Molar Heat of Combustion
To find the molar heat of combustion for four different alkanols: 1.
- And to compare the experimental value with the theoretical. Background:
The Molar Heat of Combustion of a substance is the heat liberated when 1 mole of the substance undergoes complete combustion with oxygen at standard atmospheric pressure, with the final products being carbon dioxide gas and liquid water. (Ref. “Conquering Chemistry, Roland Smith, 2005”) The Heat Capacity of a substance is the amount of heat energy it must consume in order to raise its temperature by 1 Kelvin or 1° Celsius. The heat capacity of 1 mol of a pure substance is known as its molar heat capacity, which can be expressed in J K-1 g-1. The heat capacity of 1 gram of a substance is known as its specific heat, which can also be expressed in J g-1 K-1.
Specific Heat Capacities of:
Water – 4.18 J g-1 K-1
Copper – 0.385 J g-1 K-1
The equation below relates the specific heat of a substance, the temperature change, and the mass of the substance and how much energy was put into the system. q = mCΔT
q = quantity of heat (joules)
m = Mass (grams)
C = specific heat capacity J g-1 K-1
ΔT = change in temperature (final – initial) (K or °C)
It is hypothesised that as the number of carbon chains in each alkanol increases, the energy required to raise the temperature of the water will also increase.
Type of alkanol used; Methanol, Ethanol, 1-Propanol, 1-Butanol. •
Mass of spirit burner.
Maximum Temperature of water reached. (This is the maximum temperature in which the water reaches after it has risen 20°C and the spirit burner put out). o
Energy released per mole of the fuel burnt.
Change in temperature of water; 20°C rise in temperature.
Boss head x 2
Clamp x 2
Methanol Spirit Burner
Ethanol Spirit Burner
1-Propanol Spirit Burner
1-Butanol Spirit Burner
Placed on a pair of safety goggles.
Drew up 4 different tables, one for each alkanol, for results to be recorded. Included in each table of results were: three trials of the experiment, with each trial recording -
The mass of the can,
Mass of can plus water,
Mass of spirit burner at the start, the end and the difference between, -
The temperature of the water at the start, the end and the difference between. 3.
Recorded mass of empty copper can.
Set up experiment as shown below:
Using a measuring cylinder, added 100ml of water to copper can, weighed the can with water using an electronic scale, and then recorded the mass in table of results.
Hung the copper can from the clamp so that there was no more than 3cm gap between the can and the wick of spirit burner.
Recorded the mass of spirit burner (methanol), with the cap on.
Recorded temperature of water. (The bulb of the thermometer was positioned in the centre of the volume of water, and held in place with a clamp.)
Lit the spirit burner (methanol), keeping the flame in the centre of the base of the copper can.
Allowed the temperature of the water to rise 20°C before extinguishing the flame with the cap.
The maximum temperature reached was then recorded (after flame was extinguished), as well as the difference recorded.
Re-weighed the spirit burner (methanol) with the cap on, recorded the mass and difference in initial and final mass, which is the amount of fuel combusted in grams.
The copper can was then rinsed with water and dried with paper towel (to cool back down and to keep the empty mass of the can at a constant.)
Steps 5 – 13 were repeated (using fresh water each time), with 3 trials for each alkanol.
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