Safety and Types of Fires

Transcription

1 Cookin' Chem Activity 2 Safety and Types of Fires CHEM POETRY Oliver learned all those Don t s and Do s, The ones that get passed down through moles of O 2 s; Each new generation did soon learn to trust That its purpose on Earth was to make things combust. The highest honor an O 2 could earn Came from causing organic matter to burn. A kamikaze maneuver, this game, The O 2 destroyed in creating the flame. One day, while diffusing out in the park, He saw an object that started to spark. Twas no matter what caused this electrical prank; It had gone and ignited a propane tank! Though prospects for honor had started to tire, Twas his turn now to add fuel to the fire: I just need to get close, to come into their view, And then we ll make water and CO 2! Mala Radhakrishnan GOALS In this activity you will: Discover what a combustion reaction is and what the products of a combustion reaction are. Generate carbon dioxide and oxygen gases. Practice balancing chemical equations. What Do You Think? Humans have been using fire to cook food for thousands of years. What is necessary for a fire? What can be used to put out an unwanted fire in a kitchen? Record your ideas about these questions in your log. Be prepared to discuss your responses with your group and the class. Investigate Many of the activities of science involve making observations with your senses, mainly sight, but to a lesser extent smell, touch, and sound. Observations can be either qualitative or quantitative. Qualitative observations are descriptions of what is occurring. Quantitative observations involve the use of measurements. Interpretations are based on your observations. They are an explanation of the observations. In this lab activity, you will make qualitative and quantitative observations about a candle before, during, and after it is lit. You will then interpret several of your observations. This will give you information that could be used as a part of your cooking show. For each step in the procedure, write a brief description of what you are doing, followed by your observations in your log. Leave room for interpretations (three or four lines), but do not attempt to make interpretations yet. 522

2 Activity 2 Safety and Types of Fires 1. Obtain a ruler, a candle, and a foil-covered cardboard square. Place the candle in a holder on the square, or fix it to the square using putty. Make between six and ten observations about the unlit candle. a) Record your observations in your log. 2. Light the candle. Make an additional six to ten observations. Be sure to make observations about the flame. 3. Light a match. Blow out the candle and quickly bring the lit match into the smoke. Repeat once or twice and make observations. b) Leave room in your log for the interpretations that you will make later. 4. Light the candle and then invert a clean, dry 250-mL beaker over the flame. Slowly lower the beaker over the flame. Repeat once or twice and make observations. Be sure to look carefully into the beaker too. Note: The beaker will become hot over time! b) Leave room for your interpretations. 5. Hold a piece of string with forceps over the table. Light the string. b) Leave room for your interpretations. 6. Place a foil collar around the wick of the candle. Light the candle. b) Leave room for your interpretations. 7. The reaction between vinegar and baking soda releases carbon dioxide gas. Place a small scoop of baking soda with about 50 ml of vinegar in a 250-mL beaker. Observe the reaction. Carefully pour the gas (but not the liquid) over the flame of the candle. b) Leave room for your interpretations. 200 ml Safety goggles and a lab apron must be worn at all times in a chemistry lab. 200 ml The decomposition of hydrogen peroxide releases oxygen gas. The reaction is catalyzed (sped up) by the addition of manganese dioxide to the hydrogen peroxide. Fill a test tube one-third of the way with hydrogen peroxide. Add a small amount of manganese dioxide to the test tube. Test the resulting gas by lighting a splint and blowing it out. 523

