Name SCS- Date Experiment 5: Hydrogen Formation and Reaction with Oxygen Aims: What are the energy changes associated with chemical bond formation and breakage? How are changes in potential energy related to overall bond strength in the system? How are the relative volumes of gases related to the relative number of gas molecules? What is an essential condition for molecules to react with on another? What must be conserved in a chemical reaction? Introduction Elemental hydrogen, H2, is widely regarded as the fuel of the future because, when it reacts with oxygen in the atmosphere, the reaction is exothermic releasing usable energy and producing only highly stable water molecules. H2 is not present in appreciable amount on Earth and must be formed by reactions that decompose water into H2 and O2. In this lab, you will (1) observe the electrolysis of water that produces hydrogen gas and oxygen gas, (2) form H2 gas by reaction of HCl with zinc metal and store it, and (3) activate the reaction of stored H2 with atmospheric oxygen. In each reaction, you should observe signs of reaction, whether energy must be added or is released, and the specific location of signs of reaction. The balanced chemical equations for these reactions are: Pre-lab 2 H2O 2H2 + O2 Zn + 2HCl ZnCl2 + H2 2H2 + O2 2H2O Electrolysis (decomposition) of water Reaction of Zn with hydrochloric acid Combustion of hydrogen 1. If energy must be added (absorbed by the atoms) to break a bond between atoms, then, when a bond is formed between atoms, a. Energy is added released b. The process is c. The stability of the atoms becomes d. The potential energy of the atoms becomes exothermic greater greater endothermic lower lower 2. For each process, choose the correct change. Bond Energy (Forms or (Added or breaks) released) H-H 2H atoms Potential Energy of system (Increases or Reaction is (Endothermic or exothermic) 2O atoms O 2
O + 2H H 2O 3. For each process, choose the correct change. Reaction is (Endothermic or exothermic) 2H 2O + energy 2H 2 + O 2 Potential Energy of system (Increases or Stability of products is (Greater or Lower) 2H 2 + O 2 2H 2O + energy 4. According to the ideal gas laws at the same temperature and pressure, the number of molecules in 20.mL of H2 gas, compared to the number of molecules in 10.mL of O2 gas should be (the same or 2 times or one half) 5. In order for molecules to react with each other, they must a. Have the same speed b. Have low potential energy c. collide with each other d. rotate freely PART 1: Formation of Hydrogen and oxygen by Electrolysis of water (DEMO) Equipment Material DC power supply or 6 volt batteries, electrolysis apparatus or 2 test tubes, trough, electrodes with wires 0.1M sodium sulfate A. Two test tubes with even graduation marks are filled with 0.1M sodium sulfate and inverted in a pool of the solution and clamped in place. B. Metal electrodes are placed in the mouth of each test tube. One is connected to the positive terminal of the 6 volt battery and the other connected to the negative terminal. C. Observe the formation of gases when one of the electrodes is disconnected from the battery and then reconnected. D. Allow gases to accumulate in each test tube and record the gas volumes from time to time. 2 H2O 2H2 + O2 1. Based on your observations, describe the decomposition of water by electrolysis in terms of energy required or released, the reaction being exothermic or endothermic, potential energy of the system increasing or decreasing, bond strengths of the products being stronger or weaker.
2. Based on the volumes of gases accumulated and the equation for electrolysis, identify which tube contains hydrogen gas. PART 2: Formation of Hydrogen by reaction of HCl with Zinc Metal Equipment 3 bottles, thistle tube, hydrogen generator, matches, glass plate, trough, ring stand, clamp Materials Zinc powder, 3M hydrochloric acid In this part, you will combine aqueous HCl with Zn metal allowing them to react to produce hydrogen gas and aqueous ZnCl2. As the H2 gas accumulates in the flask, it will force air out through the tubing. Eventually, H2 gas will be forced out of the tubing so that it can be stored in bottles. Remember that H2 gas is less dense than air. 2HCl + Zn ZnCl2 + H2 A. Connect flexible tubing from the hydrogen generator flask to the trough and fill the trough with water deep enough to fully immerse the bottles. Fill each bottle with water and stand them up with the mouth down in the trough. B. Pour 3M HCl through the thistle tube into the flask and observe the reaction between Zn and HCl. When the gas forces water out of the tubing, allow it to bubble up in the trough for 30 seconds. C. Slide the first bottle over to collect the bubbling gas until it contains only gas, then, slide it away. Slide the second bottle over to collect gas. Meanwhile, lift the first bottle slightly and place a glass plate to close its mouth temporarily. Lift the first bottle and plate out of the trough and slide the bottle, mouth down, onto the bench. Repeat this until you have bottles 1, 2, and 3 filled with gas mouth down on the bench. D. Observe the temperature of the bottom of the flask. E. To stop the formation of H2, remove the stopper from the flask and pour most of the liquid out of the flask into the sink. Do not allow Zn metal to fall into the sink. Add fresh tap water, swirl, and again pour most of the liquid into the sink. Repeat the water rinse twice more. 3. Identify at least two signs of reaction between HCl and Zn. 4. Based on your observations, describe the reaction of HCl with Zn in terms of energy required or released, the reaction being exothermic or endothermic, potential energy of the system increasing or decreasing, bond strengths of the products being stronger or weaker
PART 3: Reaction of Hydrogen with Atmospheric Oxygen Equipment Materials Safety matches, 3 jars filled with hydrogen or hydrogen air mixture In this part, you will bring a lighted match to the mouth of each jar and observe the reaction of hydrogen gas with oxygen gas in the air. Be sure to observe signs of reaction, and the specific location and duration of each reaction. A. Light a match. Lift bottle 2 keeping the mouth down. Without putting your hand under the bottle, bring the match to the edge of the jar mouth and observe the reaction. a. Signs of reaction b. Location and duration of reaction B. Repeat with bottle 1. a. Location and duration of reaction C. Repeat with bottle 3. a. Location and duration of reaction 5. Since H2 gas did not react with air until the match was brought near, identify what the lighted match provides. 6. Based on your observations, describe the reaction of H2 gas with oxygen in air in terms of energy required or released, the reaction being exothermic or endothermic, potential energy of the system increasing or decreasing, bond strengths of the products being stronger or weaker SUMMARY QUESTION Reaction is then (exothermic endothermic) If a reaction releases energy Potential energy (increases Bond strength of products (Stronger weaker) Energy is written as (Reactant product) absorbs energy constantly