# Difference between energy and enthalpy

1. Apr 16, 2006

### vmc303

Could someone give me a concise explanation of the difference between the two? I'm struggling to understand which concept to apply in a given situation, and what they actually signify.

2. Apr 16, 2006

### Andrew Mason

Enthalpy describes a state of energy of a gas - essentially the heat energy and pressure energy content of a gas: H = U + PV. A change in enthalpy denotes a change in energy content of the gas, ie. some combination of change in temperature, pressure and/or volume of the gas.

AM

3. Nov 30, 2010

### Soumen2010

Difference between Enthalpy and Internal energy:

When we add heat (energy) to a system the system’s energy will be increased. We say this energy is stored within the system. Similarly, when we remove some heat from the system, its energy will reduce.
Do we know the total effect within the system after heat removal? It will slightly shrinks its volume for solid. The shrinking will be prominent for the gas system. Why do the system shrink?
The answer is the atmospheric external pressure. External surrounding medium surrounds every object in the universe. It also exerts pressure on the system. Each object has to create the room (space) for its own occupancy by displacing the surrounding medium. As if the object is immersed in the spring system. More it displaces the spring, more it can occupy the space. This displaced medium is viewed as the compression by the system. The energy is imparted by the system similar to a spring compression; energy is stored within medium. When the system collapses, the energy is regained back into the system. A section of scientists feels to consider this displaced medium external energy (Pressure x Volume) for the counting of system's energy.
The energy stored in the system is termed as the “internal” energy to distinguish from the total energy. The total energy is the internal energy plus the external energy associated for its occupancy against the surrounding medium. While the internal energy is the net stored energy within the system, the enthalpy is system and surrounding energy associated for its existence. Enthalpy is actually including the work energy. Enthalpy is “Entire thermal and pressure occupancy energy”.

4. Dec 13, 2010

### Zeppos10

This seems almost correct, except that thermal energy is not clearly defined.

Enthalpy is a form of energy, and specifically the sum of 'internal energy'(U) and pressure energy (energy of displacement) (pV), where p is the external pressure.

This is elaborated in '2010-mannaerts-throttle' which can be found at