3 Cookin' Chem Wash hands and arms thoroughly before leaving the lab area. The end should be glowing. Quickly insert and remove the glowing splint into the mouth of the test tube. b) Leave room for your interpretations. 9. For Steps 3 through 8, write a sentence to interpret your observations. These should be an explanation of what you observed. Why do you think each observation occurred? How can you explain what you saw? 10. There are three things necessary for combustion fuel, oxygen, and a source of heat. The fuel in this case is the candle, which is made of wax and a string. The source of heat was from the match. The oxygen is found in the air. Answer the following questions in your Active Chemistry log, based upon your observations and interpretations. a) What are two methods that you used to put out the fire? Explain how each one worked in terms of the three necessities for combustion. b) What is the actual fuel for the combustion of the burning candle? What are the functions of the wax and the string? What evidence do you have to support your response? c) Suppose you were to let the candle burn until the wick was completely gone. Describe what would be left and how it would compare to the original candle. What happened to the wax? Could you make a new candle from what remained? THE COMBUSTION REACTION The Equation for Combustion Food is all about eating! Cooking food is all about heat. Fire is one way of producing heat. Creating a flame was certainly one of the great advances in the history of humankind. The first fire may have been from lightning, but some human decided to maintain the flame and use it to begin other fires. Eventually, someone invented a means to create a spark and start fires at will. When a substance burns in the presence of oxygen, a chemical change takes place. In a chemical change, new substances are produced. The substances you start with are called reactants, and the substances that form are called products. The key part of a chemical reaction is that the products are different substances from the reactants. 524

4 Activity 2 Safety and Types of Fires When a hydrocarbon (a compound composed of hydrogen and carbon) reacts with oxygen (O 2 ), a combustion reaction occurs. The products of a complete combustion reaction are carbon dioxide (CO 2 ) and water (H 2 O). There are lots of materials you can burn. Candles, wood, gasoline, and natural gas are some. Candle wax is made of relatively long-chain hydrocarbons (C 20 H 42 and larger). Other hydrocarbon fuels that you might be familiar with are methane (CH 4, the chief component of natural gas), propane (C 3 H 8 ), butane (C 4 H 10 ), and octane (C 8 H 18, a component of gasoline). When any of these compounds are burned in the presence of sufficient oxygen, the products are CO 2 and H 2 O. Consider the complete combustion of propane gas: C 3 H 8 + O 2 CO 2 + H 2 O propane oxygen carbon dioxide water reactants products (total of 3 C atoms, 8 H atoms, 2 O atoms) (total of 1 C atom, 2 H atoms, and 3 O atoms) The propane gas and the oxygen reacts to produce carbon dioxide and water. The chemical equation is one way you can describe the reaction. Another symbolic representation (ball-and-stick models) is shown in the diagram. The products are carbon dioxide and water. Balancing the Combustion Equation Look closely at the chemical equation for the combustion of propane. You can see that there are different numbers of atoms on both sides of the equation. On the reactant side, there are 3 carbon atoms (shown in black), 8 hydrogen atoms (shown in white), and 2 oxygen atoms (shown in red). On the product side of the equation, there is only 1 carbon atom, 2 hydrogen atoms, and 3 oxygen atoms. This violates the Law of Conservation of Mass. The law states that the total mass of the products of a chemical reaction is the same as the total mass of the reactants entering into the reaction. Since there are different numbers of atoms before and after the reaction, the total mass of atoms involved is not equal. To fix this, you need to balance the equation. Chem Words chemical change: a process in which new materials with different properties are formed. reactants: the materials that you initially start with in a chemical reaction. products: the materials that are produced in a chemical reaction. hydrocarbon: an organic compound that contains hydrogen and carbon atoms. combustion reaction: a chemical reaction that involves the burning of a hydrocarbon in presence of oxygen. Law of Conservation of Mass: a law that states that the total mass of the products of a chemical reaction is the same as the total mass of the reactants entering into the reaction. balanced chemical equation: an equation in which the number and kind of atoms of the reactants equals the same number and kind of atoms of the products. 525

5 Cookin' Chem Atoms are rearranged during a chemical reaction. This is the nature of a chemical change. It is not required that both sides of an equation contain the same number of molecules. It is required that they have the same number of atoms though. You can balance this equation by adding appropriate coefficients (numbers) in front of the molecular formulas of the reactants and products. It is important that you change only coefficients, not the molecular formulas. If you were to change the molecular formulas, you would no longer be working with the same chemical reaction. If you place a 3 in front of the CO 2 product, you will now have 3 carbon atoms on both sides of the equation. Placing a 4 in front of the H 2 O product will bring the total number of H atoms on both sides to 8. However, this now makes a total number of 10 oxygen atoms on the right side of the equation. If you place a 5 in front of the reactant O 2, this will make the total number of oxygen atoms on both sides equal. You now have a balanced equation that is represented as: C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) 3 carbon atoms 3 carbon atoms 8 hydrogen atoms 8 hydrogen atoms 10 oxygen atoms 10 oxygen atoms The ball-and-stick representation is shown in the diagram. This states that 1 molecule of propane reacts with 5 molecules of oxygen to yield 3 molecules of carbon dioxide and 4 molecules of water. 526

