Enthalpy
Enthalpy (H) is equal to heat (q) at constant pressure. Under conditions where pressure is constant and our system does not undergo changes in volume, enthalpy is equal to internal energy. Enthalpy is easily measured experimentally (we will study calorimetry at the end of the term) and is a common quantity you will see. When heat exits a system (loses heat), we define that as exothermic. When heat enters a system (gains heat), we define the process as endothermic. Let's look at an example you are familiar with. When your body sweats, the sweat will evaporate from your skin. Thinking of the sweat as the system, you have to put energy in for evaporation to take place. As a result, heat was transferred from the surroundings (your body) and you feel cooler! In a similar way, if you were to place your arm over a pot of boiling water, the water vapor (gas) would condense on your skin. This process would cause heat to exit the system (water molecules) and release heat to the surroundings (you!) likely causing a burn (ouch!).
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