Heats of Reaction: Summary

The answers to the ConcepTests are given below and will open in a separate window. 
Key points from this module:
  • Formation reaction: reaction to form a compound from its elements (in form normally present in nature).
  • Heats or reaction can be determined at 298 K from tabulated values of heats of formation.
  • Heats of reaction depend weakly on temperature for many reactions.
  • Heats of formation of elements are zero.
  • Heat of combustion: heat of reaction of a substance with O2 to usually yield CO2 and H2O.
  • The value of a heat of reaction depends on the phases of reactants and products.
  • ΔH > 0 for an endothermic reaction.
  • ΔH < 0 for an exothermic reaction.
  • Heats of reaction can often be calculated from heats of combustion.
From studying this module, you should now be able to:
  • Calculate a heat of reaction at standard conditions from heats of formation.
  • Given heat of reaction at 298 K, use temperature-dependent heat capacities to calculate heat of reaction at other temperatures.
  • Use Hess’s law to calculate a heat of reaction.
  • Use heats of combustion to calculate a heat of reaction. 

Prepared by John L. Falconer, Department of Chemical and Biological Engineering, University of Colorado Boulder