Operation steps and precautions for producing oxygen (potassium permanganate is used to produce oxygen and collected by drainage method)
a. Steps: connect-check-install-solidify-point- Collect - move - extinguish
b. Notes
① The mouth of the test tube is slightly tilted downward: to prevent the condensed water from flowing back and causing the test tube to break
②Place the medicine flat on the The bottom of the test tube: heated evenly
③The iron clamp is clamped about 1/3 away from the mouth of the tube
④The tube should slightly expose the rubber stopper: to facilitate gas discharge
⑤ A ball of cotton should be placed at the mouth of the test tube to prevent potassium permanganate powder from entering the tube
⑥ When collecting by drainage method, wait until the bubbles emerge evenly and continuously before collecting (the air in the test tube is initially discharged )
⑦At the end of the experiment, move the tube first and then extinguish the alcohol lamp: to prevent the water from being sucked back and causing the test tube to rupture
⑧When collecting gas by the air exhaust method, the tube should be extended to the gas collecting bottle Bottom
(6) Oxygen fullness check: Use a wooden stick with sparks on it and place it at the mouth of the gas collecting bottle
Inspection: Use a wooden stick with sparks on it and insert it into the gas collecting bottle
Laboratory method of producing oxygen
1. Heating and decomposing certain oxygen-containing compounds is the main method of producing oxygen in the laboratory. Commonly used oxygen-containing compounds are potassium chlorate or potassium permanganate. The method of using potassium permanganate to produce oxygen is simple and safe, but the price of raw materials is high and the utilization rate is low. When potassium permanganate decomposes, not all the combined oxygen elements are converted into free oxygen elements (i.e. oxygen):
2KMnO4 K2MnO4+MnO2++O2↑
2. In addition, other chemical reactions can be used to produce oxygen in the laboratory, such as hydrogen peroxide decomposition:
2H2O2 2H2O+O2↑
3. Problems when producing oxygen:
1. Principle expansion
The role of catalyst: Manganese dioxide is used in the reaction of producing oxygen from hydrogen peroxide. as a catalyst. Catalysts refer to substances that can change the reaction rate of other substances in chemical reactions, but their own mass and chemical properties remain unchanged before and after the reaction. In this reaction, manganese dioxide is the catalyst for the decomposition of hydrogen peroxide.
Comparison of two methods of producing oxygen
Using potassium permanganate and hydrogen peroxide to produce oxygen have their own characteristics. Potassium permanganate is a solid and is relatively easy to store; hydrogen peroxide is a liquid and is easy to decompose and should not be stored for a long time. However, using potassium permanganate requires heating, while hydrogen peroxide decomposes very quickly under the catalysis of manganese dioxide and does not require heating.
2. Key to success
Method for heating potassium permanganate: Do not pile potassium permanganate tightly at the bottom of the test tube, but spread it flat in the test tube; when starting to heat, Preheat the test tube so that the bottom of the test tube is evenly heated, then place the lamp flame near the end of the test tube mouth where there is medicine for heating, and move the lamp flame toward the bottom of the test tube as the reaction progresses and the amount of oxygen required increases. The purpose of this is to prevent the solid mixture from rushing to the test tube mouth with the air flow, and to facilitate control of the rate and amount of oxygen release.
3. Concentration of hydrogen peroxide
Commercially available hydrogen peroxide generally has two concentrations, one is a concentrated solution of about 30, and the other is a dilute solution of 30. The appropriate concentration for producing oxygen is around 10. If it is too dilute, the reaction will be slow and the collection time will be longer. If the hydrogen peroxide is too concentrated, the reaction will be too violent and generate too much heat, which will cause too much water vapor in the collected oxygen.
4. Notes
(1) Preparation of oxygen by potassium permanganate
The order of experimental equipment should be down first and then up, and the instruments should be assembled from left to right . And note that the iron clamp should be clamped around the middle and upper part of the test tube (1/4 from the mouth of the test tube), and the mouth of the test tube should be slightly tilted downward.
(2) Rationale and loading of solid reagents?
Add potassium permanganate and spread it at an angle at the bottom of the tube, do not let it accumulate at the bottom.
(3) Why should it tilt slightly downward?
Due to the fact that the moisture contained in the medicine turns into water vapor when heated, it condenses into water droplets at the mouth of the tube and flows back, causing the test tube to burst.
(4) Why is it not advisable to collect the gas immediately when the bubbles first emerge?
When the heating starts, the air is mainly released. When the bubbles are released continuously and relatively evenly, it is oxygen, and then it can be collected.
(5) Why is the drainage method used to collect gas? When stopping oxygen production, the tube should be taken out of the water first, and then the alcohol lamp should be removed?
If the alcohol lamp is removed first, the temperature inside the test tube will drop, the air pressure will decrease, and water will be sucked into the hot test tube along the conduit, which may cause the test tube to rupture.
(6) Why should the catheter in the test tube not be too long?
The tube extending into the test tube should not be too long to prevent gas from being difficult to discharge.
(7) How to verify that the oxygen is full when using the upward air exhaust method to collect oxygen?
Using wooden sticks with sparks to test oxygen is a common method of identifying oxygen. Place the wooden stick with sparks on the mouth of the gas collecting bottle. If the wooden stick ignites again, it means that the oxygen has been collected.
(8) How to verify that the gas produced is oxygen?
Insert the wooden stick with sparks into the gas collecting bottle. If the wooden stick re-ignites, it means that the gas is oxygen.
Experimental expansion:
When using hydrogen peroxide to produce oxygen, there are different ways to add hydrogen peroxide.
If you want to produce a small amount of oxygen in the laboratory, you can also use the device in the picture to inject 15% hydrogen peroxide aqueous solution into the test tube. After adding a small amount of manganese dioxide, tighten the stopper immediately. Oxygen is expelled along the tube. For example, using 15 mL of hydrogen peroxide aqueous solution and adding 0.1 g of manganese dioxide, 0.7 L of oxygen can be obtained in about half a minute. It is more suitable for students to conduct experiments by themselves. The experimental setup is shown in Figure A.
B
A
The A device can only collect oxygen once or intermittently, and cannot collect continuously. In order to achieve the purpose of continuous collection, the large test tube can be changed into an Erlenmeyer flask, the single-hole stopper can be changed into a double-hole stopper, and a long-neck funnel can be used. The experimental setup is shown in Figure B.
Although Device B can add hydrogen peroxide at any time, the control of the reaction rate is not very good. Especially when the concentration of hydrogen peroxide is high, the solution may easily rush out of the long-neck funnel. At the same time, the reaction cannot be stopped. Changing the long-neck funnel into a separation funnel is an ideal preparation device.
C
The functions of the separatory funnel in the device are:
(1) Control the total amount of dripping liquid to obtain the total amount of gas required, It can save medicines to the maximum extent;
(2) Control the rate of dripping liquid, thereby controlling the reaction rate;
(3) The reaction can be controlled to start and stop at any time