Comment Both propane and benzene are hydrocarbons. As a rule, the energy obtained from the combustion of a gram of hydrocar-
bon is between 40 and 50 kJ.
Practice Exercise 1
Calculate the enthalpy change for the reaction
2 H2O2(l)-→ 2 H2O(l) + O2(g)
using enthalpies of formation:
Change in enthalpy(f)H2O2(l)= -187.8 kJ/mol
Change in enthalpy (f) [H2O(l)= -285.8 kJ/mol

The enthalpy change : -196.2 kJ/mol

The change in enthalpy in the formation of 1 mole of the elements is called enthalpy of formation  

The enthalpy of formation measured in standard conditions (25 ° C, 1 atm) is called the standard enthalpy of formation (ΔHf °)  

(ΔH) can be positive (endothermic = requires heat) or negative (exothermic = releasing heat)  

The value of ° H ° can be calculated from the change in enthalpy of standard formation:  

∆H ° rxn = ∑n ∆Hf ° (product) - ∑n ∆Hf ° (reactants)  

Reaction

2 H₂O₂(l)-→ 2 H₂O(l) + O₂(g)

∆H ° rxn = 2. ∆Hf ° H₂O - 2. ∆Hf °H₂O₂

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