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How does increasing the carbon chain length of alcohols affect the molar heat of combustion?
"The molar heat of combustion is the energy released when 1 mole of a substance undergoes complete combustion" (Ck12, 2013). Calorimetry is the science of measuring the change of heat associated with the system and its surroundings (Henderson, 2010). The amount of heat released during the combustion of the alcohol can be calculated by the change in temperature of the water. An equation was developed to calculate the molar heat of combustion:
MHC = mcΔT / n
This equation calculates the amount of heat released that the water absorbs.
However, some heat was lost to the aluminum milkshake can and to the surroundings. To increase the accuracy of the molar heat of combustion calculation, the milkshake can was included in this equation, using the formula:
MHC = mwatercwaterΔTwater / n + mcanccanΔTcan / n
Where MHC = Molar Heat of Combustion (kJ/mol), m = mass of water or can (g), c = specific heat capacity of water or aluminum, ΔT = change in temperature (°C), n = number of moles of the combustion substance (mol).
A combustion reaction involves a fuel reacting with excess oxygen and produces carbon dioxide and water.
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For example, when butanol undergoes complete combustion, the equation is:
C4H9OH + 6.5O2 → 4CO2 + 5H2O
For this example, 1 mole of butanol reacts with 6.5 moles of excess oxygen and produces 4 moles of carbon dioxide and 5 moles of water.
According to Jim Clark, bond enthalpy is "the energy required to break 1 mole of the bond to give separated atoms - everything being in the gaseous state" (Clark, 2010).
Each chemical bond has a varied bond energy, and for a reaction to be exothermic, the total bond energy of the reactants must be greater than the total bond energy of the products, which ultimately releases energy. For an endothermic reaction, the products have a greater total bond energy than the reactants (Khan Academy, 2016).
Both types of reactions are represented on the graph below:
| Reaction Type | Energy Change |
|---|---|
| Exothermic | Release of heat |
| Endothermic | Absorption of heat |
The combustion reaction of the fuel is an exothermic reaction, releasing energy in the form of heat. This leads to the development of the research question. With the number of carbon atoms increasing, and subsequently the number of hydrogen atoms by one per alcohol (butanol, pentanol, hexanol, heptanol, octanol), the total number of bonds increases, and therefore, the bond enthalpy increases, resulting in an increase in the molar heat of combustion proportionally.
The original experiment needed to be modified to overcome limitations and make the results more accurate. These modifications included using a probe to measure the temperature of the water with LoggerPro, applying aluminum foil to the milkshake can to act as a lid and insulant, changing the height from the spirit burner to reduce the amount of heat loss, and increasing the water volume to reduce the rate of heat loss. Despite all these modifications, the experiment still wasn't a fair test due to the lack of accuracy, as there was still too much heat loss, significantly reducing the final calculated MHC values.
Several modifications were made to improve the accuracy and precision of the experiment:
The following equipment and materials were used in the experiment:
During the experiment, several risks associated with naked flames and fuels were managed to ensure safety:
Below is the raw data obtained from the experiments, including the molar mass, initial and final mass of the burner, and initial and final temperature of the water for each trial:
| Fuel | Trial | Molar Mass (amu) | Initial Mass Burner (g) | Final Mass Burner (g) | Initial Temp Water (°C) | Final Temp Water (°C) |
|---|---|---|---|---|---|---|
| Butanol | 1 | 74.12 | 326.48 | 325.99 | 19.1 | 26.2 |
| 2 | 74.12 | 324.52 | 323.91 | 19.7 | 28.2 | |
| 3 | 74.12 | 322.76 | 322.22 | 19.2 | 26.6 | |
| Pentanol | 1 | 88.15 | 291.03 | 290.67 | 19.3 | 25 |
| 2 | 88.15 | 290.05 | 289.71 | 19.4 | 24.5 | |
| 3 | 88.15 | 293.99 | 293.55 | 19.8 | 26.6 | |
| Hexanol | 1 | 102.162 | 323.5 | 323.25 | 19.2 | 22.7 |
| 2 | 102.162 | 325.46 | 325.1 | 19.4 | 25.4 | |
| 3 | 102.162 | 323.51 | 323.17 | 19.4 | 27.3 | |
| Heptanol | 1 | 116.88 | 326.3 | 325.87 | 19.3 | 26.2 |
| 2 | 116.88 | 325.91 | 325.5 | 19.5 | 26.1 | |
| 3 | 116.88 | 324.98 | 324.54 | 19.7 | 25.9 | |
| Octanol | 1 | 130.2279 | 314.34 | 313.79 | 19 | 28.1 |
| 2 | 130.2279 | 313.78 | 313.24 | 19.7 | 27.9 | |
| 3 | 130.2279 | 319.08 | 318.68 | 19.6 | 27.7 |
General:
Butanol:
Pentanol:
Hexanol:
Heptanol:
Octanol:
Table 1: Butanol Trial 1
| Mass of Water (g) | Initial Mass (g) | Final Mass (g) | Initial Temp (°C) | Final Temp (°C) | Mass of Can (g) | Molar Mass (amu) |
|---|---|---|---|---|---|---|
| 250 | 326.48 | 325.99 | 19.1 | 26.2 | 183.33 | 74.117 |
All calculations below use data from Butanol Trial 1:
Moles of Butanol (C4H9OH) = 0.00661...
