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IB CHEMISTRY HL

Relationship between the enthalpy

UPDATED ON - 28 OCT 2019

Relationship between the enthalpy of combustion and the number of carbons in simple alcohols belonging to the same homologous series 

 

Table of content:

 

The research question

My hypothesis

Analysis

Final Reflection

 

Word count excluding bibliography, charts, and graphs: 1126 

The research question

Alcohols are used as fuels for different applications ranging from cooking to powering cars. They have become increasingly important as components of biofuels. Even when ethanol is the only industrially relevant alcohol used in this sense, recent findings suggest that higher alcohols may prove more efficient and less corrosive (1). The study refers to branched alcohols, but I will use straight-chain alcohols to simplify the analysis.  This investigation would provide a preliminary step for later studies. It looks to establish a connection between the energy released when they combust and their structure. I will specifically focus on the impact that the number of carbon atoms in the chain of a homologous series of alcohols has on the enthalpy change of combustion.

 

Comment: The student has concisely outlined the topic and the research question.However, is the enthalpy of combustion the key property to study whenever lusting possible fuel efficiency? This is a common oversimplification. 

 

Comment: This is not an especially original research question. The student does outline a secondary question of possibly more interest below. 

 

My hypothesis

The first component of my hypothesis is that as the numbers of carbon atoms increase so will the enthalpy of combustion.  Enthalpy changes result from the difference in the amount of energy required for breaking bonds and the amount released when new bonds are formed. If the bonds formed are stronger than those broken the energy released will be larger than the energy invested and this energy will flow to the environment.   

When alcohols react with oxygen in air they form carbon dioxide and water. The bonds between C and O are double and therefore stronger than those between carbons which are simple. The more carbons the alcohol has the more C=O bonds will be formed-actually for each C two of such bonds are formed- and this leads me to say that the enthalpy change should be directly increasing. 

The second aspect of this investigation is also focused on bond aspects and addresses the differences existing between enthalpy changes experimentally determined and those calculated with Bond energies.

 

Comment: This could have been the sole research question.

 

These last values are average values over several compounds with similar Structure. In the case of alcohol, the amount of energy needed to break the bond between C-O when the C is only bonded to Hydrogens-the case of methanol- should be different than when it is bonded to other carbons.   

The second component of my hypothesis is that the value will be much lower in the case of methanol as the methyl group has a larger inductive effect than other alkyl groups (2). I also think that the difference between the others will be minimal because the C bonded with the O will be bonded to one more C  whose electronegativity is the same-therefore no inductive effect- and just two H providing some inductive effect. 

Finding the enthalpy changes using bond energies

In order to validate my hypothesis, I will be using the values of bond energies obtained from a database instead of practicing the actual experiment.  The school only has three alcohols and one of them is branched, therefore it would not be possible to establish a trend. For this purpose, I will use the RSC data bank and spreadsheets. 

The following are the balanced chemical equations: 

CH3OH(l) +  1. 5 O2 (g) → CO2 (g) +2 H2(l)

C2H5 OH (l) + 3 O2 (g) → 2 CO2 (g) + 3 H2(l)

C3H7OH (l) + 4.5 O2 (g) → 3 CO2 (g) + 4 H2(l)

C4H9OH (l) +  6 O2 (g) →  4 CO2 (g) + 5 H2(l) 

Table 1. Spreadsheet used for calculating Enthalpy changes of combustion based on BE*

Met Hanol  Bond energy (kJ/ mol) Overall energy (kJ/mol)  Overall energy (kJ/mol) ChangeHcombustion (kJ/mol)
H- O  464 464     
average C- C  347  
average C- H  413 1239   
average C- O  358 358   
C=O in CO2  805 1610  
O=O  498.3 748  0  
bonds are broken 
and
formed(KJ/mol)
 

 

2809 

 

3466 

 

-657

         
Et ethanol  Bond energy (kJ/ mol) Overall energy(kJ/mol)  Overall energy  (kJ/mol)  ChangeHcombustion (kJ/mol)
H- O  464 464  2784  
average C- C  347 347  0  
average C- H  413 2065  0  
average C- O  358 358  0  
C=O in CO2  805 3220  
O=O  498 1494  0  
bonds broken
and
formed(kj/mol)

 
 

 

4728 

 

6004 

 

-1276

         
Propan- 1- ol  Bond energy (kJ/ mol) Overall energy(kJ/mol)  Overall energy (kJ/mol)  ChangeHcombustion (kJ/mol)
H- O  464 464  3712  
average C- C  347 694  0  
average C- H  413 2891  0  
average C- O  358 358  0  
C=O in CO2  805 4830  
O=O  498 2241  0  
bonds broken
and
formed(kj/mol)

 
 

 

6648 

 

8542 

 

-1894

         
But an- 1 - ol  Bond energy (kJ/ mol) Overall energy (kJ/mol)  Overall energy  (kJ/mol)  ChangeHcombustion (kJ/mol)
H- O  464 464  4640  
average C- C  347 1041  0  
average C- H  413 3717  0  
average C- O  358 358  0  
C=O in CO2  805 6440  
O=O  498 2988  0  

bonds broken
and
formed(kj/mol)

 

 

8568 

 

11080 

 

-2512

         
Pent an- 1 - ol  Bond energy (kJ/ mol) Overall energy(kJ/mol)  Overall energy  (kJ/mol)  ChangeHcombustion (kJ/mol)
H- O  464 464  5568  
average C- C  347 1388  0  
average C- H  413 4543  0  
average C- O  358 358  0  
C=O in CO2  805 8050  
O=O  498 3735  0  
bonds broken
and
formed(kj/mol)

 
 

 

10488 

 

13618 

 

-3130

 

 

Comment: The column headings titled “Overall Energy” are ambiguous. Should specify bonds in reactants and bonds in products. 

Comment: Final enthalpies cited to more significant figures than the bond energy input data. 

 

*The bond energies (BE) were obtained from the RSC database and exported to a spreadsheet to make the calculations (3) 

 

Comparing experimental theoretical values with those calculated using bond energies

The bond energies are average values obtained from several similar compounds. As a difference to
the theoretical experimental values, they do not specifically consider variations resulting from
changes in the chemical environment surrounding specific bonds. 

Table 2. Comparing theoretical experimental values with values calculated using BE 

Alcohol  Exp values (kJ/mol)  Using BE (kJ/mol) % Difference
Methanol   -726  -667  8.85
Ethanol   -1300  -1276  1.88
Prop-1-ol  -2020  -1894  6.65 
Butan-1-ol  -2670*  -2512  6.29
Pentan-1-ol  -3329**  -3130  6.36

*Reference (4)  **Reference (5) 

Graph 1. The heat of combustion released per mol in terms of number of Catoms in the chain 

 

Comment: The two graphs could have been merged into one for better comparative effect.


 Graph 2. The heat of combustion released per mol in terms of number of Catoms in the chain 

 

Analysis

 

Both the theoretical experimental results and those resulting from calculations based on BE showing a positive linear relationship that validates my hypothesis.   

All the values resulting from using bond energies (BE) are lower than the experimental ones found in references. It is worth mentioning that the RSC Database provides the H-O value corresponding to water which should be different from that in a hydroxyl as the chemical widely varies. 

It is interesting that the experimental value of ethanol -while still on the line- appears as lower than the other experimental values. This is not observed in the values based on BE where the five energy values are perfectly aligned. Ethanol´s value is clearly lower than that of methanol and slightly lower than the other higher alcohols. 

Therefore there seems to be some structural difference between ethanol and the rest, with a more marked variation with the first member of the homologous series.I tend to believe that this may result from the significantly lower inductive effect that the ethyl group has on the C-O bond when compared with the methyl group.

 

Comment: Trying to explain findings in terms of a relevant theory which is good.

 

If the inductive effect is lower the bond is less polar, resulting in an increased covalent character and therefore a stronger bond.  As the bond is stronger more energy is needed to break it, and the enthalpy change would, therefore, be smaller.
The inductive effect is not changed by adding CH2 in the higher alcohols but still, there must be some, as they are slightly lower than methanol (but perfectly aligned with each other). Still, another possibility is that differences result from experimental errors which references do not report. 
Results may suggest that the difference in the bond O-H could be affecting alcohols to a different degree. More data are needed to clarify why the second CH2 affects the C-O bond in ethanol but not in the rest providing a satisfactory explanation for this anomaly.   

Values established for BE correspond to gaseous states of the reactants and products. The experimental values though address liquid states for the alcohols and water.

 

Comment: Valid consideration.

 

Thus, the previous analysis is limited as it has not taken into account the heat of condensation. 

 

Final reflections

 

I may finally conclude that my hypothesis has been validated both by experimental values found in cited resources and those calculated using bond energies. The investigation has evidence that there is a positive linear relationship between the ΔH of combustion and the number of C atoms in  A homologous series of simple alcohols. It has also shown that results based on bond energies are Lower than those experimentally obtained underlining the relevance of chemical environments in The energy needed to break specific bonds even when extremely similar. An unexpected small The anomaly was found in the experimental value of ethanol which is not shown in the trend based on Bond energies, reinforcing the limitations that average values may impose on accurate descriptions.

 

Bibliography


(1) http://www.nature.com/nature/journal/v451/n7174/abs/nature06450.html Accessed
February 2012
(2) http://www.transtutors.com/chemistry-homework-help/general-organicchemistry/
inductive-effect.aspx Accessed February 2012
(3) http://www.rsc.org/education/teachers/resources/databook/ Accessed February 2012
(4) http://en.wikipedia.org/wiki/Heat_of_combustion Accessed February 2012
(5) http://en.wikipedia.org/wiki/N-Butanol Accessed February 2012
(6) http://www.docbrown.info/page07/delta1Hd.htm Accessed February 2012

 

 

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