The Enthalpy of Combustion of Vegetable Oil

Purpose

The purpose of this experiment is to determine the enthalpy of the combustion of a vegetable oil.

Introduction

Humans obtain energy required for life processes from the controlled oxidation of foods. Typical examples of the reaction are:

sucrose +oxygen->carbon dioxide+water+energy

vegetable oil+oxygen->carbon dioxide+water+energy

(1)

(2)

This is the same chemical reaction that is observed during burning or combustion except that during the burning process, energy is released rapidly as heat and, to a small extent, light. The heat which exchanges with the environment during a chemical reaction at constant pressure is called the enthalpy change of the reaction. Since the reactants and products of the combustion and controlled oxidation reactions are the same, the heat released by the two processes should be the same.

Thus, by measuring the heat released during the combustion of a vegetable oil, we can estimate the fuel equivalent of the oil when it is metabolized by the body. This is commonly referred to as the calorie content of the food and is expressed in terms of nutritional calories. The nutritional calorie is equal to 1000 thermochemical calories (1 Kcal) or 4184 joules of energy. The thermochemical calorie is defined as the amount of heat required to raise the temperature of one gram of liquid water by 1 degree Celsius.

In principle, the determination of the caloric content of a food material is straightforward. A measured mass of the substance is burned and the heat which is released is used to warm a known mass of water in a container. By measuring the increase in the temperature of the water the amount of heat which has warmed in the water can be calculated. In practice, it's not quite this simple. While most of the heat given off by the reaction is absorbed by the water, a small but significant amount is absorbed in heating the container. Some is also lost to the surroundings and we have no way to measure this directly as the food is burned. We can represent this in equation form as:

Heat produced by the reaction = Heat gained by water

+ heat gained by the container

+ heat gained by the thermometer

+ heat gained by the environment

(3)
or: -qrxn = qH2 O + qcon + qtherm + qenv (4)

Since the heat absorbed by an object is the product of the specific heat (Csp) of the object, its mass (m) and its temperature change (DT),

i.e., q = CspmDT (5)

and since the enthalpy change of a reaction is the heat gained

or lost by the reaction, we can rewrite this expression as:

-DHrxn = -qrxn = Csp,H2 OmH2 ODT + qcon + qth + qenv (6)

Most of the heat given off by the flame in this experiment heats the water. In addition, if a series of experiments are carried out and the temperature changes are very similar, the amount of heat, q, which does not heat the water will be approximately the same for each experiment. So we can rewrite equation (6) as:

-DHrxn = Csp,H2 OmH2 ODT + q (7)

where q represents the heat leaving the reaction but not warming the water in the calorimeter.

The DHcomb for a vegetable oil will be measured by burning another substance for which the value of DHcomb is known

(1-dodecanol; DHcomb = -42.266 Kj/g) and measuring the heat gained by the water. The difference between the heat given off by the reaction and the heat absorbed by the water is q. Thus, we can calibrate the calorimeter. If vegetable oil, a substance for which the DHcomb is not known, but which is very similar to 1-dodecanol, is burned, it can be assumed safely that the value of q will be the same as long as the value of DT is about the same. The amount of heat given off by the combustion of the oil can be determined by measuring the change in the temperature of the water when the oil is burned, then calculating the heat gained by the water (qH2O = Csp,H2OmH2O DT, where Csp,H2O = 4.184 J/gC) and finally adding the value of q.

 

Procedure

photo of finished burnerUsing two pieces of copper wire, form two wick holders to fit in 30 ml beakers as shown in the picture to the right (click for a bigger picture).  The holder should extend across the diameter of the beaker about 1/3-1/2 of the way from the top. [Burner making demonstration movie]

Fit the wickholders and wicks in each beaker. The wick should extend from about the bottom to the top of each beaker. When the wicks are in place, fill one beaker to within 0.8 cm of the top with 1-dodecanol, taking care to avoid getting it on the outside surface of the beaker. Fill the other one in the same way with vegetable oil. These same two burners will be used throughout the experiment. You do not need to make new burners between trials.

Light both lamps to be sure they work properly. If either lamp has an excessively sooty flame, you will need to use scissors to trim the wick. Blow out the lamps and allow them to cool. Trim the wicks if necessary. Mount the aluminum can as shown in the figure so that the bottom of the can will be about 1 cm above the top of the flame.

