Energy and Enthalpy

Enthalpy is a measure of heat in the system. 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 Enthalpy

There 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. 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. (Note: When you have numbers only, the idea of direction (as in vectors, for example) is difficult to convey. With numbers, we convey direction by using signs. One way is "positive" and the opposite way is "negative"). When a system is in equilibrium the speed of forward reactions equals the speed 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 (isolated) system can be figured out by adding the changes in enthalpy for each step. This recognizes that energy is conserved in an isolated system. Many reactions occur in steps. Only after looking at each step, and combining their effects, are you able to understand and measure the entire process.

Next page on thermodynamics and heat.
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