Question

Calculate the enthalpy change for the isomerization of butene-1 to butene-2 given that the heat of combustion of butene-1 and butene-2 are -2874 kJ/mol and -2854 kJ/mol respectively. Assume all reactants and products are in the gaseous state and that the reaction is carried out at constant pressure.

Answer 1

To calculate the enthalpy change for the isomerization of butene-1 to butene-2, we can use Hess's Law. Hess's Law states that the enthalpy change of a reaction is the same whether it occurs in one step or several steps. In this case, we can use the heat of combustion values for butene-1 and butene-2 to determine the enthalpy change for the isomerization reaction.First, let's write the combustion reactions for butene-1 and butene-2:Butene-1:C4H8 g  + 6O2 g   4CO2 g  + 4H2O l ; H1 = -2874 kJ/molButene-2:C4H8 g  + 6O2 g   4CO2 g  + 4H2O l ; H2 = -2854 kJ/molNow, we want to find the enthalpy change for the isomerization reaction:Butene-1 g   Butene-2 g ; H = ?To do this, we can use the combustion reactions to create a reaction pathway:1. Butene-1 g  + 6O2 g   4CO2 g  + 4H2O l ; H1 = -2874 kJ/mol2. 4CO2 g  + 4H2O l   Butene-2 g  + 6O2 g ; H3 = -H2 = 2854 kJ/molNow, we can add the two reactions together to get the isomerization reaction:Butene-1 g   Butene-2 g ; H = H1 + H3H = -2874 kJ/mol + 2854 kJ/mol = -20 kJ/molSo, the enthalpy change for the isomerization of butene-1 to butene-2 is -20 kJ/mol.