An experiment to determine the enthalpy of combustion for alcohols of differing numbers of carbon atoms PDF

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An experiment to determine the enthalpy of combustion for alcohols of differing numbers of carbon atoms.

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Abstract: This experiment aimed to find the value for the enthalpy of combustion for Methanol, ethanol and propanol. The literature values for these three alcohols were; Methanol = (−715.0 kJ/mol), Ethanol = (−1370.7 kJ/mol), Propanol = (–2021 kJ/mol). The experimental values obtained were; Methanol=11.14 kJmol−1 , ethanol and propanol (put values in). This showed a % difference of (put in your % error values) and therefore lowers the validity of this experiment and requires an improvement in the method.

Introduction: This experiment aims to determine a value for the enthalpy change of different alcohols of varying numbers of carbon atoms within the organic chain. The alcohols used were methanol, ethanol and propanol as they differ by 1 carbon atom per molecule and therefore provide an adequate difference in determination of energy exchange per molecule.

Fig 1: This image defines the skeletal structure of methanol, ethanol and propanol highlighting the increase in one carbon atom per molecule. (Google images, 2020) The energy transfer per molecule is known as the enthalpy change of a substance and is defined by the symbol, ∆ H . A subscript is usually added to define the type of enthalpy change, in this case, the enthalpy of combustion. Here, the subscript c is added to highlight combustion: ∆ H c . The enthalpy of combustion is calculated using the following equation: ∆ H c=

−Q [ kJ mol−1 ] n

Where Q Is the heat energy transferred by burning the alcohol and n is the number of molecules of fuel burnt. The minus sign convention states the loss of energy from the alcohol and therefore how this reaction is an exothermic process. The amount of energy transferred by burning the alcohol is given by the energy transfer equation: Q=mc ∆ T [ J ] Here m is the mass of the substance being heated by the burning fuel, in this case water, c is the specific heat capacity of water, given as 4.18 [ kJ kg−1 k −1 ] and ∆ T is the change in temperature of the water in [K] Literature values for the enthalpy of combustion of the three alcohols are given as:

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Methanol = (−715.0 kJ/mol), Ethanol = (−1370.7 kJ/mol), Propanol = (–2021 kJ/mol) Experimental values will be compared to these literature values and the uncertainty will be determined, highlighting the effectiveness and accuracy of this experiment. Hypothesis: Following the information found in the introduction a prediction can be made that states that the higher the number of carbon atoms in a molecule, the greater the energy release and therefore the greater the enthalpy of combustion. Propanol should provide the highest enthalpy value when burnt. Equipment list: �Spirit burner containing an alcohol �Splints �Calorimeter �Heat shield �100-200 cm3 water �50 cm3 or 100 cm3 measuring cylinder or 50 cm3 pipette �Protective mat �Eye protection �Thermometer reading 0-110 °C in 0.1 °C increments �Access to a balance �Retort stand, boss and clamp.

Method: Put 100 cm3 of water in the calorimeter make sure to record the temperature of the water, writing this down in a pre-constructed table. Clamp the calorimeter in position about 10 cm above the protective mat. Find the mass of a spirit burner containing an alcohol by placing it on a balance to a high enough resolution. Make a note of this mass. Place the burner under the calorimeter and light the wick. Place the heat shield around the apparatus. Stir the water with the thermometer to distribute the heat evenly throughout the sample. Stop heating when the temperature has risen by 20 °C or a set temperature that has been predetermined. Extinguish the flame by replacing the cap on the burner. Reweigh the burner as soon as possible, making a note of this new mass. Keep noting the temperature – record the highest value reached. Repeat the experiment with other alcohols recording the results in the pre constructed table. Results: Having conducted the experiment the following values for energy transfer and enthalpy were obtained:

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. Methanol (Lit value

715 kjmol−1 ¿ :

Moles of MethanolBurnt=

m methanol 34.53 = =0.5746 moles Mr 32.04

Energy transfered ¿ water =Q=mc ∆ T =0.15 x 4200 x 10=6300 J =6.4 kJ

Enthalpy of Methanol= % difference=

−Q −6.3 =11.14 kJmol−1 = 0.5746 n

715 −11.14 x 100=98 % 715

Ethanol (Lit value

−1 1366.8 kjmol ¿ :

Moles of MethanolBurnt=

m methanol ❑ = =moles ❑ Mr

Energy transfered ¿ water=Q=mc ∆ T =J = kJ Enthalpy of Methanol=

−Q −❑ = =kJmol−1 ❑ n

% difference= ❑ x 100=99.59 % ❑ Propanol (Lit value

−1 2021 kjmol ¿ :

Moles of MethanolBurnt=

m methanol ❑ = =moles ❑ Mr

Energy transfered ¿ water =Q=mc ∆ T =¿ J = kJ Enthalpy of Methanol=

−Q −❑ −1 = ❑ =kJmol n

% difference= ❑ x 100=98 % ❑ The % difference calculated for each of these alcohols shows a large variation from literature value. This indicates that there is a significant amount of heat lost from the system due to the radial nature of energy transfer from an open flame. This indicates that only a small portion of the heat energy provided by the burning alcohol was actually transferred to the water. The results for enthalpy can now be plotted as a function of the number of carbon atoms and a relationship determined.

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Results table showing the enthalpy of alcohols with varying numbers of carbon atoms. Alcohol

[ kJ mol− Methano l Ethanol Propanol

Number of carbon atoms

Enthalpy

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715

2 3

1366.8 2021

Enthalpy of alcohols of increasing number of carbon atoms 2500

Enthalpy kJmol^-1

2000 1500 1000 500 0 0.5

1

1.5

2

2.5

3

3.5

Number of carbon atoms

This graph shows the relationship between the number of carbon atoms found in an alcohol and the enthalpy released by combustion. This graph shows a positive correlation and as the number of carbon atoms increases so does the enthalpy or energy released per molecule. This corresponds and confirms the hypothesis. Evaluation:

This experiment has shown how alcohols with a varying number of carbon increase in enthalpy. The values obtained show a high level of inaccuracy which is proven by the % error and lowers the validity of this experiment, though the results do prove the stated hypothesis. The reasons for this loss of energy can be accounted for in the following way, radiation, being radial, emits heat in all directions and accounts for a significant loss of heat transfer due to the lack of insulation and having no closed system between the flame and the water sample. As the calculations show, the amount of heat loss is between 98% and 99% for all three experiments. A small portion of this heat is also transferred to the glassware and does not contribute to the overall heating of the water. This will affect the results for energy 6

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transfer and therefore enthalpy by reducing the amount of Q and therefore the amount of enthalpy per alcohol. There are additional errors within this experiment including, not stirring the water which would cause an uneven heat distribution, lack of insulation, allowing additional heat to escape and not allowing the water to reach room temperature before applying the heat. This would mean energy is being used to heat up the water to room temperature, effecting the temperature change. Conclusion: To improve this experiment the following measures can be put in place, creating an isolated, insulated system that allows a greater portion of heat transfer to the water. Continuously stirring the water to dissipate heat into the sample. And ensuring water is at a consistent starting temperature before attempting the experiment.

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Reference list: Googl ei mages .2020.Met hanol ,et hanol andpr opanol i mage.[ ONLI NE]Av ai l abl eat :. [ Acc ess ed17Januar y2020] . # Wi ki pedi a.2020.Ent hal pyofcombust i onl i tv al ues .[ ONLI NE]Av ai l abl e at :ht t ps : / / en. wi k i pedi a. or g/ wi ki / Pr opan1ol _( dat a_page) .[ Acc ess ed17Januar y2020] .

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