Terribly confused!! Enthalpy and Enthalpy Change>?

Random98

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What exactly IS Enthalpy? And Enthalpy change? What makes 'energy' and 'enthalpy' different?(are they even related?)

Why can we not measure enthalpy alone but can measure enthalpy change(i don't really understand enthalpy itself so I'm confused here as well)

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charco

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(Original post by Wishah)
What exactly IS Enthalpy? And Enthalpy change? What makes 'energy' and 'enthalpy' different?(are they even related?)

Why can we not measure enthalpy alone but can measure enthalpy change(i don't really understand enthalpy itself so I'm confused here as well)

Enthalpy can be thought of as the energy within a substance that can be released (or absorbed) as heat, when no other work is done on, or by, a system.

It is equal to the energy change at constant pressure.

The absolute chemical potential energy of a substance is a function of how the substance is bonded together and its relative interaction with all of the matter in the universe. This is clearly impossible to know.

However, by measuring the change in heat energy during thermodynamic processes we can directly relate this to change in chemical potential energy of a substance.

We call the energy involved in these changes, enthalpy.

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Random98

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(Original post by charco)
Enthalpy can be thought of as the energy within a substance that can be released (or absorbed) as heat, when no other work is done on, or by, a system.

It is equal to the energy change at constant pressure.

The absolute chemical potential energy of a substance is a function of how the substance is bonded together and its relative interaction with all of the matter in the universe. This is clearly impossible to know.

However, by measuring the change in heat energy during thermodynamic processes we can directly relate this to change in chemical potential energy of a substance.

We call the energy involved in these changes, enthalpy.

Can you please explain the bold.
And if possible, can you give an example just to clear it all up?

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charco

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(Original post by Wishah)
Can you please explain the bold.
And if possible, can you give an example just to clear it all up?

It's just a definition and useful as the majority of experiments are carried out at ambient pressure.

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Random98

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(Original post by charco)
It's just a definition and useful as the majority of experiments are carried out at ambient pressure.

I see.. So basically, "enthalpy" is the chemical potential energy of a substance(reactants or products). And when a reaction occurs, there is an overall gain or loss of energy(depending upon whether a reaction is endothermic or exothermic) which is "enthalpy change"...? And THAT "enthalpy change" can be measured because it has an impact on the surroundings.

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charco

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(Original post by Wishah)
I see.. So basically, "enthalpy" is the chemical potential energy of a substance(reactants or products). And when a reaction occurs, there is an overall gain or loss of energy(depending upon whether a reaction is endothermic or exothermic) which is "enthalpy change"...? And THAT "enthalpy change" can be measured because it has an impact on the surroundings.

Yes.

There may be a loss of heat energy in the environment (system) as more chemical potential energy is stored - endothermic reaction

or an increase in heat energy in the system as chemical potential energy is transformed into heat energy - exothermic reaction

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langlitz

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(Original post by Wishah)
What exactly IS Enthalpy? And Enthalpy change? What makes 'energy' and 'enthalpy' different?(are they even related?)

Why can we not measure enthalpy alone but can measure enthalpy change(i don't really understand enthalpy itself so I'm confused here as well)

This is how it's defined more formally:
Enthalpy, H, is

where U is the internal energy of the system.

thus

so using the first law of thermodynamics

so at constant pressure

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Random98

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(Original post by charco)
Yes.

There may be a loss of heat energy in the environment (system) as more chemical potential energy is stored - endothermic reaction

or an increase in heat energy in the system as chemical potential energy is transformed into heat energy - exothermic reaction

Thanks a lot! Its all cleared up for me now!

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If you point a hairdryer at an inflated balloon, it will expand.

Some of the heat energy goes into raising the temperature (making it hotter),

Some of the heat energy going into stretching the rubber (work is done to expand the balloon). This will increase the pressure on the inside.

The heat energy needed to raise the temperature is the enthalpy.

Had you done the hairdryer thing with a gas syringe, the plunger thingy could move (effectively) effortlessly, i.e. there would have been a change in volume, but not pressure, so all of the heat energy went into temperature change and so it was all enthalpy.

I hope no one tells me I'm wrong.

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langlitz

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(Original post by Pigster)
If you point a hairdryer at an inflated balloon, it will expand.

Some of the heat energy goes into raising the temperature (making it hotter),

Some of the heat energy going into stretching the rubber (work is done to expand the balloon). This will increase the pressure on the inside.

The heat energy needed to raise the temperature is the enthalpy.

Had you done the hairdryer thing with a gas syringe, the plunger thingy could move (effectively) effortlessly, i.e. there would have been a change in volume, but not pressure, so all of the heat energy went into temperature change and so it was all enthalpy.

I hope no one tells me I'm wrong.

Surely the pressure is changing in that scenario?

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Pigster

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(Original post by langlitz)
Surely the pressure is changing in that scenario?

In which scenario? the second? I guess a fuller explanation (which was warranted) would be to say that pressure increased which forces the plunger out. Assuming a frictionless contact and no momentum, the plunger would move effortlessly and no work would have to be done.

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(Original post by Pigster)
In which scenario? the second? I guess a fuller explanation (which was warranted) would be to say that pressure increased which forces the plunger out. Assuming a frictionless contact and no momentum, the plunger would move effortlessly and no work would have to be done.

I meant the first one but I misinterpreted what you were saying, I think you're right

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