Mass of Water (mwater) = 250 ± 1g
Change in Mass (Δm) = 0.49 ± 0.02g
Change in Temperature (ΔT) = 7.1 ± 0.2°C
Mass of Can (mcan) = 183.33 ± 0.01g
Relative Uncertainty:
mwater = 250 ± 0.4%g
Δm = 0.49 ± 4.08%g
ΔT = 7.1 ± 2.82%°C
mcan = 183.33 ± 5.46x10-3g
Absolute Error of MHC = Accepted – Experimental
Absolute Error of MHC = 2676 – 1299.488
Absolute Error of MHC = 1376.51
Relative Error of MHC:
Relative Error of MHC = (Absolute Error of MHC / Accepted) x 100
Relative Error of MHC = (1376.51 / 2676) x 100
Relative Error of MHC = 51.44%
Average MHC of Butanol Trials:
Average MHC of Butanol Trials = 1259.386 kJ/mol
Table 2: Calculated MHC values
| Fuel | Trial | MHC (kJ/mol) | Average (kJ/mol) | Average (kJ/mol) |
|---|---|---|---|---|
| Butanol | 1 | 1299.488 | 1259.386 | 1259.386 |
| 2 | 1249.681 | |||
| 3 | 1228.989 | |||
| Pentanol | 1 | 1688.765 | 1645.671 | 1645.671 |
| 2 | 1599.883 | |||
| 3 | 1648.364 | |||
| Hexanol | 1 | 1730.581 | 2220.994 | 1895.399 |
| 2 | 2060.216 | |||
| 3 | 2872.183 | |||
| Heptanol | 1 | 2269.318 | 2179.537 | 2179.537 |
| 2 | 2276.538 | |||
| 3 | 1992.755 | |||
| Octanol | 1 | 2607.098 | 2730.229 | 2499.928 |
| 2 | 2392.758 | |||
| 3 | 3190.830 |
The outliers, Hexanol T3 and Octanol T3, did not significantly impact the results, but they were excluded from averaged calculations and certain graphs to ensure accuracy. The primary limitation of the evidence was the substantial heat loss due to experimental methods, not the accuracy of the materials used. The equipment, including the LoggerPro temperature probe with an absolute uncertainty of ±0.1ºC, the balance with an absolute uncertainty of ±0.01g, and the 250mL measuring cylinder with an absolute uncertainty of ±1mL, was precise. There was a sufficient variety of fuels used to establish the relationship between chain length and MHC.
The experiment demonstrated reliability in some trials, such as Butanol and Pentanol, which showed precision with results within 100kJ/mol of each other. However, Hexanol, Heptanol, and Octanol exhibited a wider range of 300kJ/mol for each fuel, excluding outliers. The uncertainty was relatively high, with the highest absolute uncertainty being ±350.17kJ/mol for Hexanol T1 and the lowest being ±109.85kJ/mol for Butanol T2. Factors contributing to random errors included air drafts caused by fast movement around the apparatus, inconsistent starting temperatures, and variations in the centring of the spirit burner under the can.
The experiment generated results that addressed the research question but yielded inaccurate MHC values, with a large average relative error of 52%. The primary systematic error was the substantial heat loss to the surrounding environment, primarily due to the open flame and insufficient oxygen supply. This resulted in incomplete combustion, visible through the flame's orange appearance, and substantial soot buildup. Incomplete combustion led to reduced heat transfer to the water and lower MHC values, rendering the results invalid.
To minimize heat loss and improve accuracy, several enhancements can be made:
To extend the investigation, compare the combustion of alcohols to alkanes, alkenes, and alkynes to explore the impact of functional groups and hydrogen atoms on MHC.
The aim of this investigation was to examine the impact of carbon chain length in alcohols on the molar heat of combustion (MHC). It was hypothesized that increasing the number of carbon atoms in the chain would proportionally increase the MHC. The results supported this hypothesis, demonstrating that alcohols with longer carbon chains indeed exhibit higher MHC values than those with shorter chains.
For instance, Butanol with 4 carbon atoms displayed the lowest average MHC of 1259.39 ± 122.8 kJ/mol, while Octanol with 8 carbon atoms exhibited the highest average MHC of 2499.93 ± 227.5 kJ/mol. The graphical representation of the results confirmed a linear trend, suggesting that for each additional carbon atom, the MHC increased by approximately 301.5 kJ/mol.
However, the experiment encountered a significant limitation due to a major systematic error: heat loss to the surrounding environment. This error resulted from the absence of insulation and insufficient oxygen supply, leading to incomplete combustion. As a consequence, the heat transferred to the water was lower than expected, causing all experimental MHC values to be substantially lower than the accepted values. The average MHC for Butanol, for instance, was 1259.39 ± 122.8 kJ/mol, while the accepted value was 2676 kJ/mol, resulting in an error of 53%. Octanol, excluding outliers, had an average MHC of 2499.93 ± 227.5 kJ/mol, compared to the accepted value of 5294 kJ/mol, with an error of 52%. These significant errors rendered the data inaccurate and the results unreliable.
Therefore, a method with more controlled conditions is required to obtain conclusive results that align with accepted MHC values for alcohols.
| MHC (kJ/mol) |
|---|
| 122.805 |
| 214.525 |
| 318.693 |
| 252.646 |
| 265.942 |
| 122.805 |
| 214.525 |
| 318.693 |
| 252.646 |
| 265.942 |
| MHC (kJ/mol) |
|---|
| 122.805 |
| 214.525 |
| 318.693 |
| 252.646 |
| 265.942 |
| 4 | 5 | 6 | 7 | 8 | |
|---|---|---|---|---|---|
| MHC (kJ/mol) | 1259.386 | 1645.671 | 1895.398 | 2179.537 | 2499.928 |
| T1 | T2 | T3 | |||
|---|---|---|---|---|---|
| 4 | 5 | 6 | 7 | 8 | |
| MHC (kJ/mol) | 1299.488 | 1688.765 | 1730.581 | 2269.318 | 2607.098 |
| T2 | 4 | 5 | 6 | 7 | 8 |
| MHC (kJ/mol) | 1249.681 | 1599.883 | 2060.216 | 2276.538 | 2392.758 |
| T3 | 4 | 5 | 6 | 7 | 8 |
| MHC (kJ/mol) | 1228.989 | 1648.364 | 2872.183 | 1992.755 | 3190.830 |
| Experimental | Accepted | ||||
|---|---|---|---|---|---|
| 4 | 5 | 6 | 7 | 8 | |
| MHC (kJ/mol) | 1259.386 | 1645.671 | 1895.398 | 2179.537 | 2499.928 |
| Accepted | 4 | 5 | 6 | 7 | 8 |
| MHC (kJ/mol) | 2676 | 3331 | 3984 | 4638 | 5294 |
Effect of Carbon Chain Length on MHC. (2024, Jan 11). Retrieved from https://studymoose.com/document/effect-of-carbon-chain-length-on-mhc
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