Enthalpy of combustion of methanol by spirit burner
A student determines the enthalpy of combustion of methanol using a spirit burner. She places 100.0 g of water in a copper can, records the initial temperature as 21.0 C, burns methanol until the temperature reaches 46.0 C, and finds the burner has lost 1.20 g. She uses c(water) = 4.18 J g-1 K-1 and M(methanol) = 32.04 g mol-1. Her calculated value is much less exothermic than the literature value of -726 kJ mol-1. She concludes that methanol is a poor fuel and that her result is accurate because the thermometer uncertainty is only +/-0.5 C. Her teacher points out that the thermometer uncertainty is not the main problem. The student must decide which errors are systematic, which are random, and whether her data can still be used to compare methanol with ethanol.
1. Calculate the enthalpy of combustion of methanol from her data and compare it with the literature value.
Read the worked answer
Heat absorbed by water:
q = m c ΔT = 100.0 x 4.18 x (46.0 - 21.0)
q = 100.0 x 4.18 x 25.0 = 10450 J = 10.45 kJ
Moles of methanol burned:
n = 1.20 / 32.04 = 0.03745 mol
Enthalpy change per mole:
ΔH = -q/n = -10.45 / 0.03745 = -279 kJ mol-1
The experimental value is -279 kJ mol-1, compared with -726 kJ mol-1. The experimental value is less exothermic by:
(726 - 279) / 726 x 100 = 61.6%
So the student's method has underestimated the magnitude of the enthalpy change by about 62%.
2. Identify the main sources of error and classify them as random or systematic; explain why thermometer uncertainty is minor.
Read the worked answer
The main errors are systematic underestimation of the heat released to the water. Heat is lost to the surroundings, the copper can, and the air; some methanol evaporates from the wick without burning; and combustion may be incomplete, producing soot or CO rather than only CO2 and H2O. These errors all make the measured temperature rise smaller than it should be for the mass of methanol burned, so they are systematic. Thermometer uncertainty is random and small: a +/-0.5 C uncertainty on a 25.0 C rise is about 2%, whereas the discrepancy is about 62%. Therefore the thermometer is not the main problem. The student's precision may be reasonable, but her accuracy is poor because of heat loss and incomplete combustion.
3. Recommend two improvements and evaluate whether the student's conclusion that methanol is a poor fuel is valid.
Read the worked answer
Two improvements are: use a bomb calorimeter to reduce heat loss and ensure complete combustion under excess oxygen; and use a draught shield or lid, place the burner closer to the can, and weigh the burner before and after to determine the mass burned more reliably. Repeating the experiment and averaging would improve reliability, though it would not remove systematic heat loss. The conclusion that methanol is a poor fuel is not valid from this experiment alone. The method underestimates the enthalpy release for any alcohol because heat is lost. Methanol's literature enthalpy of combustion is -726 kJ mol-1, which is substantial. The data could still be used to rank methanol against ethanol if the same flawed apparatus and procedure were used for both, because the systematic error would be similar, but the absolute values would remain unreliable.