Molar heat of combustion Question (1 Viewer)

[nsbrando]

Q1 Assuming that the calorimeter and the water have the same initial and final temperatures, calculate the heat of combustion of pentan-1-ol.



Q2 Explain why the experimental value for the molar heat of combustion deviates significantly from the literature value (3331kJ/mol).

 

[Amundies]

Answer to Q2:
The hot air flows around the flask and some of the heat is absorbed by the equipment (e.g. tripod, gauze mat, etc). Also, if incomplete combustion occurs then carbon soot forms at the bottom of the flask which acts as an insulator and so less heat is absorbed by the water.

 

[Dan895]

We don't have enough info to work out Q1.

There is a few reasons for the expermental value being lower. E.g. Heat escaping into the surroundings, incomplete combustion of the substance, inaccurate apparatus.

 

[Amundies]

We don't have enough info to work out Q1.

There is a few reasons for the expermental value being lower. E.g. Heat escaping into the surroundings, incomplete combustion of the substance, inaccurate apparatus.

My tutor said that while the writing in bold is true, it's too broad. You need to be more specific like what I said in my first post. Just so you know

 

[Dan895]

My tutor said that while the writing in bold is true, it's too broad. You need to be more specific like what I said in my first post. Just so you know

Yeah, fair enough I really suppose also how many marks the question is. But for the most part I agree.

 

[someth1ng]

Answer to Q2:
The hot air flows around the flask and some of the heat is absorbed by the equipment (e.g. tripod, gauze mat, etc). Also, if incomplete combustion occurs then carbon soot forms at the bottom of the flask which acts as an insulator and so less heat is absorbed by the water.

If it was a 3 marker, you'd get 1.

There is a few reasons for the expermental value being lower. E.g. Heat escaping into the surroundings, incomplete combustion of the substance, inaccurate apparatus.

Same goes for you.

Q2 Explain why the experimental value for the molar heat of combustion deviates significantly from the literature value (3331kJ/mol).

There are many factors that contribute to the significant difference in the experimental value to the literature value.
Factors include:
- Incomplete combustion of fuel, characterised by the production of soot, liberates less energy than complete combustion of fuel and hence, a lower experimental value is found.
- Likewise, incomplete combustion can produce soot on the bottom on the can and act as an insulator, decreasing the efficiency of thermal energy transfer and thus decreasing the experimental value of the molar heat of combustion further - deviating it from the literature value.
- In addition, the inefficient transfer of heat from the flame to the water through the aluminium as some thermal energy is not transferred to the can and some thermal energy from the can is not transferred to the water causing a lower experimental value to be determined.
- Furthermore, the flame from the spirit burner heats the air surrounding the flame causing thermal energy to be lost by convection, further decreasing the experimental value from the literature value.
Evidently, many factors can contribute to the lowered and deviated experimental value from the literature value.

 

[Dan895]

If it was a 3 marker, you'd get 1.



There are many factors that contribute to the significant difference in the experimental value to the literature value.
Factors include:
- Incomplete combustion of fuel, characterised by the production of soot, liberates less energy than complete combustion of fuel and hence, a lower experimental value is found.
- Likewise, incomplete combustion can produce soot on the bottom on the can and act as an insulator, decreasing the efficiency of thermal energy transfer and thus decreasing the experimental value of the molar heat of combustion further - deviating it from the literature value.
- In addition, the inefficient transfer of heat from the flame to the water through the aluminium as some thermal energy is not transferred to the can and some thermal energy from the can is not transferred to the water causing a lower experimental value to be determined.
- Furthermore, the flame from the spirit burner heats the air surrounding the flame causing thermal energy to be lost by convection, further decreasing the experimental value from the literature value.
Evidently, many factors can contribute to the lowered and deviated experimental value from the literature value.

There is no doubt out of the three answers yours is the best, however do you honestly believe that your anwser is the minimum respose required to get 3/3?

 

[someth1ng]

Definitely not the 'minimum' but it should get you 3/3.

Surprisingly, if you don't state that the experiment yielded a LOWER value, you can get penalised quite hard - I believe there was a similar question that penalised 2 marks.

The other problem with the other answers is that it does not follow the cause/effect requirement. You can't simply state "incomplete combustion", you need to say "incomplete combustion causes less heat to be liberated per mole of fuel combusted" or something like that.

Anyway, for 3 marks, I'd say you would need at least TWO of the points that I raised (I did 4) as well as stating that it produced a lower experimental value than the literature value.

 

[Dan895]

Ok thanks.