CHEMISTRY - Enthalpy of Combustion of Alcohol

IB CHEMISTRY

LAB REPORT

ENTHALPIES OF COMBUSTION OF ALCOHOLS

SUBMITTED BY: Ankon Rahman

Introduction:

The alcohols are a type of organic compounds which contain a hydroxyl (-OH) ion. The standard enthalpy of combustion is the enthalpy change when one mole of a reactant completely burns in oxygen under standard thermodynamic conditions. Therefore, enthalpy of combustion of an alcohol means the enthalpy change when one mole of an alcohol is burnt in presence of oxygen under standard conditions. For this experiment, three alcohols i.e. methanol (CH3OH), ethanol (C2H5OH) and propanol (C3H7OH) are heated in presence of oxygen. Apparatus:         

Clamp and stand Spirit lamp Test tubes Tongs Measuring cylinder Thermometer Balance Lab coat Safety glasses

Method:          

A measuring cylinder should be filled up with water and the volume should be recorded. A test tube should be filled up with the water in the cylinder methanol afterwards. With a clamp and a stand, the test tube should be suspended in a way so that there is space below it to place the spirit lamp. The initial weight of the spirit lamp filled with methanol is recorded with the help of a balance. The spirit lamp then is place right below the suspended test tube. A thermometer is placed in the test tube and the initial temperature is recorded. The spirit is then lit and observed as the temperature of water goes up. After reaching a particular temperature, the spirit lamp is then blown out and then its weight is recorded again. The temperature of water is recorded again. The above steps are repeated again for ethanol and propanol.

Data Collection And Processing:

Raw data: In the first trial in case all the three alcohols, the lamps were blown out at 50 oC. In the second trials, the same was done at 65 oC. The two following raw data tables contain the set of raw data recorded at 50 oC and 65 oC respectively. Mass (g) ± 0.01g

Methanol Ethanol Propanol

Initial 173.48 166.11 151.63

Final 171.67 164.97 150.89

Temperature of water (oC) ±0.5oC Initial Final 20.0 57.0 20.0 58.0 21.0 57.0

Table 1: Experimental values recorded when lamps were removed at 50oC

Mass (g) ± 0.01g

Methanol Ethanol Propanol

Initial 171.58 162.59 157.93

Final 169.62 161.17 156.97

Temperature of water (oC) ±0.5oC Initial Final 21.0 71.0 21.0 70.0 21.0 71.0

Table 2: Experimental values recorded when lamps were removed at 65oC Processed Data:

From the raw data, the amount of consumed mass of the alcohols can be calculated by subtracting the final mass of the spirit lamp from the initial mass.

Methanol Ethanol Propanol

Mass consumed (g) ± 0.02g Final temperature: 50oC Final temperature: 65oC 1.81 1.96 1.14 1.42 0.74 0.96 Table 3: Masses of the alcohols consumed in the combustion

The difference in temperature of the water is processed.

Methanol Ethanol Propanol

Take-off temp: 50oC 37 38 36

ΔT (K) ± 1oC Take-off temp: 65oC 50 49 50

Table 4: Difference in Temperature Now we can plug in these values into our equation to calculate the Where specific heat of water or c = 4.18 K J-1 mol-1

o

Methanol Ethanol Propanol

Take-off temp: 50 C 6.2 6.4 6.0

ΔE (kJ) Take-off temp: 65oC 8.4 8.2 8.4

Table 5: ΔE in Reaction of Combustion In order to calculate the enthalpy of combustion, we need one final piece of data: the number of moles of alcohol used up by the lamp during the process of combustion.

o

Methanol Ethanol Propanol

Take-off temp: 50 C 0.057 0.025 0.012

n(alc) Take-off temp: 65oC 0.061 0.031 0.016

Table 6: Number of Moles of Alcohol used up in Combustion

With this data, we can derive the enthalpy of combustion (

Methanol Ethanol Propanol

Take-off temp: 50oC -110 -260 -490 Table 7:

) by using the equation:

