This is a good question, but it does have a somewhat long answer, so here goes:
The first law of thermodynamics states that energy is conserved. This means that it any transfer, the amount of energy in the closed system does not change. This also means that any increase or decrease in the energy of the system must be because energy came into or out of (respectively) the system.
The result of this is that we can *track* energy (imagine an auditor or an accountant keeping tabs of how much energy went in or out). So we can *invent* a tracking term called "internal energy". Notice that internal energy is always written as a "delta" E - this is because we want to emphasize that all we are doing is monitoring the changes in the energy of the system (whether something went in or out).
It is important to remember that we can not speak of "E" (without the delta) only. This is because we have no access to how much energy is in the system. We can only know how much *change* happened.
Enthalpy is derived from internal energy. Enthalpy is another invented term and is another bookkeeping term or a way of tracking energy changes. We talk of enthalpy as the change in internal energy when the only changes to speak of is heat going in an out of the system.
In this sense we can talk of how much heat went into a mole of substance in a particular process. For example: when 18g of ice (one mole of water) melts, we can talk of the enthalpy of melting - how much heat had to have gotten into the system in order for 1 mole of ice to change to 1 mole of liquid water.
So - enthalpy is a measure of changes in heat into or out of a system. As such, it is not as tangible as 11 inches of paper, because we are talking about a change, the amount of heat that caused a particular change.original source:http://www.newton.dep.anl.gov/askasci/chem07/chem07161.htm
The first law of thermodynamics states that energy is conserved. This means that it any transfer, the amount of energy in the closed system does not change. This also means that any increase or decrease in the energy of the system must be because energy came into or out of (respectively) the system.
The result of this is that we can *track* energy (imagine an auditor or an accountant keeping tabs of how much energy went in or out). So we can *invent* a tracking term called "internal energy". Notice that internal energy is always written as a "delta" E - this is because we want to emphasize that all we are doing is monitoring the changes in the energy of the system (whether something went in or out).
It is important to remember that we can not speak of "E" (without the delta) only. This is because we have no access to how much energy is in the system. We can only know how much *change* happened.
Enthalpy is derived from internal energy. Enthalpy is another invented term and is another bookkeeping term or a way of tracking energy changes. We talk of enthalpy as the change in internal energy when the only changes to speak of is heat going in an out of the system.
In this sense we can talk of how much heat went into a mole of substance in a particular process. For example: when 18g of ice (one mole of water) melts, we can talk of the enthalpy of melting - how much heat had to have gotten into the system in order for 1 mole of ice to change to 1 mole of liquid water.
So - enthalpy is a measure of changes in heat into or out of a system. As such, it is not as tangible as 11 inches of paper, because we are talking about a change, the amount of heat that caused a particular change.original source:http://www.newton.dep.anl.gov/askasci/chem07/chem07161.htm
