STATEMENT: Alcohols are liquid fuels with energy values (ΔHc) that are related to their relative molecular mass (Mr). Plan and design an experiment that would allow you to establish the relationship between Mr and ΔHc value for C2 to C5 straight chain alcohols.
HYPOTHESIS: As Mr increases (number of carbon atoms per molecule) in straight chain alcohols from C2 to C5, the heat of combustion increases (ΔHc decreases).
APPARATUS + MATERIALS: 400cm3 water; 50cm3 each of: ethanol, propanol, butanol, pentanol; thermometer; 50cm3 spirit lamp; 100cm3 measuring cylinder; 250cm3 beaker; tripod stand; wire gauze; electronic scale; matches
METHOD: Fill the spirit lamp with 50cm3 ethanol. Weigh the lamp with contents included. Record this value. Set up the apparatus as shown below, using 100cm3 of water (measure using the measuring cylinder). Record the temperature of the ...view middle of the document...
So heat of combustion of 1 mole of ethanol = db = _e_ J = e100 kJ = _f_ kJ
Since reaction was exothermic, ΔHc is negative. ΔHc = -f kJ mol-1
* Repeat these calculations for propanol, butanol and pentanol.
EXPECTED RESULTS: If ΔHc decreases from ethanol to pentanol (as number of carbon atoms per molecule increases), the hypothesis is supported. Otherwise, it is invalid.
SOURCES OF ERROR:
* Heat is lost to the environment and in heating the beaker, since not all the energy produced is transferred to the water. Hence, the calculated value for the heats of combustion of the alcohols are less than the actual values.
* Some water may have evaporated out of the beaker when heating the alcohol, and thus the mass of 100g of water may not have been maintained throughout the entire heating process.
[ 1 ]. Mass of lamp with contents before heating – mass of lamp with contents after heating
[ 2 ]. Temperature of water after heating – temperature of water before heating
[ 3 ]. Heat was released to the surroundings, and the water increased in temperature.