- In this experiment you will determine the amount of energy (heat) involved in a chemical change. When alcohol burns it produces carbon dioxide and water as products. Energy is also released in the reaction. Recall that the type of change in which new products are formed is called a chemical change. Let us determine the amount of heat (in calories) liberated when three alcohols burn, and compare their energy output (heat of combustion). We will be using methyl alcohol CH3OH , ethyl alcohol C2H5OH , and 2-propyl alcohol CH3CHOHCH3 . The heat obtained when a known mass of alcohol burns will be used to warm a measured volume of water. (We shall assume that 1 ml of water has a mass of 1 gram).
- Because the calorimeter used is very crude, you will not get data very close to the actual heat of combustion for the three alcohols. Some of the heat will escape around the can, and not be measured. You will, however, get good comparative data.
- Try designing a calorimeter that will be more accurate. This would make a good class project.
MATERIALS NEEDED:
Alcohol samples in burners
Stirring rod
Standard vegetable can (calorimeter)
Ring stand with large ring
Ice
250-ml graduated cylinder
ThermometerPROCEDURE:
1. Obtain a burner with one of the test fuels in it. Find the mass of the burner and fuel to the nearest 0.01 g and record the mass on your DATA TABLE.
2. Set up the calorimeter apparatus as follows: Poke 2 holes just inside the rim of the open end of the can, straight across from each other. Suspend the can from the ring, using the stirring rod through the holes and across the ring. Adjust the height so that the flame of the burner when lit (do not light it yet) will almost, but not quite, touch the bottom of the can.
3. Using a 250-ml graduated cylinder, obtain approximately 200 ml of cold water, and put it into the can, which will be the calorimeter.
4. Cool the water with ice, if necessary, so that its temperature is about 10 deg. C to 15 deg. C below room temperature. Add the ice directly to the water. Remove any remaining ice when the desired temperature has been reached.
5. Read and record on the DATA TABLE, the temperature of the water to the nearest 0.5 deg. C. Light the burner and heat the water. Stir gently with the thermometer until the water reaches a temperature as much above room temperature as it was below at the start. Carefully blow out the burner, but continue to stir the water and watch the thermometer reading. Record on the DATA TABLE the highest temperature that is reached to the nearest 0.5 deg. C.
6. Find the mass of the burner and fuel on the same balance as before, to the nearest 0.01 g and record it on the DATA TABLE.
7. Carefully pour the water from the calorimeter, into the 250-ml graduated cylinder, and measure the volume to the nearest 1 ml. In measuring the volume of a liquid in any graduated container, always have your eye at the same level as the bottom of the meniscus (the lens-shaped surface the liquid forms in the container). Record this volume on the DATA TABLE.
DATA TABLE: for METHYL ALCOHOL CH3OH 
Mass of methyl alcohol and burner before heating. ________ g Temperature of water before heating. ________ deg. C Temperature of water after heating. ________ deg. C Mass of methyl alcohol and burner after heating. ________ g Volume of water heated. ________ ml DATA TABLE: for ETHYL ALCOHOL C2H5OH Mass of ethyl alcohol and burner before heating. ________ g Temperature of water before heating. ________ deg. C Temperature of water after heating. ________ deg. C Mass of ethyl alcohol and burner after heating. ________ g Volume of water heated. ________ ml DATA TABLE: for 2-PROPYL ALCOHOL CH3CHOHCH3 Mass of 2-propyl alcohol and burner before heating. ________ g Temperature of water before heating. ________ deg. C Temperature of water after heating. ________ deg. C Mass of 2-propyl alcohol and burner after heating. ________ g Volume of water heated. ________ ml CALCULATIONS:
- When heat is absorbed by liquid water, the temperature of the water rises. The amount of energy necessary to raise the temperature of 1 gram of water 1 degree Celsius is defined as 1 calorie. This is called the specific heat for water. Consequently, we can calculate the amount of energy released or absorbed during a process. We have measured the mass of the water in grams and the temperature change of the water in degrees Celsius. By definition, the energy gained by the water is equal to the mass of water multiplied by the temperature change multiplied by the specific heat for water. For example:
heat associated with
temperature change = mass of water x change in temperature x 1 calorie / 1g 1deg. C8. From the data obtained, calculate the mass of alcohol burned. This is simply the mass of the burner and alcohol before burning, minus the mass of the burner and alcohol after burning.
9. Calculate the mass of water heated. By definition one milliliter of water has a mass of one gram.
10. Calculate the temperature change of the water.
11. Calculate the quantity of heat absorbed by the water (assume that all the heat from the burning alcohol went into the water). This is the formula given above.
12. Calculate the heat of combustion per gram of alcohol. This is the heat absorbed by the water, divided by the mass of the alcohol burned. This gives the unit calories / g .
Repeat for the other two alcohols.
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- Because the calorimeter used is very crude, you will not get data very close to the actual heat of combustion for the three alcohols. Some of the heat will escape around the can, and not be measured. You will, however, get good comparative data.
HEAT OF COMBUSTION
FOR THREE ALCOHOL FUELS
INTRODUCTION: