Molar Heat combustion chemistry - investigate the effect of molar mass on the molar heat of combustion of adjacent members of a homologous alcohol series.

Observations:

Table 2. Qualitative Data

Processing Raw Data:

Sample calculation: 1. Finding the  in mass

2. Finding the  in temperature

3. Finding the heat of combustion

4. Finding molar mass

5. Finding the number of moles

6. Finding the molar heat of combustion

7. Finding the average molar heat of combustion for each fuel

8. Finding the uncertainties of the average

Sample calculation 1. Finding the  mass:

To find the  mass, use the following equation:

E.g. (values from methanol trial 1):

Calculating uncertainties:

Table 3.  mass

Sample calculation 2. Finding the  temperature:

E.g. (values from methanol trial 1):

Calculating uncertainties:

Table 4. temperature

Sample Calculations 3. Finding the heat of combustion

To find heat of combustion, use the following equation:

1. Convert all masses to kg
2. Use the equation,
3. Use error propagation to calculate uncertainties (adding the percentage errors)
4. Change back to absolute errors and round to 1 significant figure

E.g. (values from methanol trial 1):

 

)

Table 5. Heat of combustion

Sample Calculations 4. Finding molar mass

Table 6. Molar mass

Sample Calculations 5. Finding the number of moles

E.g. (values from methanol trial 1):

Table 7. Number of moles:

Sample Calculations 6. Finding the molar heat of combustion

E.g. (from methanol trial 1)

Table 8. Molar heat of combustion:

Sample Calculation 6: Finding average molar heat of combustion:

E.g. (values from methanol):

Table 9. Average molar heat of combustion:

Processed Data:

Table 10. Processed Data Table

Graph:

Graph 1. Molar mass vs. Heat of Combustion (average trend):

Graph 2. Molar Mass vs. Molar Heat of Combustion (max and min trend line):

Graph 3. Accepted Values for Molar Heat of Combustion

Table 11. Percentage difference of experimental values from experimented values:

Conclusion and Evaluation:

The aim of this experiment is to investigate the effect of molar mass on the molar heat of combustion of adjacent members of a homologous alcohol series, with the first 5 being the adjacent members. Methanol, ethanol, 1-propanol, 1-butanol and 1-pentanol, which follows the rule CnHn+1OH, where n > 1. It is evident from graphs 1 and 2 that as the molar mass increases, the molar heat of combustion increases. This can be explained that as the series number ‘n’ increases, the carbon chain length also increases. Therefore, the number of bonds and the molar mass must also increase. During the combustion reaction, bonds are being broken and energy is being released. The more bonds that are being broken, the more energy is being realised. This explains why the molar heat of combustion increases as the molar mass increases. The linear relationship could also be explained that the bonds in this series are only single bonds and does not create double or triple bonds as the series number increases. This would have created an exponential relationship as the mass increases.

Table 11 shows the percentage difference from the experimented values compared with the accepted values of the molar heat of combustion. The percentage varies from approximately 95% to 195%. This means for example, the value of methanol must increase by approximately 132% to reach the accepted value of molar heat of combustion. The ranges of percentage difference is very large, which tells us that there are many inaccuracies with the experiment. Despite this fact, the trend of the experimental results were still found to be linear when graphed with molar mass vs. the molar heat of combustion as the errors remained consistent. The graph of the accepted values support that it is a linear trend.

The rules from error propagation was used to find the uncertainties. This is a good method to finding the uncertainties as it accounts for every possible error when adding, subtracting, multiplying and dividing. The errors of values must increase as they become manipulated. Error propagation accounts for all of these errors.

Limitations and Improvements: