ΔH°rxn for this combustion ΔH°f of C₁₈H₃₆O₂ = -948 KJ/mol is -7612.5 kJ.
Octadecanoic acid, commonly known as stearic acid, is one of the most prevalent long-chain fatty acids and is present in combination form in natural animal and vegetable fats. Commercial "stearic acid" is a blend of stearic and palmitic acids in roughly equal quantities, with trace amounts of oleic acid.
Equation of the reaction:
C₁₈H₃₆O₂ + 26O₂(g) --> 18H₂O(g) + 18CO₂(g)
ΔHrxn = ΔHp - ΔHr
ΔH (combustion) = [18*(ΔHf of CO2) + 18*(ΔHf of H2O)] - [1*(ΔHf of C18H36O2) + 26*(ΔHf of O2)]
= [18 × (-394) + 18 × (-242)] - [1 × (-948) + 26 × (0)]
= [(-7092) + (-4356)] - (-948)]
= -10500 kJ/mol
Molar mass:
C18H36O2;
= (18×12) + (36×1) + (2×16)
= 284 g/mol
O2:
= (2 × 16)
= 32 g/mol
Number of moles = mass/molar mass
C18H36O2:
= 206/284
= 0.725 mol.
O2:
= 943.2/32
= 29.475 mol
Finding the limiting reagent,
By stoichiometry, 1 mole of steric acid combusted in 26 moles of oxygen.
Therefore,
= 29.475 mol of O2 * 1 mol of stearic acid/26 mol of O2
= 1.134 mol of steric acid(only 0.725 mol is present)
Stearic acid is the limiting reagent.
q = n × ΔHrxn
= 0.725 * -10500
= -7612.5 kJ.
To know more about Stearic acid refer to: brainly.com/question/13200535
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