Experiment 7
| The Heat of Combustion of Vegetable Oil |  |
Background
Humans obtain energy required for life processes from the controlled oxidation of foods. Typical examples of the reactions which are used are:
sucrose + oxygen ____> carbon dioxide gas + liquid water + energy
( C6H12O6 + O2 ____> CO2 + H2O + energy )
and
vegetable oil + oxygen ____> carbon dioxide gas + liquid water + energy
These are the same chemical reactions that are observed during burning or combustion, except that during the burning process, energy is released rapidly as heat and to a small extent, light.
By measuring the heat released during the combustion of a vegetable oil, we can estimate the amount of heat which would be released when it is metabolized by the body. This is commonly referred to as the caloric content of the food and is expressed in terms of nutritional calories (Cal).
The calorie (cal) is a convenient unit of heat energy. One calorie is defined as the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. A kilocalorie (kcal) is equal to 1000 calories. One nutritional calorie is also equal to one kilocalorie. If, for example, a 4 oz. serving of ice cream contains 295 nutritional calories or 295,000 calories and if all of the energy stored in the ice cream were converted to heat, enough heat would be produced to raise the temperature of 295,000 g (650 lb. or 74 gallons) of water by 1 degree Celsius.
The measurement of the heat evolved by a chemical reaction is measured using a device called a calorimeter. Calorimeters vary in design. The one to be used in this experiment is shown in Figure 1.
A burner containing a pre-weighed amount of a liquid fuel, such as vegetable oil, is placed under a 250 ml Erlenmeyer flask containing 200 ml of water and a thermometer. The burner is shielded from drafts by a coffee can which hangs from the flask. The burner flame causes the temperature of the water to rise. When the water temperature has risen by a sufficient amount, the burner is extinguished, and the weight of the fuel is re-measured. Thus, the temperature rise produced by the burning of a known mass of fuel is determined, and from this information, the heat produced by the reaction can be calculated.
Calculation of Heat of Combustion
The first step in this calculation is to calibrate the calorimeter. This means we will determine the number of calories needed to raise the temperature of all of the parts of the calorimeter by 1°C by burning a known substance (dodecanol) and using the fact that the burning of each gram of dodecanol releases 10,102 calories.
The total heat released by the burning of the dodecanol is equal to the mass of the dodecanol burned times 10,102 cal/g.
heat of combustion of dodecanol (cal) = mass burned x 10,102 cal/g
Next, calculate the number of calories used to raise the calorimeter temperature by 1°C.
calibration value = heat of combustion of the dodecanol (cal)/ temperature change of the calorimeter (°C)
Do this for each trial and calculate the average value.
The next step is to calculate the heat of combustion of the unknown oil. After measuring the mass of the unknown oil burned and the temperature change, calculate the number of calories released by the burning of the oil. This is done by multiplying the temperature change (in °C) by your average calibration value, the number of calories needed to raise the temperature of the calorimeter by 1°C.
heat of combustion of oil (cal) = temperature change (°C) x average calibration value (cal/°C)
Finally, since we are interested in the amount of heat released by the burning of one gram of the oil, calculate the number of calories per gram by dividing the heat of combustion of the oil by the number of grams of oil burned.
heat of combustion per gram = heat of combustion of oil (cal) / mass of oil (g)
Usually, the average value for several trials is then calculated and reported.