Question

Calculate the standard enthalpy change for the formation of ethylene glycol (C2H6O2) from its elements given that the standard enthalpies of formation of carbon dioxide, water, and ethylene glycol are -393.5 kJ/mol, -285.8 kJ/mol, and -421.1 kJ/mol respectively.

Answer 1

To calculate the standard enthalpy change for the formation of ethylene glycol  C2H6O2  from its elements, we need to consider the balanced chemical equation for the formation reaction:2 C  s  + 3 H2  g  + O2  g   C2H6O2  l Now, we can use Hess's Law, which states that the enthalpy change for a reaction is the same whether it occurs in one step or several steps. We can break down the formation of ethylene glycol into two steps:Step 1: Formation of CO2 and H2O from their elementsC  s  + O2  g   CO2  g  Hf = -393.5 kJ/mol2 H2  g  + O2  g   2 H2O  l  Hf = 2 *  -285.8 kJ/mol  = -571.6 kJ/molStep 2: Formation of ethylene glycol from CO2 and H2OCO2  g  + 2 H2O  l   C2H6O2  l  Hf = -421.1 kJ/molNow, we can use Hess's Law to find the enthalpy change for the formation of ethylene glycol from its elements:Hf  C2H6O2  = Hf  CO2  + Hf  2 H2O  - Hf  C2H6O2 Hf  C2H6O2  =  -393.5 kJ/mol  +  -571.6 kJ/mol  -  -421.1 kJ/mol Hf  C2H6O2  = -543.0 kJ/molThe standard enthalpy change for the formation of ethylene glycol from its elements is -543.0 kJ/mol.