Monday, November 15, 2010

Tutorial: Specific Evaporation Heat of Water

The Tutorial Part of this post is written in English. German Version may be updated soon. Or not. XD

Nachdem ich nach verzweifelter Suche heute Mittag gottseindank doch noch mein Lab-Notebook wiedergefunden habe (nein ich hab keinen Laptop im Labor mit, es geht hier um ein normales Notizbuch XD) hatte ich heute die freudige Aufgabe das Protokoll, das eigentlich mein Laborpartner hätte schreiben sollen, nochmal komplett zu überarbeiten.
Weil ich das jetzt zum x-ten Mal machen muss und das Gefühl habe das nett gemeinte Verbesserungsvorschläge von mir sowieso einfach nicht gehört (bzw. gelesen werden), hab ich auch endgültig die Nerven verloren und ein nicht ganz so nettes E-Mail verfasst =/

Eigentlich find ich es schade, dass ich so viel Ärger mit dem Praktikum habe, denn eigentlich finde ich es Wahnsinnig spannend, und sogar der Mechanik/Thermodynamik-Kram wird mir zum ersten Mal auf interessante Weise vermittelt.

Zueltzt hatten wir ein nettes Experiment, das eigentlich sehr einfach zu verstehen war, und das ich euch kurz vorstellen möchte :) Dazu gitb es natürlich eine kleine Erklärung des grundlegenden Theoriewissens, und zwar das Ganze diesmal in Englisch. Denn für viele der Begriffe kenne ich das deutsche Äquivalent gar nicht erst XD Aber ich verspreche euch, es ist nicht schwer zu verstehen worum es hier geht, auch ohne Vorwissen :) (Übrigens möchte ich von vorneherein klarstellen, dass ich kein "Native" bin und auch nur "halb" Zweisprachig aufgewachsen. Also erwartet hier bitte kein perfektes Englisch ohne Fehler. Vielleicht werd ich das ganze irgendwann noch auf deutsch übersetzen wenn ich dazukomm.)

Wer also bis jetzt noch nicht schreiend die Flucht ergriffen hat, den will ich hiermit noch einmal klar und deutlich warnen:

Es geht hier um Physik.

 kein Science-Tutorial ohne Glasses-pic! erstes Varis'sches Gesetz :D

Und wen auch das nicht abgeschreckt hat (auch das Foto *lol*), den heiße ich herzlich willkommen zu meinem zweiten Tutorial - vTeacher im Einsatz! Diesmal ist das Thema etwas spezifischer als mein letztes Tutorial, und auch eher theoretischer Natur.
Heute geht es um Hitze, Dampf, und Energie. Und da sag noch einmal wer Physik sei unsexy ;D




Specific Evaporation Heat of Water

So, first of all, we should talk about temperature itself, so we can more easily understand what this all is about.
Temperature basically decribes the "speed" or intensity at which the atoms of a substance (be it air or a solid like a metal or whatever) "vibrate". At the absolute zero point for temperature of 0K (this equals −273°C), they are almost not moving at all. This is also the reason why there can be something like an absolute zero point at all. If they atoms (almost) stop moving, they are unable to pass on their energy to other systems. Simply put: nothing happens anymore.

So the temperature is only the (thermal) energy that the atoms of a substance "hold". It is in fact the (kinetic) energy of the atoms' movements.
Nevertheless, "heat" and "temperature" are not the same. "Heat" is a kind of energy, it has the unit calorie [cal]. The basic relation bewteen the two of them: To raise the temperature 1g of liquid water by 1°C, 1 cal is needed. Expressed in Joule [J], a unit you are probably more familiar with if you only had physics in school, 1cal = 4,184J

As you all probably know, substances can have three different states, the so called states of matter or phases:
Solid : where the atoms hold togethe tightly, and simply do not have enough energy to bounce around as they wish. The atoms hold together by strong bonds. The substance stays the same in shape and volume.
★ Liquid: where the atoms are connected more loosely, but still hold together by weak bonds. The shape of the substance depends on its container.
★ Gas: the gaseous state is very special. The molecules of the substance are now "for themselves", and they tend to fill whatever room that is available.

You can transform a substance from one state to another by applying heat energy - by heating it. This causes the atoms to "move" more and more until enough energy is gained so that the atoms can "break free".

The peculiar thing is this:
If you heat a sbustance, its temperature rises constantly. For example, if you put a thermometer in a bowl of water and start to heat the water, you will observe a rather constant rise in temperature - as heat is applied, the temperature rises.
But there is a specific point, where the increase in temperature suddenly "stops" and stays constant for a while. This is the point where the substance starts to change in phase. The applied heat energy is used by the atoms/molecules to change in phase, just until the point where all molecules have traversed from solid to liquid state or liquid to gaseous state. So during this "process", no further increase in temperature (on the thermometer) is observed! All the energy applied by heating is used by the atoms to "break free".

From all this knowledge, we can conclude that there is a relation between the temperature, the heat energy and the mass. We could also say - we need a specific amount of heat energy per mass unit to liquify/evaporate a substance.

