The definition of enthalpy, H<\/em>, is<\/p>\n H=Eint<\/sub>+PV<\/em><\/p>\n where Eint<\/sub><\/em> is the internal energy content of the product, while PV<\/em> is work done by volume increasing against pressure (relating to gases) as the reaction proceeds. Note that enthalpy is called by the variable H<\/em> rather than E<\/em>.<\/p>\n What does enthalpy mean, conceptually? For the concept to be useful, I think the internal energy content is the important part to focus on. After the reaction that forms a given substance is over, the PV<\/em> part vanishes. The substance’s internal energy content is then what might distinguish it from another substance in terms of enthalpy.<\/p>\n Ethane (C2<\/sub>H6<\/sub>), which is found in natural gas, has a standard enthalpy of -84.68 kJ\/mol. Oxygen gas, O2<\/sub>, has a standard enthalpy of 0. CO2<\/sub> has a standard enthalpy of -393.5 kJ\/mol, while H2<\/sub>Ogas<\/sub> has -241.8 kJ\/mol. The precise numbers aren’t important. The point to realize is that when ethane burns thus:<\/p>\n 2C2<\/sub>H6<\/sub>+702<\/sub>\u21924CO2<\/sub>+6H2<\/sub>0<\/p>\n you can perceive the reaction as a change in enthalpy like so:<\/p>\n Notice that, from left to right, the enthalpy values decrease.<\/p>\n When you’re near a natural gas heater that’s burning, the enthalpy being lost is released to the environment: it’s the heat you feel. What’s actually being lost is internal energy from the fuel’s bonds as they are broken and replaced with lower energy ones. A reaction that has negative enthalpy change, aka, -\u0394H, releases heat to the environment. A reaction with +\u0394H draws in heat from the environment. In that case, the heat is used to form higher energy bonds than the original molecules had; the products’ internal energy goes up.<\/p>\n HTH:)<\/p>\n Sources:<\/p>\n www.energy.alberta.ca\/naturalgas<\/a><\/p>\n Mortimer, Charles E. Chemistry<\/em>, sixth Ed. Belmont: Wadsworth, 1986.<\/p>\n\n
\n molecules<\/td>\n 2C2<\/sub>H6<\/sub><\/td>\n \n702<\/sub><\/td>\n \u2192<\/td>\n 4CO2<\/sub><\/td>\n 6H2<\/sub>0<\/td>\n<\/tr>\n \n enthalpies (kJ\/mol)<\/td>\n -84.68<\/td>\n 0<\/td>\n \u2192<\/td>\n -393.5<\/td>\n -241.8<\/td>\n<\/tr>\n<\/table>\n