Calculating Vaporization

Ender · Post by **Ender** » 2002-09-23 06:10pm

I need help calculating how much energy it needs to vaporize something. Unfortunatly, I've forgotten all my HS Chem. Can I get a little help? Specifically, I need to know how to convert cubic meters to mols so I can do the math. Thanks.

Shadow WarChief · Post by **Shadow WarChief** » 2002-09-23 06:13pm

Well what is the calculation that you need to be done? It's a little hard for me to just bring the formula out of mid air.

Can you actually give me the numbers/instance in question?

Ender · Post by **Ender** » 2002-09-23 06:19pm

HoV for Iron, on a 24300000000 cubic meter mass.

Iron is 349.60 kJ/mol HoV.

Shadow WarChief · Post by **Shadow WarChief** » 2002-09-23 07:03pm

Ender wrote:HoV for Iron, on a 24 300 000 000 cubic meter mass.

Iron is 349.60 kJ/mol HoV.

Well since you didn't specificy whether this is solid iron at state X going to state Y, I'll just assume that this is the amount of iron you specificed at it's boiling point, needing the necessary amount of energy needed to vaporize it.

All right then, according to this place, cast (liquid) Iron has a density of 7000 kg per cubic meter or 7 megagrams

so multiple the density by the volume and you have the mass.

Volume is...let me count those zeroes.....24.3 E9 cubic meters

(24.3 E9 m^3) X (7 E6 grams / meter^3 ) = 1.7 E17 grams

55.85 is the atomic weight of an atom of iron, making the mass per mole 55.85 grams

So take the mass of your liquid iron that you're vaporizing , and divid it by the atomic weight to figure out the moles

1.7 E17 grams ÷ 55.85 = 3.05 E15 moles

multiply the moles above by the heat of vaporization and you'll get the answer, which is

1.065 E18 joules

OR

1 million terajoules

OR

254 Megatons of TNT

Have fun.

Shadow WarChief · Post by **Shadow WarChief** » 2002-09-23 07:56pm

Ack. Made a mistake. I used 349.6 J /mol instead of the given 349.6 kJ / mol. So the numbers should be

1.065 E21 joules

OR

1 billion terajoules

OR

254 gigatons of TNT

Ender · Post by **Ender** » 2002-09-23 11:38pm

solid state, sorry

Crown · Post by **Crown** » 2002-09-23 11:40pm

Ender I assume that you have checked out Mike's asteroid destruction calculator?

Ender · Post by **Ender** » 2002-09-23 11:49pm

Tried it. I am trying to calculate how to vape a borg cube, and since it is a cube and not a sphere, I don't have a diameter to use.

If there is a way to get it to work just punching in the volume, let me know because I spent a good 20 mins tring to get it to work

Crown · Post by **Crown** » 2002-09-24 12:00am

If you know the Borg Cube's volume, then you can just use that to find out the required radius of a sphere with an equal volume! Also you can be like specific and say it's only 50% solid, and reduce it's volume again, now give me a second to find the equation of volume for a sphere..

Crown · Post by **Crown** » 2002-09-24 12:03am

Got it!

Volume = (4/3)pi*r^3

So a sphere with the same volume as a borg cube would have a radius of;

r^3 = (3*Volume)/(4*pi)

And then you just take the cube root of that to get the 'spheres' equivalent radius...

Master of Ossus · Post by **Master of Ossus** » 2002-09-24 12:36am

Calculating vaporization is not that big of a deal. It involves:

converting from either moles or mass to determine how much energy is required to heat the entire thing 1K. (if it is not already at the boiling point)

multiplying that by the number of Kelvins needed to raise the temperature to the first state change point

multiplying to find out how much energy is needed to change the substance's state (if necessary)

Then repeating that if you are going from solid to gas, and adding the results together.

Zoink · Post by **Zoink** » 2002-09-24 11:49am

There are two methods:

1) Send $5 in a self-addressed stamped envelope to Mr. Wong.

or

2) Like M.O.O. said:

MASS * VAPORIZATION TEMP. * SPECIFIC HEAT + MASS * ENERGY_PHASECHANGE_SOLID_to_LIQUID + MASS * ENERGY_PHASECHANGE_LIQUID_to_GAS

MASS is determined by you. Other values can be had from www.google.com using keywords like:
"Vaporization temperature"
"Specific Heat"
"Phase change energy"

Make sure your units match up: all kelvins, grams, joules, etc.

victorhadin · Post by **victorhadin** » 2002-09-24 12:11pm

Of course the problem is that specific heat capacity changes as you increase the temperature, and changes a lot when a substance changes state.

Zoink · Post by **Zoink** » 2002-09-24 01:22pm

victorhadin wrote:Of course the problem is that specific heat capacity changes as you increase the temperature, and changes a lot when a substance changes state.

But as long as the change is less than a factor of 10, you can approximate values that have a proper order of magnitude. When discussing ST vs SW, 3.2GW vs 5.3GW is pretty meaningless, but 3TW vs 3GW is something to talk about. I believe that solid and gas specific heats only differ by a factor of about 2 (24 J/mole vs 12 J/mole).

Although, if you can provide a source of specific heat vs temperature for iron, it would be much appreciated.

Captain Hornblower · Post by **Captain Hornblower** » 2002-09-24 08:01pm

You might want to consider other factors in your calcs, such as: the percentage of the borg cubes volume that is made of the the iron (or what ever material you want to use, personally I calculated it's Q using titanium), volume of the borg occupants and volume of air, or what ever they breath, then sum up all those energies. Though I think you will find the last 2 insignificant. Oh and check your units

victorhadin · Post by **victorhadin** » 2002-09-25 12:54pm

Well Zoink, I am perhaps being a touch anal retentive, but accuracy is to be desired, I feel.

Alas I have no such lists for iron (though I do for water, atmospheric air and others) but I do have more 'average' specific heat capacities for iron in its solid and liquid states.

Not on me right now though.

Zoink · Post by **Zoink** » 2002-09-26 01:37pm

Well Zoink, I am perhaps being a touch anal retentive, but accuracy is to be desired, I feel.

Yes, I'm not faulting you for pointing out that fact. If you have average values for solid and liquid phases, then you should split those phases and calculate each seperately. Just don't want to let the lack of data stop anyone from trying to get a result.

There are other problems as well. For example: the specific heat values you have are probably for constant volume. Is something vaporizing in space at constant volume? Is constant pressure any better, considering its expanding in a vacuum? At some point you'll have to make assumptions in order to obtain a value.

In real life accuracy is not always desired. Sometimes the effort needed to obtain accurate input data (like measurements, specific heats, etc) is not justified by the required output. This is true in industry and in everyday life.

In the case of vaporizing a borg cube, the biggest problem is determining the cubes material and density (the biggest error will be here). For me to make a special trip to my old university library, just so that one number is a little more accurate just isn't going to happen (and is not needed)

So I'll use whatever specific heat values I find on the net. The goal in this case is to make conservative assumptions and establish a conservative value.

StarDestroyer.Net BBS

Calculating Vaporization

Calculating Vaporization

Also Ender,