This is the so called specific evaporation heat Qv!
It can be calculated by using the following formula, which is basically just the mathematical expression for what we thought about in the paragraph above:



Looks confusing, but it's not hard to understand :) First, let's make clear what the variables mean. I will explain the use of the formula by the example of an experiment, where we determined the specific evaporation heat of water.
Just imagine that we have a (almost) closed vessel containing a known amount of cold water. It has one opening, where can put in a thermometer and measure the temperature of the water inside. Through another opening, water vapor (water in the gaseous state) can be induced into the vessel.
The hot vapor will condense when it comes in contact with the cold water inside the vessel, and therefore the vapor changes to liquid phase again. This causes the temperature of the water in the vessel to increase, aswell as the mass of the water inside to increase. We can measure the increase in mass by simply putting the vessel (it's a calorimeter btw) on a balance. To calculate the amount of water that has evaporated, we simply perform a first measurement before we begin with the heating. From that point on the mass of the evaporated water can always be calculated by weighing the vessel and then subtracting the mass of the vessel plus the cold water.
So basically we induce the water vapour, and measure a few times the increase in mass and the increase in temperature, then calculate a mean value.

To summarize:


T1 is the boiling point of the substance! e.g. for water it would be 100°C
T2 is the temperature of the cold water inside the vessel before we begin the experiment.
Tm is the temperature of the mixture (cold water + condensed water vapour) inside the vessel.

m1 is the mass of the evaporated water (remember - just subtract the mass of the calorimeter + mass of cold water form the beginning)
m2 is the mass of the cold water at the beginning of the experiment.

c ist the thermal capacity. For water, c = 4190J/kg (just think of it as the amout of energy in J that 1kg of water can take up)

Knowing this, let's have a look on the formula again:



So we see that the values for the temperature in the brackets are just temperature differences. The formula can be therefore equally used with Celsius and Kelvin values.

So the formula means.
mass of the  evaporated water (times thermal capacity) times temperature dfference between temperature of the mixture and temperature at boiling point plus  mass of the cold water (times thermal capacity) times increase in temperature inside the vessel...
....per mass of evaporated water.

Okay, I give in, still sounds terribly complicated X'D However, it works out perfectly fine when you conduct the experiment described above, and fill in the corresponding values :)
What you get from that forumla für Qv would probably be values between 1400 and 1900 kJ/kg (at least we got such values when performing the experiment)
Notice that when using the formula, you have to keep track of the units! You either have to converse the masses into [kg] or convert c into [J/g]. However, you should end up with something of the magnitude as the value presented above.

What do we learn from this?
We know now, that to cause 1kg of water to evaporate, approx. 1650kJ of energy are necessary.

We can also express this in kJ/mol.
Since the molecular mass of water is 18g/mol, 1kg of water contains 55.5mol of water.
So 1650kJ/kg are the same as 1650kJ/55.5mol which would be approx. 30 kJ/mol.
Qv for water is known (thanks to scientists perform these experiments a whole lot more accurately than we don in the lab XD) to be approx. 40kJ/mol. So the value we get from the experiment is actually not that bad at all :)



Okay, so today we learned something about the difference between heat (energy) and temperature, we heard what temperature actually is and the change of states of matter. I hope you enjoyed our little game of numbers here, and I hope I was able to fulfill my promise to keep it easy :)

An upcoming tutorial I want to do is one on Differntiation. So it would probably be math next time. Maybe I'll be able to keep it a bit more funny next time :'D Always depends on the copics but it can be really hard to write such stuff in an interesting way! XD Nevertheless I hope you liked the tutorial. If you have any questions, don't hesitate to leave a comment or ask me on formspring.


I'm open for suggestions all the time, if there is any specific topic you're interested in, maybe I can write a tutorial about it that can help you. Furthermore I hope I did not make any mistakes in my explanation XD Well, if I did and you found one, please tell me immediately ^-^"


I guess it's hard to understand why I'm so fascinated about physics... I know that a lot of people don't like the subject at school and later on, but pleeease just try to see the beauty in all this.
Everything on this planet, everything in this universe - there is a reason why things are like that. Laws they obey or just some basic rules they follow. And then there are topics where we have actually no idea how stuff works or why it happends... and nevertheless it's there and works perfectly finde, we just don't know how or why.
And it works. It all works, like a giant, precise but still chaotic and unpredictable clockwork.
This is our world, and I think it never hurts to know a little bit about how things work :)

That's it for today! I'ma go to bed know, neeeeed more sleep (in preparation for another phys lap session tomorrow -_-)

Science-Nerdgirl says Byebye! 

4 comments:

  1. I like Science-Tutorials, you should make MOAR
    of them - can't tell why, but they are awesome ;3

    grussli ^^

    ReplyDelete
  2. Ich hab bei dem Satz "Hier geht es um Physik" abgeschaltet. War schon immer ne Niete in Naturwissenschaften (ausser Mathe) :D
    Aber "Varis'sches Gesetz" is der Knüller :'D

    ReplyDelete
  3. ein undichtes auge bedeutet das flüssigkeit die im inneren des auges durchsickert..es ist undich geworden weil die fäden sich im auge gelöst haben ;)

    ReplyDelete

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