthetic CrCl3 (THF) 3

Known: ① Tetrahydrofuran (THF) is a common organic solvent with a boiling point of 66 ° C.

② The main reaction to prepare CrCl3 (THF) 3:

③ Both CrCl3 and CrCl3 (THF) 3 react easily with water, and divalent chromium ions have a catalytic effect on the synthesis of CrCl3 (THF) 3

The experimental steps are as follows: The prepared anhydrous CrCl3 and a very small amount of zinc powder are put into the filter paper sleeve, and a sufficient amount of anhydrous THF is added to the double-necked flask. During the experiment, the THF in the flask is heated and evaporated, and the steam rises along the "Soxhlet extractor" conduit 2 to the spherical condenser tube. After condensation, it is dripped into the filter paper sleeve and reacts with the solid matter in the sleeve. When the liquid level reaches the top of the "Soxhlet extractor" siphon 3, it is returned to the double-necked flask through the siphon 3. Thereby, the continuous reaction of THF and CrCl3 and the continuous extraction of the product are realized.

1. (6) The purpose of adding a small amount of Zn powder is (expressed by the ionic equation).
2. (7) Reagent A should read (fill in the number).
3. a. H2O b. NaOH solution c. Concentrated sulfuric acid
4. (8) The role of tetrahydrofuran in double-necked flasks is ①________、②________。
5. (9) After the synthesis reaction is completed, remove the double-necked flask and steam out the THF to obtain a solid product of 4.494g. The yield of this experiment is ________ %。

This experimental question has prepared two unfamiliar substances, one inorganic substance and one organic substance. From the principle of preparation of substances, the control of reaction conditions, the collection, purification, and quantitative analysis of substances are investigated by using the properties of substances, and the application of new devices is also involved. The examination is more comprehensive and the difficulty is moderate.

If the known condition of the second experiment "③ CrCl3 and CrCl3 (THF) 3 are both highly reactive with water" can be placed in the known condition of the first experiment, then the student answers the question " (2) The purpose of this experiment is to continue to pass into N2 ________。” will be easier. Otherwise, it will be difficult for students to complete the two functions.

Therefore, in the review and preparation of the test, teachers should cultivate students' habit of reading the information on the question stem, and contact them back and forth, and analyze the methods and reasons for inductive reaction condition control.

Test preparation review points

• 1. Temperature control
• (1) Heating method: direct heating, indirect heating (terracotta mesh heating, water bath heating and oil bath heating, etc.).
• (2) Purpose of temperature control
• Increase reaction rate, increase yield and conversion, reduce side reactions, and improve product purity.
• Frequently asked questions about temperature control:
• ① The purpose of controlling the temperature within a certain range:
• a. If the temperature is too low, the reaction rate (or dissolution rate) is slow;
• b. If the temperature is too high, certain substances will decompose or evaporate (e.g. H2O2, NH4HCO3, ammonia, hydrochloric acid, concentrated nitric acid, etc.).
• ② Purpose of ice water cooling:
• a. to liquefy a substance, reducing the solubility of the product;
• B. Reduce other side reactions and improve product purity.
• 2. Purpose of solution pH control
• (1) Separation and removal of impurities: to fully precipitate or dissolve impurities to improve purity.
• (2) Regulation of the reaction: make the solution acidic, increase the oxidizability, or make the solution alkaline, to control the occurrence of the reaction.
• 3. Exhaust method and function
• (1) Anti-oxidation, waterproof solution
• In order to prevent the components of the air, such as oxygen, CO2, and water vapor, from interfering with the experiment, other stable gases (such as nitrogen) are often used to exhaust the air in the device; sometimes reactant gases (such as nitrogen, chlorine, sulfur dioxide) can be fully utilized to exhaust the air in the device, and the gas generated in the device can be expelled into the rear device, etc.
• (2) Quantitative determination of gases
• In order to fully absorb a certain gas to be analyzed or measured in the experiment, other stable gases (such as nitrogen) are often used as the carrier gas; sometimes reactant gases (such as oxygen) can also be fully utilized as the carrier gas, and the generated gas is pressed into the designated absorption device.

In addition, there is another question in this experimental question. When preparing CrCl3 (THF) 3, it is said that "divalent chromium ions have a catalytic effect on the synthesis of CrCl3 (THF) 3." But the laboratory should also pass nitrogen first, and also use concentrated sulfuric acid at the end of the air isolation device. But it is an anhydrous environment, so how does anhydrous CrCl3 and a very small amount of zinc powder react to generate divalent chromium ions? The information in the question stem is not enough, or whether the reaction equation is better than the ion equation.

So in response to this situation, how should teachers develop students' test-taking ability in test preparation review? Hope to get the views of the majority of netizens.