Artikel Kimia

Apakah Perbedaan antara SiCl4 dan GeCl4

Posted: 02 Mar 2012 02:05 AM PST

Pertanyaan:

SiCl₄ dan GeCl₄ digunakan untuk sintesis unsur murni, namun mereka berbeda. Mengapa?

Jawaban:

SiCl₄ berbeda dari GeCl₄ dalam hal ini, ia dapat didistilasi dan dipisahkan dari HCl pekat.

Why a certain materials properties has conductivity properties

Posted: 01 Mar 2012 02:05 PM PST

The valence electrons are bound to individual atoms, as opposed to conduction electrons (found in conductors and semiconductors), which can move freely within the atomic lattice of the material.

On a graph of the electronic band structure of a material, the valence band is located below the conduction band, separated from it in insulators and semiconductors by a band gap. In metals, the conduction band has no energy gap separating it from the valence band.

Conductivity

A conductivity allows an electric current to flow through it equally well in either direction. The amount of current which flows depends only on the amount of resistance of the conductor and on the amount of voltage applied across it. The direction of flow can always be considered as being from the positive to the negative pole of the source of the voltage applied, so the direction of flow through a conductor is always determined by which end of the conductor is connected to the positive pole of the source.

Semiconductivity

The overlapping depends on the interatomic distance (rd) and also on the energy level of the orbitals. If (rd) is large or the orbitals are of large energy level then there may be small overlapping or no overlapping leaving a band gap (Eg). The electrical conductivity of a metal depends on its capability to flow electrons from valence band to conduction band. Hence in case of a metal with large overlapped region the electrical conductivity is high along with good metallic property. If there is a small forbidden zone then the flow of electron from valence to conduction band is only possible if an external energy (thermal etc.) is supplied and these groups with small Eg are called semiconductivity.

Superconductivity

Superconductivity is an electrical resistance of exactly zero which occurs in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes in 1911. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is also characterized by a phenomenon called the Meissner effect, the ejection of any sufficiently weak magnetic field from the interior of the superconductor as it transitions into the superconducting state. The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of “perfect conductivity” in classical physics.

The electrical resistivity of a metallic conductor decreases gradually as the temperature is lowered. However, in ordinary conductors such as copper and silver, this decrease is limited by impurities and other defects. Even near absolute zero, a real sample of copper shows some resistance. Despite these imperfections, in a superconductor the resistance drops abruptly to zero when the material is cooled below its critical temperature. An electric current flowing in a loop of superconducting wire can persist indefinitely with no power source

Artikel Kimia

Tekanan Osmotik

Posted: 01 Mar 2012 08:41 AM PST

Tekanan Osmotik

Tekanan osmotik adalah sebuah sifat dari larutan organik atau pelarut anorganik dan tampak pada sejumlah konteks dalam studi sistem koloidal. Adalah penting untuk menganggap bahwa konsep tekanan osmotik adalah lebih umum dari eksperimen yang dijelaskan. Secara khusus tekanan osmotik tidak harus membutuhkan keberadaan membran (atau bahkan perbedaan konsentrasi) untuk menentuksn tekanan osmotik. Tekanan osmotik sebagai sifat dari larutan, selalu hadir dan sesuai untuk melawan tendensi dari potensi kimia untuk menyeimbangkannya. Adalah tidak penting bagaimana perbedaan pada potensi kimia itu terjadi. Perbedaan dapat timbul mengacu pada faktor lain seperti medan elektrik atau gravitasi.

Fenomena terkait terjadi saat membran dalam eksperimen yang disebutkan adalah permeable terhadap pelarut dan ion-ion kecil tetapi tidak terhadap makroion seperti oleh elektrolit atau partikel koloid bermuatan yang dapat berada dalam larutan. Polielektrolit, terlindung dari bergerak ke sisi lain, mengusik distribusi konsentrasi dari ion kecil dan memberikan kenaikan kepada kesetimbangan ionik (dengan bantuan perbedaan potensial) yang berbeda dari yang kita harapkan dari ketiadaan polielektrolit. Kesetimbangan yang dihasilkan dikenal sebagai kesetimbangan Donnan (atau kesetimbangan Gibbs-Donnan) dan memainkan peran penting dalam sejumlah masalah menarik dalam kimia koloid dan permukaan, elektokimia dan biologi.