General Chemistry Questions

[Ruhana]

I am a curious junior in high school. I just finished my first

semester of chemistry but have always loved it. Here is a LARGE block of

questions I would love to have answered by anyone who can. Here goes:

1) Why is mercury a liquid? It is surrounded by solids!

2) How are cyclics (cyclopropane, cyclohexane, etc...) made from ordinary

hydrocarbons? Why can cyclopropane be used as an anesthetic?

3) Why would KNO2 reduce the number of H+ ions in a solution of HNO2?

4) How can arsenic be considered to have 5 valence electrons? Are not valence

electrons those in the s and p orbitals?

5) Does exothermic truly mean the reaction releases heat? Could it also be

light or is it always heat, then converted into light?

 

 

wht you think

 

Thanks!

[hermanntrude]

I can help with a few of those questions:

 

1) To explain that mercury is a liquid requires the use of one of einstein's theories of relativity. I can't remember which one. It was a big mystery until relativity was introduced. Apparently relativity also explains the colour of gold. How exactly, i'm not sure.

 

2) not sure. good questions :0) If you tried a search on wikipedia I imagine you could probably find information on synthesis. I suspect it'd be different for some of the more strained cycloalkanes like cyclopropane and cyclobutane

 

3) [ce]KNO2[/ce] contains [ce]NO2-[/ce], which is the conjugate base of the weak acid [ce]HNO2[/ce].

 

The acid and base exist in equilibrium:

 

[ce]HNO2 + H2O <=> H3O+ + NO2-[/ce]

 

If you add [ce]NO2-[/ce] to the equilibrium, you are adding a product. According to le chatelier's principle, addition of a product of an equilibrium results in a shift to the left-hand-side of the equilibrium. This means an increase in the quantity of [ce]HNO2[/ce] and a decrease in the quantity of [ce]H3O+[/ce]. The only reason it doesn't result in a decrease in the quantity of [ce]NO2-[/ce] is that you ADDED some of it. Nevertheless, there will be less after the equilibrium is re-established than there was immediately after the addition.

 

4) Your question and your objection to the answer are the correct answer:

 

Arsenic DOES have 5 valence electrons. Remember that the number of valence electrons is the number in the highest energy shell, ie the shell with the highest value of "n". In the case of arsenic, the highest shell is n=4. If you look up the electron configuration of As (or work it out for yourself), you will see that it ends in 4s2 4p3, which means there are 2s electrons and 3p electrons in the 4th shell, giving a total of 5 valence electrons.

 

5) i'm not sure of the semantics here but i've always felt that exothermic could technically be extended to include reactions which release energy in general, rather than specifically heat. Whether this is a belief shared by the rest of the scientific community, i'm not sure. Perhaps there is another word for our idea, but i don't know it. Perhaps exoergic?

 

any further questions, give me a few days :0)

[McCrunchy]

1) Why is mercury a liquid? It is surrounded by solids!

 

http://antoine.frostburg.edu/chem/senese/101/periodic/faq/why-is-mercury-liquid.shtml

 

Reading this explanation, I was wondering about something else: how does one compute the band diagram of liquid mercury ? Conduction is explained in solid state physics by calculating so-called valence and conduction-band diagrams; is the bands overlap, the material is a conductor, if they do not we have either a semiconductor or an insulator, depending on the width of the gap (http://en.wikipedia.org/wiki/Electronic_band_structure). This calculation requires knowledge of the unit cell geometry of the atoms constituting the solid, as for example cubic close-packed for copper (http://www.chemicalelements.com/elements/cu.html). So what do people do with a liquid ?

 

Being both a conductor and a liquid is a wonderful thing: consider for example what would happen is you were to charge a glass baguette and waggler it near a drop of mercury. I 'll let you ponder upon various phenomena that follow from these two combined properties.

 

4) How can arsenic be considered to have 5 valence electrons? Are not valence electrons those in the s and p orbitals?

 

From http://en.wikipedia.org/wiki/Valence_electron

 

"For main group elements, valence electrons can be defined as those in the electronic shell of highest principal quantum number n."

 

As has 33 electrons : 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p3

 

This makes 2+3 = 5 electrons in the 4-th orbital.

 

 

5) Does exothermic truly mean the reaction releases heat?

 

Yes. You could try dissolving NaOH in water to get a feel for this, or maybe just plainly light a match (the reaction involves potassium chlorate and sulfur).

 

> Could it also be

> light or is it always heat, then converted into light?

 

Combustion is a typical example of heat producing light. A candle's light is essentially the result of glowing soot particles that emit as black-body radiators at a temperature of 1400 K.

 

The enthalpy of reactions you'll find in tables ( for example 46 kJ/g for a burning candle : paraffin + O2 --> CO2 + H20) will or will not include energy released as light, depending on how the measurement is made. For example if you measure it by seeing by how many degrees a kettle full of water warms up when heated by a candle, some of the light will radiate away from the kettle, and will thus not be included in your measurement. This example is not a good one, as the ratio of light energy/heat for a candle is 0.04 % (http://en.wikipedia.org/wiki/Lighting_efficiency#Lighting_efficiency)

 

Can there be light without heat ? See

http://scifun.chem.wisc.edu/HOMEEXPTS/Chemilum.html

Also, in LEDs, light is triggered by application of an electric field - heat only comes as a secondary effect.

 

Can light heat ? Definitely, see : the sun

 

 

I'll let chemists answer your other 3 questions.

 

McCrunchy

[max.yevs]

i believe mercury is a liquid because one of its subshells is completely filled, which makes it not very reactive with neighboring mercury atoms (its right at the end of the transition metals)... on the other hand titanium and tungsten have those shells just half filled which makes the atoms more chained with each other and results in higher boiling and melting temp.

[CaptainPanic]

2.

Cyclo-alkanes are products from the oil refineries.

 

For some part, these are already formed in the crude oil... although aromatics (with a benzene ring) are more common.

 

Some heavy components (stuff that resembles asphalt) of the crude are quite useless, so these are sent to the crackers. The Catalytic cracker breaks up molecules. These are then radicals which are able to recombine. Sometimes this forms a ring structure. It's all quite random and violent I believe, although I'm sure that lots and lots of research has been done to improve the catalysts and to influence the products. Some of the ring structures are cyclo alkanes.

 

It's possible to add hydrogen to the cracking process (it's then called a hydrocracker). This will reduce the amount of double bonds, and also the amount of rings that are formed.

 

You can also hydrogenate benzene or some cyclic alkenes (olefins). This is also why a component such as cyclohexane is so cheap, I guess. It's a large scale bulk product.

 

There exist also methods to produce specific cyclo-alkanes, but that is something which you can better search for on wikipedia.