(kJ mol-1) Take-off temp: 65oC -140 -270 -520

Average -125 -265 -505

of the 3 Alcohols

Uncertainties and Errors:

For calculating the uncertainty in , the uncertainty in the mass m and the change in temperature are to be calculated. As c, which is the specific heat capacity of the alcohols is a constant for each alcohol, it has no uncertainty. As the calculation is a multiplication, the percentage errors in the mass and the temperature change are to be added to find the percentage error in the energy change. Uncertainty in Mass: As the mass is calculated with a digital balance, the uncertainty should be its smallest unit which is ± 0.01g. However, the mass used in the calculation is the difference of two masses, i.e. the difference between the initial and final mass of the spirit lamp. Therefore, to find the absolute error, the uncertainty is multiplied by 2. Absolute error = (± 0.01g * 2) = ± 0.02g Percentage error = (0.02/1.34) * 100 = 1.50%

Uncertainty in temperature: As the temperature is calculated with an analog thermometer, the uncertainty should be half of its smallest unit which is ± 0.5 oC. However, the temperature used in the calculation is the difference of two temperatures, i.e. the difference between the initial and final temperature of the water. Therefore, to find the absolute error, the uncertainty is multiplied by 2. Absolute error = ± 0.5 K * 2 = ± 1 oC Percentage Error = (1/43.3) * 100 = 2.30%

The total percentage error of the calculation is = 1.50 + 2.30 = 3.80 % Absolute Error of Alcohol Methanol Ethanol Propanol

:

Average -125 -265 -505

Absolute Error to 1 s.f. ±5 ± 10 ± 20

Conclusion: The comparison of the actual values of enthalpies of combustion of alcohols and the values calculated from the experimental data are shown: Alcohol Methanol Ethanol Propanol

Calculated value from the experiment -125 -265 -505

Actual Value -726 -1367 -2021

Percentage Deviation 82.78% 80.61% 75.01%

Table 9: Comparison of Experimental and Actual Values of Alcohols As observed from the above table, it is seen that with the increase of a (–CH2-) group, the values of enthalpies increase. From this it can be deduced that there might be a proportional relationship between the increasing molecular mass of the alcohols to the enthalpies of combustion. Theoretically, this deduction can be said to be true, because as there are a higher number of bonds involved in breaking per mole in the combustion process. As methanol has the least number of bonds, therefore it has the lowest enthalpy of combustion. Similarly, as propanol has the highest number of bonds among the alcohols, it has the highest enthalpy of combustion. The following scatter plots show a comparison between the experimental and actual values of enthalpies, as the molecular mass increases.

Graph 1: A comparison of the mass versus enthalpies graph of the actual and experimental values.

From the graphs it can be seen that the actual values of the enthalpies are proportional to the increasing molecular mass, as the R2 value is equal to 1. But it is also observed that the relation between the enthalpies calculated from the experiment are closely proportional as well, as the R 2 value of 0.997 is very close to 1. It is to be noted that the more the value is closer to 1, the more is the possibility that the y-axis and the x-axis are proportional to each other.

Evaluation Although the derived values of the enthalpies are significantly deviated, the proportional nature of the experimental data suggests that the experimental inaccuracies have remained consistent throughout the experiment. The first apparent source of such inaccuracies can be said to be the measurement of temperature. The temperature readings from the experimental data were found to be very low. But it is already known that a very high amount of heat is released. This high amount was not calculated to complete extent because heat energy had escaped the system through multiple ways which were not possible to consider into calculation. A very high amount of heat had been lost to the surroundings. By taking various measures, the amount of enthalpy escaped to surroundings can be minimized. During the process of heating, a higher amount of heat could be propagated to the test tube. This can minimize the heat loss because it would take a lower amount of time for the process to complete and in a shorter amount of time, there would definitely be a shorter amount of heat loss.

The process of measuring the heat change could have been done electronically with the help of a data logging instrument. In that way, from the cooling curve of the alcohols, the initial and final temperature could be derived through extrapolation. In this way, more exact values of temperatures could be calculated which would minimize inaccuracies.