# Hess's Law - Lompoc Unified School District Hesss Law Definition of Hesss Law If a reaction is carried out in a series of steps, H is the sum of all the enthalpy H is the sum of all the enthalpy changes

CH4(g) + O2(g) CO2(g) + 2H2O(g) H is the sum of all the enthalpy H=-802 2H2O(g) 2H2O(l) H is the sum of all the enthalpy H=-88 -----------------------------------------------------------CH4(g) + O2(g) CO2(g) + H2O(l) H is the sum of all the enthalpy H=-890 Example

The heat of combustion of C to CO2 is 393.5 kJ/mole and the heat of combustion of CO to CO2 is 283.0 kJ/ mole. Calculate the heat of combustion of C to CO. Example 2 Carbon

occurs in two forms graphite and diamond. The heats of combustion are shown: C(graphite) + O2 CO2 H is the sum of all the enthalpy H = -393.5kJ C(diamond) + O2 CO2 H is the sum of all the enthalpy H =-395.4kJ Calculate the H is the sum of all the enthalpy H for the conversion from graphite to diamond. Example 3 Calculate

the H is the sum of all the enthalpy H for 2C(s) + H2(g) C2H2(g) given that C2H2(g) + 2.5O2 2CO2 + H2O(l) H is the sum of all the enthalpy H=-1299.6 C + O2 CO2 H2(g) + .5O2(g) H2O(l) H is the sum of all the enthalpy H=-393.5 H is the sum of all the enthalpy H=-285.9

Whats It Mean Practically Can never obtain more or less energy form a chemical reaction by changing the method or number of steps in the reaction