Enthalpy Change Example Problem

• Problem
  
  Hydrogen peroxide decomposes according to the following thermochemical reaction:
  
  H₂O₂(l) → H₂O(l) + 1/2 O₂(g); ΔH = -98.2 kJ
  
  Calculate the change in enthalpy, ΔH, when 1.00 g of hydrogen peroxide decomposes.
  
  
• Solution
  
  The thermochemical equation tells us that ΔH for the decomposition of 1 mole of H₂O₂ is -98.2 kJ, so this relationship can be used as a conversion factor. Using the Periodic Table, the molecular mass of H₂O₂ is 34.0, which means that 1 mol H₂O₂ = 34.0 g H₂O₂.
  
  Using these values:
  
  ΔH = 1.00 g H₂O₂ x 1 mol H₂O₂ / 34.0 g H₂O₂ x -98.2 kJ / 1 mol H₂O₂
  
  ΔH = -2.89 kJ
  
  
• Answer
  
  The change in enthalpy, ΔH, when 1.00 g of hydrogen peroxide decomposes = -2.89 kJ