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Energy and EnthalpyEnthalpy is a measure of heat and energy in the system. Scientists figure out the mass of a substance when it is under a constant pressure. Once they figure out the mass, they measure the internal energy of the system. All together, that energy is the enthalpy. They use the formula H = U + PV. H is the enthalpy value, U is the amount of internal energy, and P and V are pressure and volume of the system. This system works really well for gases.Affecting EnthalpyThere are factors that affect the level of enthalpy in a system. The enthalpy is directly proportional to the amount of substance you have. Chances are if you have more of a substance, you have more energy. More energy means higher enthalpy. If you visualize on a large scale, you can compare the enthalpy in a glass of water to the enthalpy in the ocean. The ocean has more total energy.The second thing to remember is that the value for H (enthalpy) changes sign when the reactions or values are reversed. When a reaction moves in one direction, the sign is positive. When a reaction moves in the opposite direction, the value is negative. When a system is in equilibrium the number of forward reactions equals the number of reverse reactions. The third idea to remember is called Hess's Law. If a process happens in stages or steps, the enthalpic change for the overall system can be figured out by adding the changes in enthalpy for each step. Many reactions occur in steps. Only after looking at each step are you able to understand and measure the entire process. 
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Go for site help or list of physics topics at the site map!©copyright 1997-2007 Andrew Rader Studios, All rights reserved. Current Page: Physics4Kids.com | Thermodynamics & Heat | Enthalpy |
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