METHOD
•The heat of combustion of methanol, ethanol, and 1-proponol were measured
•Each of the above alkanols was placed in a spirit burner, and their initial mass was recorded. 200mL of water was poured into a tin can, and the initial temperature was recorded. One spirit burner was placed under the tin can, and then lighted so that the flame touched the bottom of the can. Monitor the temperature of the water, and once it has increased by 10°C, extinguish the flame. The burner was immediately weighted again, and the mass change was recorded. The above procedure was repeated for the other alkanols. The results were then analysed with appropriate calculations to determine the heat of combustion of each alkanol.
•Draw a diagram of the set-up if possible
SAFETY
•Methanol is toxic by all routes of exposure (can cause blindness if ingested) => wear eye and skin protection
RESULTS
•MASS LOSSES
oMethanol: 1.22g
oEthanol: 0.66g
o1-proponol: 0.62g
•HEATS OF COMBUSTION
oMethanol: 220kJmol-1
oEthanol: 584kJmol-1
o1-proponol: 1147kJmol-1
•THEORETICAL HEATS OF COMBUSTION
oMethanol: 726kJmol-1
Ethanol: 1360kJmol-1
o1-proponol: 2020kJmol-1
•Methanol had the lowest heat of combustion, 1-proponol had the highest heat of combustion
oThis is because more bonds are created in the formation of the products (CO2, H2O) of 1-proponol than methanol (remember creating bonds releases energy, breaking bonds absorbs energy)
The combustion equations are:
•The heat of combustion calculations required the specific heat capacity equation
•This gave the change in enthalpy, which was then divided by mass of alkanol and moles of alkanol to give both heat of combustion per gram, and molar heat of combustion
•Remember heat of combustion is POSTIVE, whilst change in enthalpy is NEGATIVE (by convention)
RELIABILITY
•The experiment was not repeated and no average value was obtained
VALIDITY/ACCURACY
•The investigation tested the aim by comparing the heats of combustion
•The burner was measured with an electronic scale (accuracy)
•The temperature could have been measured with a thermal sensor and data logger (improving accuracy)
•A tin can is a better thermal conductor than a glass beaker
•There was not 100% efficiency in heat transfer between flame and can however => a draught shield around the burner could reduce this
•Soot under the tin can for 1-proponol indicated incomplete combustion, reducing the heat emitted
•Heat was lost from the water to the surroundings due to radiation and evaporation => a lid on the can would improve validity
•All of the above problems contributed to a lower measured molar heat of combustion than the theoretical value
