Molar Heat of Combustion

Aim:
To find the molar heat of combustion for four different alkanols:
1. Methanol
2. Ethanol
3. 1-Propanol
4. 1-Butanol
- 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 …show more content…

78.2 178.4 258.4 258.3 0.5 19 41.5 22.5

Data Analysis:
Combined Specific Heat Capacity of water and copper = 4.18 + 0.385 = 4.565 J g-1 K-1

Methanol (molar mass = 32.04 g mol-1)

o Number of moles of fuel used = Mass of methanol burnt ÷ Molar mass of fuel

(i) n = 1.0 ÷ 32.04 (ii) n = 1.3 ÷ 32.04 = 0.0312 mol = 0.0405 mol (ii) n = 1.0 ÷ 32.04 = 0.0312 mol

Average = 2(0.0312) + 0.0405 = 0.1029 ÷ 3 = 0.0343 mol

o Calculate energy required to change temperature of water using:

ΔH = -mCΔt (i) ΔH = -178.3 x 4.565 x 23.5
= -19127.58 J
= -19.1276 KJ
(ii) ΔH = -178.4 x 4.565 x 22
= -17916.72 J
= -17.1967 KJ (iii) ΔH = -178.3 x 4.565 x 22.5
= -18313.64 J
= -18.3136 KJ

Average = -19.1276 + -17.1967 + -18.3136 = -55.3579 ÷ 3 = -18.4526 KJ

o Molar heat of combustion (M.H.C)

M.H.C = -18.4526 ÷ 0.0343
= -537.97 KJ mol-1

Ethanol (molar mass = 46.068 g mol-1)

o Number of moles of fuel used = Mass of methanol burnt ÷ Molar mass of fuel

(i) n = 0.8 ÷ 46.068 = 0.0174 mol

(ii) n = 0.7 ÷ 46.068 = 0.0152 mol (iii) n = 0.7 ÷ 46.068 Average = 0.0174 + 2(0.0152) = 0.0152 mol = 0.0478 ÷ 3 = 0.0159 mol
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