6 Activity 2 Safety and Types of Fires There are two important understandings for balancing equations that you should have: You need to understand that you can balance equations because mass is conserved during a chemical reaction. The mass before the reaction must be identical to the mass after the reaction. You need to understand that you can balance equations because atoms are conserved during a chemical reaction. For example, the oxygen atoms entering the reaction must be the same oxygen atoms after the reaction. There are two skills concerning balancing equations that you should also develop: You should be able to recognize whether a chemical equation is balanced by checking if the numbers of each atom are identical in the product and reactant sides of the equation. You should be able to balance the equations so the numbers of each atom are identical in the product and reactant sides of the equation. This skill has more to do with solving a math puzzle than the essence of chemistry and chemical principles. All chemists understand why equations must be balanced. They can check to see if an equation is balanced. Some chemists enjoy balancing equations, while others do not. The Role of Oxygen and Carbon Dioxide in Combustion In Step 4 of this activity, you placed a beaker over the candle flame. This reduced the amount of O 2 entering into the reaction. Without a source of O 2, the combustion reaction eventually stops. In addition to the requirement that oxygen be present for fuel to burn, energy must be provided in order to start the reaction. In lighting a candle, a burning match provides the energy required to initiate the reaction. In Step 7, you prepared CO 2 gas by reacting vinegar with baking soda. This happens in two steps. Vinegar contains acetic acid. This acetic acid reacts with baking soda (sodium hydrogen carbonate), and produces carbonic acid and sodium acetate. The carbonic acid is an unstable compound. It decomposes into CO 2 and H 2 O. 527

7 Cookin' Chem Chem Words catalyst: a substance that lowers the amount of energy needed to start a reaction by providing an alternate reaction pathway but is itself not changed in the reaction. qualitative observation: an observation of features that do not involve measurement. quantitative observation: an observation of features that involve the use of measurements. HC 2 H 3 O 2 (aq) + NaHCO 3 (s) H 2 CO 3 (aq) + NaC 2 H 3 O 2 (aq) acetic acid sodium hydrogen carbonic acid sodium acetate carbonate H 2 CO 3 (aq) CO 2 (g) + H 2 O(l) carbonic acid carbon dioxide water In the activity, the CO 2 produced was poured over the flame and extinguished it. The formation of O 2 from the decomposition of hydrogen peroxide (H 2 O 2 ) is represented by the following chemical equation: 2H 2 O 2 (aq) MnO 2 O 2 (g) + 2H 2 O(l) hydrogen peroxide oxygen water The rate of this reaction was sped up by adding manganese dioxide (MnO 2 ) to act as a catalyst. A catalyst is something that lowers the amount of energy needed to start and maintain a reaction by providing an alternate reaction pathway. True catalysts are regenerated (not used up) in a reaction. In the activity, the O 2 produced was able to light a glowing splint. Checking Up 1. What is the difference between a quantitative and a qualitative observation? 2. Define a chemical change. 3. In a combustion reaction, what are the reactants and what are the products? 4. Name three hydrocarbons with which you are familiar. 5. Explain how the Law of Conservation of Mass helps you balance an equation. 6. Is the following a balanced equation? Justify your answer. CH 4 + 2O 2 CO 2 + 2H 2 O 7. What is a catalyst? In Step 4 of this activity, you placed a beaker over the candle flame. Before the reaction stopped completely, however, an interesting thing took place. Smoke was produced and black soot was deposited on the beaker. This was carbon, which was released from the wax due to incomplete combustion. The other product of incomplete combustion is carbon monoxide (CO). You would not have been able to see or smell the carbon monoxide. C(s) + O 2 (g) CO(g) carbon oxygen carbon monoxide Carbon monoxide is a poison. It is one reason why incomplete combustion can be dangerous. At this point you will probably notice that the equation above is not balanced. To fix this, you must place a 2 in front of the CO on the right and a two in front of the C on the left: 2C(s) + O 2 (g) 2CO(g) 528

