الرئيسية » Simplified Chemistry (صفحه 18)

Simplified Chemistry

crossed-beam reaction

A chemi­cal reaction in which two molecular beams are crossed; one beam is re­garded as the incident beam of gas and the other as the target gas. This technique enables a great deal of in­formation to be gained about the chemical reaction since the states of both the target and …

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cryptands

Compounds with large three-dimensional molecular struc­tures containing ether chains linked by three-coordinate nitrogen atoms. Thus cryptands are macropolycyclic polyaza-polyethers. For example, the compound (2,2,2)-cryptand has three chains of the form -CH2CH2OCH2CH2OCH2CH2-. These chains are linked at each end by a nitrogen atom. Cryptands, like the *crown ethers, can form coordi­nation …

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crystal test

A type of presump­tive test in which a substance is identified by the formation of charac­teristic crystals when a certain reagent is added. Usually, such tests are conducted using a microscope (microcrystal test). An example is the acetone-chlor-haemin test for blood.

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curare

A resin obtained from the bark of South American trees of the genera Strychnos and Chondrodendron that causes paralysis of voluntary muscle. It acts by blocking the action of the neurotransmitter  acetyl­choline at neuromuscular junctions. Curare is used as an arrow poison by South American Indians and was for­merly used …

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Curie’s law

The susceptibility (X) of a paramagnetic substance is propor­tional to the thermodynamic temper­ature (T), i.e. x = C/T, where C is the Curie constant. A modification of this law, the Curie-Weiss law, is more generally applicable. It states that  X = C/(T- θ), where θ is the Weiss constant, a …

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cycloaddition

A reaction in which two or more unsaturated compounds form a cyclic adduct or in which a cyclic compound is formed by addi­tion between unsaturated parts of the same molecule. In cycloaddition, there is no net reduction in bond multiplicity. The Diels-Alder reac­tion is an example. Cycloadditions may be stepwise …

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cyclophane

A compound consist­ing of one or more aromatic rings forming part of a larger ring system in which aliphatic chains of the CH2 groups link the aromatic rings. Com­pounds of this type have the suffix phane in their names. Depending on the sizes of the (CH2)11 chains, the aro­matic rings …

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buffer

A solution that resists change in pH when small amounts of an acid or alkali are added over a cer­tain range or when the solution is di­luted. Acidic buffers consist of a weak acid with a salt of the acid. The salt provides the negative ion A-, which is the …

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buna rubber

A type of synthetic rubber based in polymerization of butadiene (buta-1,3-diene). The name comes from Bu (for butadiene) and Na (for sodium, which was used as a catalyst in the original polymeriza­tion reaction). An improved form, known as Buna-S was developed by copolymerizing butadiene with styrene. In 1934, Buna-N was …

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Butler-Volmer equation

 An equation for the rate of an electro­chemical reaction; it describes the current density at an electrode in terms of the overpotential. The But­ler-Volmer equation is given by: j = ja – jc = je[exp(1 – a)Fη/RT- exp(- aF/RT)] where ja and jc are the individual cathode and anode currents …

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