CJC Chemistry Blog

Intro to Organic Chemistry‏

2008-07-27T18:32:00.002+08:00

Hi,everyone!

welcome to Intro to Organic Chemistry‏!
Here are some links which may help you understand Organic Chemistry‏ better!

www.chemguide.co.uk/basicorg/isomermenu.html#topGood notes on the topic and examples

www.creative-chemistry.org.uk/molecules/isomers.htmVery colourful website; brief summary; good 3D animation

www.chembio.uoguelph.ca/educmat/chm19104/isomers/Interesting problems with 3D animations. Click on the links!

It will only take 20 min of your time if you attempt all the questions.
So, please go and try!!
It will help you...

2008-05-11T21:49:00.002+08:00

Thermal conductivity: why do metals conduct heat?
Everyone knows that touching a metallic surface at room temperature produces a colder sensation than touching a piece of wood or plastic at the same temperature. The very high thermal conductivity of metals allows them to draw heat out of our bodies very efficiently if they are below body temperature. In the same way, a metallic surface that is above body temperature will feel much warmer than one made of some other material. The high thermal conductivity of metals is attributed to vibrational excitations of the fluid-like electrons; this excitation spreads through the crystal far more rapidly than it does in non-metallic solids which depend on vibrational motions of atoms which are much heavier and possess greater inertia.

Appearance: why are metals shiny?
We usually recognize a metal by its “metallic lustre”, which refers to its ability of reflect light. When light falls on a metal, its rapidly changing electromagnetic field induces similar motions in the more loosely-bound electrons near the surface (this could not happen if the electrons were confined to the atomic valence shells.) A vibrating charge is itself an emitter of electromagnetic radiation, so the effect is to cause the metal to re-emit, or reflect, the incident light, producing the shiny appearance. What color is a metal? With the two exceptions of copper and gold, the closely-spaced levels in the bands allow metals to absorb all wavelengths equally well, so most metals are basically black, but this is ordinarily evident only when the metallic particles are so small that the band structure is not established.
The distinctive color of gold is a consequence of Einstein's theory of special relativity acting on the extremely high momentum of the inner-shell electrons, increasing their mass and causing the orbitals to contract. The outer (5d) electrons are less affected, and this gives rise to increased blue-light absorption, resulting in enhanced reflection of yellow and red light.

patrick

2008-05-10T19:41:00.009+08:00

A bit on chemical bonding.

Hybridisation in methane.

This is a common structure of methane that we would normally draw using dot and cross diagram, however, this is a wrong structure.

As you all know, the electronic configuration of CH₄is 1s²2s²2p_x¹2p_y¹. This electronic configuration (the 1s² is not shown on the electron in box diagram) shows that only 2 electrons are available for sharing. So why is methane CH₄ and not CH₂?

When bonds are form, it is an exothermic reaction and as you all know, energy is released and the whole system is more stable. (Remember your notes? Less energy = more stable). When carbon forms 4 bonds (4 C-H bonds), the energy released is twice the energy released when carbon forming 2 bonds (2 C-H bonds). The energy gap between the 2s orbital and the 2p orbital is very small and so it pays the carbon to provide a small amount of energy to promote an electron from the 2s to the empty 2p to give 4 unpaired electrons. The extra energy released when the bonds form more than compensates for the initial input.

If you are unsure and curious about why the arrow is drawn upwards, look down.

Now that we've got 4 unpaired electrons ready for bonding, another problem arises. In methane all the carbon-hydrogen bonds are identical, but our electrons are in two different kinds of orbitals. You aren't going to get four identical bonds unless you start from four identical orbitals.

Now the fun part

Hybridisation

The electrons rearrange themselves again in a process called hybridisation. This reorganises the electrons into four identical hybrid orbitals called sp³ hybrids (because they are made from one s orbital and three p orbitals). You should read "sp3" as "s p three" - not as "s p cubed".

sp³ hybrid orbitals look a bit like half a p orbital, and they arrange themselves in space so that they are as far apart as possible. You can picture the nucleus as being at the centre of a tetrahedron (a triangularly based pyramid) with the orbitals pointing to the corners. This explains why methane is tetrahedral in shape.

What happens when the bonds are formed?

Remember that hydrogen's electron is in a 1s orbital - a spherically symmetric region of space surrounding the nucleus where there is some fixed chance (say 95%) of finding the electron. When a covalent bond is formed, the atomic orbitals (the orbitals in the individual atoms) merge to produce a new molecular orbital which contains the electron pair which creates the bond.

Four molecular orbitals are formed, looking rather like the original sp3 hybrids, but with a hydrogen nucleus embedded in each lobe. Each orbital holds the 2 electrons that we've previously drawn as a dot and a cross.

The principles involved - promotion of electrons if necessary, then hybridisation, followed by the formation of molecular orbitals - can be applied to any covalently-bound molecule.

So why are the arrows drawn all pointing upwards?

1. Because the arrow indicates the direction in which the electron spins, either clockwise or anticlockwise and the direction of the magnetic moment it creates when the charged electrons are spinning.

2. If the arrow in 2p_z is pointing downwards, the charged particle will create a magnetic field that attracts another electron, but since the 2s electron has a lower energy level, it will attract the 2p_z electron and we will be back to square one.

Michael

Application of the Ideal Gas Law

2008-05-08T21:19:00.008+08:00

Using the Ideal Gas Law, estimate the reduction of pressure in this tire if there is a change of temperature from 20oC to -20oC. Assume the tire to be mounted on a car with an initial pressure of 32 psi (220640 pa or 4611 psf).

The Ideal Gas Law can be expressed as PV = NkT where P, V & T are the Pressure, Volume and Temperature, respectively and N is the number of molecules contained within the Volume V and k is a constant.

We have two situations, 1) when the temperature is 20oC and 2) when it is -20oC. For each of these situations the Ideal Gas Law applies and we can write
P1V1 = NkT1
P2V2 = NkT2

Let's assume the volume of the tire is not affected by the change of temperature. Since V1 = V2, then it follows that
NkT1/P1 = NkT2/P2

and after canceling Nk from both sides，then we have

T1/P1 = T2/P2
Or
P2 = P1T2/T1

(It is also the Gay Lussac's Law)

Since we know both temperatures and the initial pressure we can solve this problem once we make some simple unit conversions.

When using the Ideal Gas Law we must convert the temperatures to Kelvin. Recall that
Tk = Tc + 273
then T1 = 20 + 273 = 293 K and T2 = -20 + 273 = 253 K.
Therefore T2/T1=1.16and P2 = 32 (1.16) = 37.1psi (255805pa or 5346psf)

You see that the reduction of pressure is significant. Low tire pressure will result in poor gas-mileage. In the case that you are traveling a long distance it is best to check your tire pressure, especially at the beginning of the winter months. The onset of warmer summer days will instead provoke a pressure increase。

CJC 1T21 Wei Feng

2008-05-03T11:35:00.000+08:00

This Chemistry blog is for all and please comment (by clicking on the comment link at the end of every post.)

2008-05-01T18:37:00.002+08:00

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2008-05-01T17:55:00.000+08:00

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