Burning Acetylene Achievement Standard for C11-3-05, 06, 10, 12, 13, 14, 15 The following activity is developed to assess aspects of the following learning outcomes of the Grade 11 Manitoba Curriculum. C11-3-05 Write and classify balanced chemical equations from written descriptions of reactions. Include: polyatomic ions C11-3-06 Predict the products of chemical reactions, given the reactants and type of reaction. Include: polyatomic ions C11-3-10 Solve problems requiring interconversions between moles, mass, volume, and number of particles. C11-3-12 Interpret a balanced equation in terms of moles, mass, and volumes of gases. C11-3-13 Solve stoichiometric problems involving moles, mass, and volume, given the reactants and products in a balanced chemical reaction. Include: heat of reaction problems C11-3-14 Identify the limiting reactant and calculate the mass of a product, given the reaction equation and reactant data. C11-3-15 Perform a lab involving mass-mass or mass-volume relations, identifying the limiting reactant and calculating the mole ratio. Include: theoretical yield, experimental yield The focus of this collection of learning outcomes is for students to show they can (1) write and balance equations; (2) make inferences about what is observed in a reaction based upon the equations written; and (3) explain observations and justify inferences and observations based upon a sound knowledge of what is happening at the molecular level in the reaction. Assessment Criteria: Students will be awarded a Not Achieved, Achieved, Merit or Excellence for this based upon the criteria below. The criteria are cumulative. To obtain an excellence, the achieved and merit criteria must be attained. Not Achieved Achieved Merit Excellence Unable to balance equations write and balance equations make inferences about what is observed in a reaction based upon the equations written explain observations and justify inferences based upon a sound knowledge of what is happening at the molecular level in the reaction. 1 Burning Acetylene Burning Acetylene C11-3, 5, 6, 10, 12-15 From Nelson Chemistry 11 & Trevor Shackleton Student Information The focus of this investigation is for you to show you can: (1) write and balance equations; (2) make inferences about what is observed in a reaction based upon the equations written; and (3) explain observations and justify inferences and observations based upon a sound knowledge of what is happening at the molecular level in the reaction. Assessment Criteria: You will be awarded a Not Achieved (N), Achieved (A), Merit (M) or Excellence (E) for this based upon the criteria below. The criteria are cumulative. That is, to obtain an excellence, the achieved and merit criteria must be attained. Not Achieved Achieved Merit Excellence Unable to balance equations Able to write and balance equations Also able to make inferences about what is observed in a reaction based upon the equations written Also able to explain observations and justify inferences based upon a sound knowledge of what is happening at the molecular level in the reaction. Introduction: A variety of fuels are used in everyday life. We are dependent primarily on propane as a liquefied petroleum gas in home heating and octane and ethanol as fuels for vehicles. A further fuel is acetylene, an alkyne with the chemical formula C2H2. Acetylene is commonly used in welding because when it combusts it burns at a high temperature as long as there is sufficient oxygen for combustion. If there isn’t sufficient oxygen it burns incompletely at a low temperature and produces lots of soot. In this investigation we will be looking at the ideal burning conditions for acetylene. We will work towards understanding the ideal conditions for the burning of acetylene, especially by thinking about the balanced chemical equation infers about the ideal conditions for combustion. 2 Burning Acetylene Materials Required: Eye Protection Granules of Calcium Carbide Tweezers or spatula to handle the calcium carbide granule 6 test tubes of similar size with well fitting stoppers Test tube rack Test tube tongs A marker that can mark a test tube Wooden splint and matches 1 L beaker 3/4 full of water Caution: Calcium carbide must be kept away from water until required to be placed in the water. It must be handled by tongs. Acetylene (ethyne) the gas produced is very flammable. Ignite only small volumes, and, if possible, ignite the gas when required in a fume hood. Pre-lab Exercise: 1. In this lab we will produce acetylene gas by placing solid calcium carbide (CaC2) in water. Write the chemical formula for the reaction between calcium carbide and water knowing that the product is acetylene (C2H2). Include all states of matter in the equation. 2. The acetylene in this lab will be collected in a test tube and burned. Write the balanced equation for the combustion of acetylene. 3. Look at the balanced equation. Note the coefficients of the reactants. What proportion of oxygen molecules to acetylene molecules are required for the most effective burning of acetylene? 