 

Calibration of the calorimeter. Determine the mass of the dodecanol lamp by weighing it on a balance. Be careful not to spill any of the oil because you must determine the mass of the oil burned in the combustion reaction. Record the mass.

Prepare about 500 ml of water which is about 10C below the room temperature. You can do this by adding ice to tapwater and stirring until the ice melts. (Be sure the water is thoroughly mixed). Using your graduated cylinder, transfer exactly 100 ml of the water (no ice) to the aluminum can. Stir the water thoroughly with the thermometer, read the temperature to the nearest 0.2OC and record it in your notebook.

Quickly relight the dodecanol lamp and place it under the can of water, centering the flame under the bottom of the can. Stir the water with the thermometer (be careful, it is fragile), and when the temperature of the water in the can has risen about as many degrees above room temperature as it was below, blow out the flame. Continue to stir the water, recording the highest temperature reached. Remove the lamp and when it has cooled to room temperature, re-weigh it and record the mass. While the lamp is cooling, empty the water from the can and clean the soot off of the bottom. Repeat this calibration, using the same lamp.

(When you are done, the dodecanol and vegetable oil can be washed down the drain. Wash the beakers with soapy water; return the equipment to the Stockroom).

In all trials, the temperature change, DT, should be as nearly equal as possible. The distance from the lamp to the can should also be constant.

Enthalpy of combustion of the vegetable oil: Record the type of vegetable oil you use. Weigh and record the mass of your vegetable oil lamp. Determine the heat given off by the combustion of the vegetable oil twice, using the same procedure used to calibrate the calorimeter, except that the dodecanol lamp should be replaced by the vegetable oil lamp.

(The dodecanol and vegetable oil can be washed down the drain. Wash the beaker with soapy water; return the equipment to the Stockroom).

 

CALCULATIONS---Use the Table on the next page as a guide for organizing your calculations.

Calibration: For each trial, calculate the heat liberated by the combustion of dodecanol, the heat absorbed by the water, and q in equation (7) which represents the heat liberated by the combustion of dodecanol, but not absorbed by the water. Calculate the average value of q.

Enthalpy of combustion of the vegetable oil: For each trial, calculate the heat absorbed by the water, and add to this the average value of q obtained in the calibration step. Use this result and the mass of vegetable oil to calculate the enthalpy of combustion per gram of vegetable oil.

For your CONCLUSION, report the average experimental value of

DHcomb in joules per gram of vegetable oil.

 

 

Calibration by Combustion of 1-dodecanol

Data Trial 1 Trial 2
a) Initial mass of lamp (in grams) _______ _______
b) Final mass of lamp (in grams) _______ _______
c) Volume of H2O placed in calorimeter (ml) _______ _______
d) Ti, initial temp. of H2O (C) _______ _______
e) Tf, final temp. of H2O (C) _______ _______
Calculations    
a) Mass of 1-dodecanol burned (in grams) _______ _______
b) DT, the temperature change of the H2O

(DT = Tf - Ti)

_______ _______
c) Mass of H2O (in grams)

(mH2O = dH2O VH2O )

where density H2O = 1.00 g/ml

_______ _______
d) Heat absorbed by the water (in joules)

(qH2O = csp,H2OmH2O DT)

where Csp = 4.184 J/(gC)

_______ _______
e) Heat given off by combustion of 1-dodecanol

(in joules)

(qdodec = mdodecDHcomb)

where DHcomb = -42.266x103 J/g or -42.266 kJ/g

_______ _______
f) Heat given off but not used to warm the

water (in joules)

(q = -qdodec - qH2O)

_______ _______
g) Average Value of q (in joules)_______________    
Enthalpy of combustion of vegetable oil:    
Data Trial 1 Trial 2
a) Name of vegetable oil used: _________________    
b) Initial mass of oil lamp (in grams) _______ _______
c) Final mass of oil lamp (in grams) _______ _______
d) Vol. of H2O placed in calorimeter (ml) _______ _______
e) Initial water temperature _______ _______
f) Final water temperature _______ _______
Calculations    
a) Mass of oil burned (in grams) _______ _______
b) Temperature change of H2O _______ _______
c) Mass of H2O (in grams) _______ _______
d) Heat absorbed by H2O (in joules) _______ _______
e) Heat given off by combustion

(-qoil = qH2O + q)

_______ _______
f) Enthalpy of combustion (in J/g)

(DHcomb = qoil /moil)

_______ _______
Conclusion    

The average DHcomb (in Kj/g) ________________