Enthalpy of Combustion Lab Report

Hypothesis:

The given experimental procedure is hypothesised to be a highly exothermic reaction as when ethanol is burnt in air the reaction gives out energy in the form of heat. When the spirit lamp (ignited) is placed directly below the calorimeter containing water, the heat energy released will result in a rise in temperature in the distilled water in the calorimeter.

Variables:

a). Independent Variables:

1. The amount of ethanol used in the experimental procedure is an independent variable. In each of the five trials, the amount of ethanol used differs and this helps in providing a variety of results and making the experiment more accurate.
2. The calorimeter in which the distilled water has been placed for each trial in the experimental procedure is an independent variable. This has been changed for each trial in order to provide more accuracy to the experiment.
3. The mass of the distilled water used in the experimental procedure is an independent variable. This has also been changed for each trial in order to provide a variety of results and cover all aspects of the experimental procedure.

 

b). Dependant Variables:

In the conducted experimental procedure, the temperature of the measured mass of water in the calorimeter is the dependent variable. As the amount of ethanol used in the experimental procedure differs, the temperature of the mass of water will also change depending on the amount of ethanol used.

 

c). Controlled Variables:

1. The time for which the measured mass of distilled water is heated is kept constant as one and a half minutes throughout the experimental procedure. This is done to provide an accurate experiment.
2. The entire experiment is conducted in the same environment on the same day and in a secure room.
3. The stopwatch used for each trial has also been kept constant throughout the experimental procedure.
4. The thermometer used for each trial has also been kept constant throughout the experimental procedure.

Observations:

1. The ethanol was burnt with a yellow flame.
2. Some black soot was formed on the bottom of the calorimeter after the spirit lamp was placed underneath it in the experimental procedure.
3. It was noted that when the mass of the spirit lamp was noted in the beginning of the experiment, the lamp was not warm. However after the ethanol was burnt and the mass of the spirit lamp was once again being recorded, the lamp was warm at the end of the experiment.
4. It was noted that while the spirit lamp was placed on the beaker perpendicularly below the calorimeter containing the distilled water, one could feel the warmth of the flame when one put out the flame after a period of one and a half minutes.

Raw Data:

Initial mass of spirit lamp = M1,

Final mass of spirit lamp = M2,

Mass of distilled water used in the experimental procedure = M3,

Initial temperature of water = T1,

Final temperature of water = T2,

The following table N1 shows the raw data that has been recorded in order to determine the enthalpy change of combustion of ethanol while conducting the following experimental procedure:

Processed Data:

Mass of ethanol that has been used and burnt completely = M1 – M2 = MF,

Rise in Temperature of the distilled water when the spirit lamp is placed directly below the calorimeter = T2 – T1 = TF,

The following Table N2 gives the values of the mass of ethanol that has been used and the change in temperature of the distilled water solution.

For all five cases, molar mass of ethanol Mm, C2H5OH

Amount of ethanol burnt = MF/Mm = A1,

Heat energy required to raise the temperature of water by TF

 = M3 × TF × 4.18 = H1,

Heat energy evolved by burning A1 moles of ethanol = H2,

Heat energy evolved by burning 1 mole of ethanol = H2/A1 = H3,

Enthalpy change of combustion of ethanol =

The following table N3 gives the processed data for the enthalpy change of combustion of ethanol:

Since the combustion of ethanol is an exothermic reaction, the sign of is negative.

Hence from the above five trials an average value of the enthalpy of combustion can be taken as the following:

Fair Test:

1. The experimental procedure is carried out in a safe environment under the same environmental conditions on the same day.
2. The time taken for each trial i.e. one and a half minute time period was followed as accurately as possible and with as much efficiency to avoid errors.
3. The stopwatch is used accurately and was reset after each trial reading.
4. While stirring the water alongside the experimental procedure one made sure that one did not disturb the digital thermometer in any possible way as even a small disturbance could affect the temperature readings obtained.

Safe Test:

1. One used safety glasses and goggles while conducting the experiment to protect oneself from any chemical hazards.
2. Once the time period of one and a half minutes has elapsed, the cap of the spirit flame was placed over the wick of the spirit lamp with precision and safety.
3. Before starting the experiment, one made a double check about all the equipment being used in the experiment and whether the equipment is efficient or not.
4. One made use of the experimental hazcards that were given to one in order to prevent any chemical hazards from occurring.

Errors and Analysis:

The data booklet value for the standard enthalpy of combustion of ethanol is        -1371 KJ/mol. Hence the percentage error for this experimental procedure is:

= [(1371 – 948) ÷ 1371] × 100

= 30.9%

This error is large and cannot be accounted for by the uncertainties in the experimental measurements. The major uncertainties lie in the underlying assumptions used in the experimental procedure such as the following:

1. It is assumed that complete combustion of ethanol takes place. However the fact that when ethanol was being burnt in air, it burnt with a yellow flame and the black soot on the calorimeter indicated that carbon, one of the products of incomplete combustion was also formed.
2. It is assumed that all the heat from the reaction was transferred to the distilled water in the calorimeter. It is true that some of the heat would have indeed been lost round the sides of the calorimeter and also from the inner system to the surroundings.
3. It is assumed that the glass stirrer, the digital thermometer and the calorimeter were not absorbing any heat but in reality these instruments are also being heated.
4. It is assumed that none of the ethanol escaped through evaporation during the time the flame was extinguished and the spirit lamp and its contents were reweighed. However, ethanol is a volatile liquid and the fact that the spirit lamp was hot indicates that some of it would have evaporated.

Conclusion:

In the carried experimental procedure one has determined the enthalpy change of combustion of ethanol when one mole of ethanol is burned completely with the help of a spirit lamp for a time period of one and a half minutes.

One has successfully proven the hypothesis that has been made earlier. I had predicted beforehand that the reaction-taking place in the experimental procedure is an exothermic one. The fact that heat is given out when ethanol is burnt has been depicted by the temperature rise of the distilled water present in the calorimeter. Hence it is clearly evident that the combustion reaction of ethanol is an exothermic reaction.

Suggestions for Improvement / Evaluation:

1. There should not be any disturbance in the room where the experiment is being conducted as one is trying to prevent any interference to the internal system.
2. While conducting the experiment, one should use a shield like device in order to cover the entire apparatus and prevent any heat transfer between the surrounding and the internal system.
3. More efficient equipment should be used in the conducted experimental procedure.