<< Chapter < Page Chapter >> Page >

Figure 3

Figure 3 is an example of a complete phase diagram. This diagram shows for each temperature and pressure which phase or phases are present at equilibrium. Figure 3 is for water, but each substance has its own unique phase diagram, similar in appearance.

Observation 3: boiling points and intermolecular forces

Earlier in this study, we determined that the boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the pressure applied externally, e.g. by the atmosphere or by a piston trapping the liquid and gas in a cylinder. When the applied pressure is 1 atm, we refer to this as the “normal boiling point.” If we compare the normal boiling points of two liquids, the liquid with the higher normal boiling point clearly requires a higher temperature to reach a vapor pressure of 1 atm. From our work on dynamic equilibrium, we know that this higher temperature is required to provide sufficient kinetic energy for the molecules in the liquid to overcome stronger attractions between the molecules. Overcoming these attractions is necessary for a molecule to “escape” the liquid and join the vapor phase.

This line of reasoning means that, when we compare the normal boiling points of two liquids, we are also indirectly comparing the strengths of the intermolecular attractions in those two liquids. The liquid with a higher boiling point has stronger intermolecular attractions.

What determines the strength of these attractions? To find out, we can analyze experimental data for the boiling points of many liquids and look at the properties of the corresponding molecules. A useful set of compounds to look at are the covalent compounds formed by combining hydrogen with each of the elements in the “main group,” Groups IV to VII. For example, in the first row of the periodic table, these include CH 4 , NH 3 , H 2 O, and HF. Table 1 gives the experimentally observed normal boiling points of the sixteen hydrides from Groups IV to VII in the first four rows of the periodic table.

Boiling Point (˚C)
CH4 -164
NH3 -33
H2O 100
HF 20
SiH4 -111.8
PH3 -87.7
H2S -60.7
HCl -85
GeH4 -88.5
AsH3 -55
H2Se -41.5
HBr -67
SnH4 -52
SbH3 -17.1
H2Te -2.2
HI -35

At first glance, the values of the boiling points seem to be all over the place. Any patterns that might exist are not obvious. But there are patterns if we look at the data long enough, and those patterns can reveal to us what determines the intermolecular attractions. First, we can see that, for the compounds in each row of the periodic table, the compound with the lowest boiling point is from Group IV: CH 4 , SiH 4 , GeH 4 , and SnH 4 . Note that these are not the lowest four boiling points in Table 1. Rather, for each period, they are lowest boiling points of the compounds in each period. Notice also that the boiling points increase as we move down the table in Group IV, so that the heavier mass molecules have higher boiling points. This is easiest to see if we put them together on a chart in Figure 4.

These two observations suggest that we might find patterns if we put all of the sixteen hydrides on a chart together, sorted by the period each are in. The result is shown in Figure 5, which is just Figure 4 expanded to show all four groups in the data set of Table 1. With this chart, we see that the two patterns we described for Group IV work for Groups V to VII, but we also see three dramatic exceptions to those patterns in H 2 O, NH 3 , and HF.

Questions & Answers

How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Concept development studies in chemistry 2013. OpenStax CNX. Oct 07, 2013 Download for free at http://legacy.cnx.org/content/col11579/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Concept development studies in chemistry 2013' conversation and receive update notifications?