8 Activity 2 Safety and Types of Fires What Do You Think Now? At the beginning of the activity you were asked: What is necessary for a fire? What can be used to put out an unwanted fire in a kitchen? Now that you have completed this activity on heat-energy transfer, what are the three things required for a fire? Once a fire has started, what should you remove in order to put the fire out? What does it mean? Chemistry explains a macroscopic phenomenon (what you observe) with a description of what happens at the nanoscopic level (atoms and molecules) using symbolic structures as a way to communicate. Complete the chart below in your log. MACRO NANO SYMBOLIC List three observations that you made about the burning candle. How do you know? In an incomplete combustion reaction, carbon (soot) is formed. Where did you observe this in the activity? Why do you believe? Fires need to be controlled or extinguished. These include campfires, fires in the kitchen, and large-scale fires in the burners of an electrical power plant. Explain the importance of controlling the fire in each of these cases. Why should you care? In a combustion reaction, describe why the equation must be balanced. You may choose to use fire in your cooking-show segment. Allowing the fire to rage out of control might not go over well with the TV executives. Being able to explain the chemical reaction responsible for the fire might be of interest to your viewers. Use chemical formulas in a balanced chemical equation to represent the combustion of propane. 529

9 Cookin' Chem Reflecting on the Activity and the Challenge In this activity, you learned about the combustion of fuels and what is required to keep the fuel burning. In a kitchen, natural gas (which is mostly methane) is often used as a source of fire for cooking. You may want to discuss the combustion of methane (natural gas) that allows you to cook during the segment. You may find a creative way of describing the need for balancing the equations that describe the combustion. You may also consider some other possible sources of combustion that might be found in a kitchen or used in cooking. There are many precautions that a cook should take when working in the kitchen. You might also want to include some fire safety issues in your cooking-show segment. 1. Locate the fire extinguisher in your lab or in some other location at your school. a) What substances are in the extinguisher? b) How do they put out a fire? 2. Prove to yourself that the following equation is balanced by determining the total number of atoms on both sides of the equation: 2C 8 H 18 25O 2 16CO 2 18H 2 O (Remember that 25O 2 means that there are 25 molecules of oxygen. Each molecule of O 2 has two oxygen atoms.) How many carbon atoms on the reactant side? How many carbon atoms on the product side? Repeat for the H and O atoms. 3. Balance the following reactions: a) H 2 O 2 H 2 O b) C 3 H 8 O 2 CO 2 H 2 O 4. Show whether the following reactions are balanced. a) 2C 3 H 8 7O 2 CO 2 4CO 8H 2 O C b) 2CH 3 CH 2 OH 7O 2 4CO 2 6H 2 O 5. The products of the balanced equation for the complete combustion of butane (2C 4 H 10 13O 2 ) are: a) 4CO 2 10H 2 O b) 4CO 10H 2 O c) 8CO 2 10H 2 O d) 8CO 2 5H 2 O 530

10 Activity 2 Safety and Types of Fires 6. Which of the following must be the same before and after a chemical reaction? a) The sum of the masses of all substances involved. b) The number of molecules of all substances involved. c) The number of atoms of each type involved. d) Both (a) and (c) must be the same. 7. Describe the combustion process to someone who has not studied chemistry. 8. Balance the following equations with whole-number coefficients: a) CH 3 OH O 2 CO 2 H 2 O b) CH 4 O 2 CO 2 H 2 O c) HCl NaOH NaCl H 2 O d) C 5 H 12 O 2 CO 2 H 2 O 9. Determine which of the following equations are balanced. a) NaHCO 3 HCl NaCl H 2 O CO 2 b) Al S Al 2 S 3 c) CH 3 COOH NaOH CH 3 COONa H 2 O d) CH 2 CH 2 6O 2 2CO 2 2H 2 O 10. Preparing for the Chapter Challenge In your cooking show, how might you include a segment about fire safety? Other than the stove and oven, what are some other possible sources of combustion (expected and unexpected) in the kitchen? How can you make this segment both entertaining and informative? Inquiring Further Generic balanced chemical equation Devise a generic balanced chemical equation for the combustion of those hydrocarbons that have the general formula C n H 2n