COMBUSTION REACTION
PRELAB
1. What is the chemical formula for ethyl alcohol? Find an alternative name for ethyl alcohol.
2. Write and balance the combustion reaction that occurs when ethyl alcohol is burned in the presence of oxygen.
3. Define colloid. What chemicals are mixed to form the colloid in this lab? What are the properties of colloids?
4. BYOChocolate. If you’d like chocolate on your smore, please bring a piece to lab.
PURPOSE
To make a gel, like Sterno that will be used to heat a food sample in lab. To study the combustion reaction that occurs when ethyl alcohol is ignited.
THEORY
By mixing a concentrated calcium acetate solution with alcohol, a colloidal gel is formed. When this gel is burned, it can be used as a heat source similar to the commercial product, Sterno®. The gel is a safer source of heat since it is not as volatile and will not spill. Sterno is a commercial product used in many cafeteria and banquet serving lines. It is a favorite with restaurants because it provides a reliable, odorless and safe heat source. Sterno, also called Canned Heat, produces a flame that is relatively small and long lasting. Since it is a solid, it does not spill. Another benefit is that it does not evaporate or easily vaporize. Canned Heat is produced by mixing a concentrated solution of calcium acetate with ethyl alcohol. As the ethyl alcohol and calcium acetate solution are mixed, the calcium acetate immediately precipitates out of solution and forms a gel with the ethyl alcohol. A gel is a solid dispersed in a liquid that develops a structure that resists flow. The exact structure of the gel in unknown but it is very stable and has an even consistency. The formation of the gel is a physical phenomenon and not a chemical reaction. As the Canned Heat is ignited, the ethyl alcohol evaporates and begins to burn (in the presence of oxygen gas) to produce carbon dioxide and water. The calcium acetate also burns to produce a mixture of calcium oxide, calcium carbonate, carbon dioxide and water.
MATERIALS
Marshmallow
Graham cracker
Skewer
Evaporating dish
Graduated cylinder
NaCl
Ca(C2H3O2)2
C2H5OH (l)
PROCEDURE
SAFETY CONCERNS:
Sterno is flammable and the evaporating dish that contains the reaction will be hot for a while after the flame is out. Do not eat any food that may have been contaminated by chemicals. Please wash your hands after mixing the sterno and before moving on to toast the marshmallows.
1. Obtain an evaporating dish.
2. Add 0.75 grams of calcium acetate to the dish.
3. Add 2.5 ml of water to the calcium acetate in the dish. Stir.
4. Add a pinch of salt to the evaporating dish. Stir.
5. Add 19 ml of ethyl alcohol to the evaporating dish. DO NOT STIR.
6. Wash your hands.
7. Obtain a skewer, a marshmallow, and a graham cracker. Do not let any of these items touch the desk or interact with the chemicals.
8. Carefully hold a match to the edge of the evaporating dish to ignite the Sterno mixture prepared above.
9. Roast your marshmallows and create your smores. Enjoy.
10. After the evaporating dish has completely cooled, wipe the contents into the trash can and then wash, dry, and put away the evaporating dish. All other equipment should be washed, dried, and put away. Start your post lab questions with time remaining.
POST LAB QUESTIONS
1. What does the prefix eth- mean in organic chemistry?
2. What does the suffix –ol indicate about an organic molecule?
3. If ethanol is C2H5OH, determine the molecular formula for propanol.
4. Write and balance the reaction when ethanol undergoes combustion including phases.
5. Given the density of ethanol (0.789 g/ cm3) and the ml used in this lab, determine the grams of ethanol that were burned in this reaction.
6. Calculate the moles of ethanol that were burned (based on your grams calculated in #5).
7. The heat of combustion of ethanol is 1300 kJ/mol. Determine the number of kJ of heat released when this reaction goes to completion. This is the heat that is used to toast the marshmallow.
8. Write and balance the reaction that occurs when the calcium acetate and oxygen combine to produce calcium oxide, calcium carbonate, water and carbon dioxide. Show phases for all substances and proper coefficients.
9. Stoichiometry preview: Using the mole ratio from the coefficients of the balanced reaction, calculate the grams of CO2 that would have been released when the moles of ethanol (calculated in # 6) were reacted.
10. Stoichiometry preview: Using the mole ratio from the coefficients of the balanced reaction, calculate the number of grams of oxygen gas that would have been consumed when the moles of ethanol (calculated in # 6) were reacted.
11. Sketch the Lewis dot structure for ethanol. Determine the hybridization around each carbon. What is the bond angle around each carbon?