Chemistry 301. Fundamentals; 1. I initially forgot it when I solved this problem prior to formatting it for the web site. Learning Strategies The following formula can be used. The formulation of Hess' Law just above is the one usually used in textbooks. And the bond between hydrogen and chloride is 427 kJ/mol. When we were growing up and we were playing with our own set of building blocks, we would piece things together and break them apart, depending on what we wanted to make. Enter the change in internal energy, the change in volume, and the change in pressure of a reaction to calculate the total change in Enthalpy. You may wish to review the laws of thermochemistry and endothermic and exothermic reactions before you begin. Water (H2O) is one of many everyday examples of compounds with covalent bonds. The pressure of this reaction mus be constant for the formula to work. So, the enthalpy of reaction can be calculated by subtracting the energy cost (breaking bonds) from the energy released (making bonds). Important point: the above reaction breaks four Si−F bonds.