Sciencemadness Discussion Board

I don't understand energy (here is Hydrogen example)!

AsocialSurvival - 13-10-2014 at 06:41

I use Wikipedia for learning purposes. Anyways, here's data for Hydrogen from there:

Unit is kJ/mol.

Heat of fusion: 0.117
Heat of vaporization: 0.904
Ionization energy (1st): 1312
Enthalpy of combustion: -286

I know that minus means that we get energy, not lose.
But, what is energy of vaporization, and what is ionization energy?
Is heat of vaporization 0.904 really, or (0.904-0.117), because we already supplied some energy for melting it?
Also, does ionization energy includes the energies we use for melting and vaporizing, or it is additionaly needed?
Seems very complicated!

Also, is enthalpy of combustion always in kJ/mol, or it depends on equation near it, like: 2 H2(g) + O2(g) → 2 H2O(l) + 572 kJ (286 kJ/mol)

HgDinis25 - 13-10-2014 at 07:51

First of all, I didn't get your explanation on the minus in the energy. That minus means that energy is released instead of being absorbed. In that particular case, Enthaply of Combustion, it means that 286 kJ of energy is release in the combustion of one mole of Hydrogen.

Next, the Ionization Energy is the Eneregy requiered to remove the most energetic electron of the element, in its gas form. Meaning that the following reaction:
H --> H+ + e-
takes place when you suply 1312 kJ/mol of energy. The (1st) means it's the first energy of ionization. Atoms with more than one electron have more than one energy of ionization (one for each electron).

The heat of fusion is the energy that needs to be given to the Hydrogen that is on it's melting point. Let's bring this example to water, for the sake of simplicity. You have an ice cube. You heat it to 0ºC (you've just given it energy right?). However it won't star to melt just yet. You need to give it the Heat of Fusion so it can melt. Same thing goes for the Heat of Vaporization.

AsocialSurvival - 13-10-2014 at 14:57

But, if I need a certain amount of energy to melt, let's say, Copper, how many kJ are really needed? Energy is proportional to temperature, but what temperature is the starting point? -273 K or 0 degrees Celsius or room temperature? For example, for hydrogen we obviously don't need any energy so I guess that energy is found in room temperature around us.

HgDinis25 - 13-10-2014 at 16:48

Quote: Originally posted by AsocialSurvival  
But, if I need a certain amount of energy to melt, let's say, Copper, how many kJ are really needed? Energy is proportional to temperature, but what temperature is the starting point? -273 K or 0 degrees Celsius or room temperature? For example, for hydrogen we obviously don't need any energy so I guess that energy is found in room temperature around us.


Actually, in the case of Hydrogen room temperature provides enough energy to keep it's molecules in the unorganized state we call "gasous".

Now, how to melt copper?
Let's say you start with copper (m.p. 1084ºC). You need to provide your sample of copper enough energy to make it reach 1084ºC. You calculate this using:
E = m . c . ∆T
where E is the energy required, m is the mass of copper, c is the specific heat of copper and ∆T is the variation of temperature.
More information on the formula:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html

After you've reached 1084ºC you still have solid copper. Now you need to provide the energy known as Heat of Fusion. Only after you added such energy will the copper melt.

You can say that the energy requiered to melt a given sample is the the Energy required to bring the temperature to the m.p. of the substance plus i'ts Heat of Fusion.