Question

When 10 g C₂H₅OH_(I) are formed, 51 kJ heat is liberated. Calculate standard enthalpy of formation of C₂H₅OH_(I).

Answer 1

Given : 

Mass of C₂H₅OH_(I) = m = 10 g

Heat liberated = ΔH₁ = -51 kJ

Molar mass of C₂H₅OH = 46 gmol^-1

Standard enthalpy of formation of C₂H₅OH_(I)

= Δ_fH = ?

Standard enthalpy of formation is the enthalpy change for the formation of 1 mole C₂H₅OH_(I) 

i.e., 46 g C₂H₅OH_(I).

Now,

∵ For the formation of 10 g C₂H₅OH_(I)

ΔH₁ = -51 kJ

∴ For the formation of 46 g C₂H₅OH,

Δ_fH⁰ = \(\frac{-51\times 46}{10}\) = – 234.6 kJ mol^-1

∴ Standard enthalpy of formation of C₂H₅OH

= Δ_fH⁰ = – 234.6 kJ mol^-1