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РЕГИСТРАЦИЯ ЭКСКУРСИЯ

Text 21 A

~~Factors Influencing the Rate of Reactions

 

Factors Influencing the Rate of Reactions - раздел Образование, Part One THE PREDICATE Eveiy Chemical Reaction Requires Some Time For Its Complctiqn, But Some React...

 
 

   

Eveiy chemical reaction requires some time for its complctiQn, but some reactions arc very fast and some arc very slow. Reactions between ions in solution without changc in oxidation state arc usually extremely fast. An example is the neutralization of an acid by a base, which proceeds as fast as the solutions can be mixed. Presumably nearly every time a hydronium ion collides with a hydroxide ion reaction occurs, and the number of collisions is vciy great, so that there is little delay in the reaction. The formulation of a precipitate, such as that of silver chloridc when a solution containing silver ion is mixed with a solution containing chloride ion, may require a few seconds, to permit the ions to diffuse together to form the crystalline grains of the precipitate:

Ag*(aq) + Cl-(aq) AgCl(c).

On the other hand, ionic oxidation-reduction reactions arc sometimes very slow. An example is the oxidation of a stannous ion by a ferric ion:

гРс^ + Зп^гРс^ + Бп4'.

This reaction does not occur every time a stannous ion collides with one or two ferric ions. In order for a reaction to take placc, the collision must be of such a nature that electrons can be transferred from one ion to another, and collisions that permit this electron transfer to occur may be rare.

An example of a reaction that is extremely slow at room temperature is that between hydrogen and oxygen:

2H2 + 02->2H20.

A mixture of hydrogen and oxygen can be kept for years but we can observe no appreciable reaction taking placc.

That is why it is not unusual to hear chemists speaking about the factors that determine the rate of a reaction. These arc manifold. The rate depends not only upon the composition of the reacting substances, but also upon their physical form, the intimacy of their mixture, the temperature and pressure, the concentrations of the rcactants, special physical circumstances such as irradiation with visible light, ultraviolet light, X-rays, neutrons, or other waves or particlcs, and the presence of other substances that affect the reaction but are not changcd by it.

 


19.01.2015; 01:20
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