Molar Enthelpy question with Combustion

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Discussion Overview

The discussion revolves around calculating the molar enthalpy of combustion for 3-pentanol based on experimental data. Participants analyze the calculations provided, considering the implications of temperature changes in both the water and the remaining alcohol.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • The initial calculation of molar enthalpy using the formula Q=mcΔT is presented, with specific values for mass, temperature change, and specific heat capacity.
  • One participant questions whether the remaining alcohol also experienced a temperature increase of 21°C, suggesting this could affect the final enthalpy calculation.
  • Another participant agrees that the remaining alcohol likely increased in temperature and suggests that this should be accounted for, although they note that the effect may be small enough to neglect.
  • There is uncertainty about whether the initial calculation of -823 kJ/mol is correct without an answer key for verification.

Areas of Agreement / Disagreement

Participants generally agree on the method of calculation but express differing views on the significance of the temperature change in the remaining alcohol, indicating an unresolved aspect of the discussion.

Contextual Notes

There are assumptions regarding the heat capacity of the remaining alcohol and its temperature change, which may affect the accuracy of the molar enthalpy calculation. The discussion does not resolve these assumptions.

Who May Find This Useful

Students working on thermochemistry problems, particularly those involving combustion and enthalpy calculations, may find this discussion relevant.

staka
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Homework Statement


Using the data for the combustion of 3-pentanol calculate the molar enthalpy for the alcohol.

Initial mass of alcohol: 27.35g
Final mass of alcohol: 26.41g
Change in temperature: 21.0C
Volume of water: 100.0mL (100.0g)
Specific heat capacity of water: 4.18 J/gC

Homework Equations


Q=mcΔT

The Attempt at a Solution


3-pentanol: C5H12O (88.14818 g/mol)
27.35g-26.41g=0.94g
0.94g/88.14818g/mol=0.010663861mol

Q=mcΔT
=(100)(4.18)(21.0)
=8778J
ΔH=-8778J
ΔH/n=-8778J/0.0106638161mol
=-823154.0152J/mol
=-8.23x10^2kJ/mol ←is this the correct answer? (I don't have an answer key.)
 
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It looks good. The only complicating factor is to determine if the 3-pentanol underwent an increase in temperature as well. You used about a gram of the alcohol and about 26 grams are left. Did that remaining alcohol undergo an increase in temperature of 21oK?
 
I believe the remaining alcohol increased 21 celsius.
I just want to know if I determined the molar enthalpy of alcohol in the end.
 
staka said:
I believe the remaining alcohol increased 21 celsius.

If that is the case you will need to revise (very slightly) your answer to account for the heat required to heat 26.41 g of the alcohol through 21o. It's probably small enough to neglect.
 

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