Is the Energy Output from Different Alcohols Proportional to Their RMM?

[sueanne]

does this make any sense at all ? I am so stressed out by this, pls someone verify it, its GCSE level...
ANALYSIS

Apart from propanol, the graph shows the increase of energy output is proportional to the increase of RMM for alcohol. According to the line of best fit, butanol produced the highest amount of energy output. However, without looking at the line of best fit, it states that propanol produces the highest amount of energy output, and this does not support my findings. I will discuss this in the evaluation section.

With reference from the above calculations, it was shown that the above analysis agrees to the fact that the energy released followed the order that methanol to ethanol to propanol to butanol. When comparing my predicted results to the actual results from the experiment I have noticed that my results form the experiment were much lower, this could be because of the huge amount of heat energy released into the air. I know from my research that C=O bonds have the highest bond energy of 805, these bonds in butanol were not broken therefore the overall energy output was so low. In the first place, ample of heat energy (that is more than 805 kJ) will have to be made before the bonds are broken as they are so hard to break due to the fact that they have strong double bonds.

The gradient of the line is quite steep. There are plenty of spaces in between the points. This must mean that more alcohols can be experimented for to produce a more accurate line of best fit. The general trend of the graph is that the more bonds of an alcohol will guarantee a higher energy output, especially when most of the bonds are C=O bonds.
- basically, i would like to know whether if all the alcohols (methanol,ethanol,butanol,propanol) have the highest bond of C=O, its bond energy is 805J.

 

[Astronuc]

Firstly, to what graph are you referring.

Secondly, the simplest alcohols contain R-O-H, where R = longest chain of (C,H). For example methanol has formula (CH₃)-OH and ethanol (CH₃-CH₂)-OH.

C=O bond is found in aldehydes, amides, amino and carboxylic acids, esters and ketones.

The C=O bond energy is approximately 804-805 kJ/mol.


Useful reference - http://www.chem.queensu.ca/people/faculty/mombourquette/FirstYrChem/Molecular/BondEne/

 

[xanthym]

During complete combustion of Alcohol, Combustion Energy (or Combustion Heat) is produced from formation of combustion products CO₂ and H₂O. Thus, Heat of Combustion (expressed in kJoules/mole) should increase approx linearly with number of Alcohol CARBON atoms. In fact, this general relationship can be seen from published data:

Alcohol     # C Atoms          Heat of Combustion (kJoules/mole)
-------    -----------      ----------------------------------------

Methanol        1                              726
Ethanol         2                             1367
Propanol        3                             2021
Butanol         4                             2676

The Oxygen atom in Alcohols is present with a SINGLE BOND to the Carbon atom (not a double bond mentioned in your msg). This means Alcohols are already "partially oxidized" and will have lower heats of combustion than their corresponding alkanes. Of course, some heat energy is absorbed in breaking Oxygen bonds during Alcohol combustion. However, the Heats of Formation for all the CO₂ and H₂O produced are much greater in magnitude than the absorbed energy related to Oxygen bonds, resulting in a net release of heat energy.

For your information, following are some approx bond energies (in kJoules/mole):
C-H ... 400
O-H ... 450
C-C ... 300
C-O ... 300
C=O ... 800
C=C ... 600

Refs:
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/B/BondEnergy.html
http://64.233.161.104/search?q=cache:ZX29iPCyYI0J:www.creative-chemistry.org.uk/gcse/documents/Module7/N-m07-24.pdf+alcohol+combustion+heat&hl=en&ie=UTF-8
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[sueanne]

analysis of chem coursework

okay, i get it now. i have re-edited my analysis, so is this a better version?
does it have enough "scientific knowledge and understanding"?

--Apart from propanol, the graph shows the increase of energy output is proportional to the increase of RMM for alcohol. According to the line of best fit, butanol produced the highest amount of energy output. However, without looking at the line of best fit, it states that propanol produces the highest amount of energy output, and this does not support my findings. This means that butanol and propanol are anomalies. I will discuss this in the evaluation section.

With reference from the above calculations, it was shown that the above analysis agrees to the fact that the energy released followed the order that methanol  ethanol  propanol  butanol. When comparing my predicted results to the actual results from the experiment I have noticed that my results form the experiment were much lower, this could be because of the huge amount of heat energy released into the air. Alcohols are partially and this contributes to the lower heat energy. This leaves the oxygen molecules with the highest bond energy of 498 kJ. I have learned at school that oxygen is double bonded; it takes up a lot of energy to break these bonds. Perhaps not all of the oxygen bonds were broken and also it takes up a lot of heat energy to break these bonds first.

The general trend from the graph is that as the number of carbon atoms increases, so does the amount of heat combustion because according to my research part of my prediction, the equation stated that heat combustion is produced from the formation of H2O and CO2 so the number of carbon atoms should increase.

Alcohol Carbon Atoms Heat of Combustion(values taken from the experiment)
Methanol 1 212.4 kJ / moles
Ethanol 2 322.5 kJ / moles
Propanol 3 582.0 kJ / moles
Butanol 4 636.6 kJ / moles

The gradient of the line is quite steep. There are plenty of spaces in between the points. This must mean that more alcohols can be experimented for to produce a more accurate line of best fit--

 

[xanthym]

.

Your report is somewhat unclear. At first, you state the results do not show the proper order of increasing heats of combustion for the measured Alcohols. However, you then indicate that the Best-Fit Line is OK, and the data you present near the last paragraph seems OK also. Perhaps you should present the actual results {Alcohol versus Actual Measured Combustion Heat (ave for each alcohol)}, and then present the Best-Fit Line equation. Also, you again mention double-bonded Oxygen, which is NOT correct for Alcohols.
(Note: You're certainly correct that significant energy is lost to the air. However, the data should still show the correct order of increasing released energy: methanol, ethanol, propanol, butanol.)


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