Hydroxide

Hydroxides are salt- like substances that hydroxide ions ( [ OH] - ) as a negative grid blocks ( anions). Soluble hydroxides such as sodium hydroxide and potassium hydroxide with water form strong alkaline solutions ( lye), which are known as caustic soda and caustic potash. Less soluble hydroxides, such as barium hydroxide and calcium hydroxide form weak alkaline suspensions with water. The saturated solutions are called baryta water and lime water. When these two substances come into contact with carbon dioxide, they become cloudy. In chemistry lab metal hydroxides are generally prepared by salt solutions mixed with sodium or potassium hydroxide and precipitation is then filtered off, washed and dried in air. In part, do not make the pure hydroxides but after precipitation oxide hydroxides such as iron ( III) oxide-hydroxide.

Reaction equation

Example:

Structure of aqueous hydroxide solutions

In aqueous solution, the hydroxide ion is generally surrounded by four to five water molecules. There are four water molecules are around the oxygen atom of the OH - arranged so that they each can form a hydrogen bond to this (ie, show it with one of its hydrogen atoms on the OH - ). These four water molecules are located approximately in one plane with the OH - ion, ie in a different geometry than the (as with sp3 hybridization expected) approximately tetrahedral arrangement of the electron pairs in the water and H3O . The OH - ion can with its proton and an - albeit weak - form hydrogen, so that the complexes [ OH - (H2O) 4] and [ OH - (H2O ) 5] occur, depending on whether it is configured or not. For this reason hydroxides are often very bulky and sediment - unlike crystalline precipitates - very slowly.

Precipitation / formation of hydroxides

Metal hydroxides are formed in a certain pH range, which is dependent on the solubility of the hydroxide and the concentration of the precipitating cation. For example, for a divalent metal ion, the following reaction equation:

Amphoteric hydroxides at higher pH values ​​go back into solution. example:

Some metal hydroxides are oxidized after precipitation by atmospheric oxygen to form hydroxides with a higher oxidation state. Thus, manganese (II ) hydroxide quickly to manganese (III) - and manganese converted (IV ) oxide hydroxide, which exploiting the oximetry for the fixation of oxygen. Similarly, freshly precipitated green iron (II ) hydroxide is oxidized by atmospheric oxygen present to brown iron ( III) oxide hydroxide:

Hydroxides in organic chemistry,

In organic chemistry hydroxide are used as nucleophiles. Reacting suitable bromine - or chlorine- alkanes with sodium hydroxide or potassium hydroxide gives the corresponding alkanols and alkali. As a competing reaction to the substitution reaction can also take place an elimination, resulting in alkenes.

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