4. Other fuels that undergo combustion in the presence of oxygen are propane gas and liquid octane. Write and balance equations including states for these two fuels: a) combustion of propane C3H8, b) combustion of octane C8H18 3 Burning Acetylene 5. In this lab you will be filling test tubes with acetylene. Some test tubes will have only acetylene (C2H2,) in them and some will have both acetylene and air (O2, and N2) and acetylene in them. If air is approximately 80 % nitrogen and 20 % oxygen, draw a molecular representation using a key to explain your diagram of the contents of (1) a test tube with contents of pure acetylene and (2) a mixture of acetylene and air. 1. Test tube with pure acetylene 2. Test tube with acetylene and air Procedure: 1. Teacher Demonstration: The teacher will demonstrate how the acetylene gas will be produced and collected. The teacher will demonstrate how to get a test tube completely filled with acetylene and, then, partially filled with acetylene. The beaker is ¾ filled with water. Using tweezers a calcium carbide granule is dropped in the water in the container and bubbles of acetylene gas begin to be produced. A test tube full of water is inverted and placed over the evolving gas. The gas displaces the water and the entire test tube fills with acetylene. The test tube can be stoppered to retain the gas. The teacher may also demonstrate how it burns when the gas is ignited by a splint. Any black soot evidenced indicates that there is carbon being produced as there is not enough oxygen present for complete combustion to occur. If the test tube is pulled out before the water is fully displaced we can get a test tube partially filled with acetylene. As the water flows out of the test tube, air flows in so that the test tube has an acetylene : air mix. 2. Student Investigation: In the steps that follow you will be filling 6 test tubes with different amounts of acetylene. That is, in each test tube will contain a different amount of acetylene. 4 Burning Acetylene 3. Begin by marking a line on the 6 test tubes that are equivalent to 3/4, 2/3, 1/2, 1/3, 1/4, 1/10. 4. Fill all the test tubes with water and ensure there are no bubbles in the test tubes when the stoppers are placed on the tubes. Place the test tubes when stoppered in the rack. 5. Drop a granule of calcium carbide and note the evolution of the acetylene. 6. Place the 3/4 inverted test tube still stoppered in the water and remove the stopper and fill the test tube with acetylene to the marked line. Keeping the test tube inverted take the test tube out of the water and let the excess water drain from the test tube Quickly stopper the test tube while inverted. This tube will now be 3/4 acetylene and ¼ air. That is a 4:1 ratio of acetylene to air.. 7. Repeat for the other 5 test tubes. Don’t ignite the acetylene yet! Making Some Hypotheses: Answer the following questions based upon your knowledge of the molecular composition of the test tubes. Remember that air is a mixture and that acetylene requires oxygen to burn. 1. At this point you should have 6 test tubes with varying amounts of acetylene. 2. Which of the 6 test tubes do you think will burn the best when ignited? Why do you think this? 3. Which of the 6 test tubes do you think will burn the poorest when ignited? Why do you think this? 5 Burning Acetylene 4. Use the chart below to rank your predictions (1- most reactive, 6-least reactive) Burning the Acetylene: Test tube Predicted Acetylene Amount Reactivity 1 (most)- 6 (least) Actual Reactivity 1 (most)- 6 (least) Observations of Burning 3/4 2/3 1/2 1/3 1/4 1/10 1. Once you have made your prediction ignite each of the test tubes and record observations pertaining to their burning rate and any evidence complete or incomplete combustion. 2. How do your actual results compare to the predicted results (M)? 3. Which acetylene sample burned the best? Why do you suggest that is the case? Two things are necessary for you to answer this question. First consider the balanced chemical equation 2C2H2 + 5 02 -> 4CO2 + 2H20 Second consider that air is only about 20% oxygen. 4. Suggest why the sample with the least acetylene actually burned the best (E). 6 Burning Acetylene 5. Explain why when there is the least amount of air (and oxygen), more soot (carbon) is produced. Your answer should make reference to the fact that carbon (C) gets produced rather than just carbon dioxide (E). 6. Explain why this reaction clearly shows how the concept of limiting reagents applies to the burning of acetylene. In your explanation you should refer to the test tubes in which a reagent is limited and how it influences combustion (E). 7. Write a chemical equation for the incomplete combustion of acetylene. (M) 8. Based upon this answer, explain why there is a high hazard for carbon monoxide (CO) poisoning from gas stoves and vehicles that are not wellmaintained. Answer your question in regards to the combustion efficiency and oxygen availability (E). Your Achievement: Not Achieved Achieved Merit Excellence Unable to balance equations Able to write and balance equations Also able to make inferences about what is observed in a reaction based upon the equations written Also able to explain observations and justify inferences based upon a sound knowledge of what is happening at the molecular level in the reaction. In the future: 7
