43. SPEED OF CHEMICAL REACTIONS. DEPENDENCE ON RALICHNY FACTORS.

You are familiar with the concept "speed" from physics course. In general view speed is the size showing as any characteristic for unit of time changes.
Speed of chemical reaction is the size showing as concentration of mother substances or reaction products for unit of time change.
It is necessary for assessment of speed izmereneny concentration of one of substances.
The reactions proceeding in the homogeneous (homogeneous) environment are of the greatest interest. Mathematically the speed of chemical homogeneous reaction can be presented by means of formula:

For heterogeneous reaction, reaction rate is defined by number of moths of the substances which have entered in or formed as a result of reaction in unit of time on surface unit:

Dependence of speed of reactions on different factors

Conditions	Examples
concentration	When strengthening at least of one of reactants the speed of chemical reaction increases according to the kinetic equation. Let's consider the general equation of reaction: aA +bB = cC + dD. For this reaction the kinetic equation takes form: Increase in collision number of the reacting particles due to increase in particles in unit volume is the reason of increase in speed.
temperature	The chemical reactions proceeding in homogeneous systems (mixes of gases, liquid solutions) it is carried out due to collision of particles. However, not any collision of particles of reagents leads to formation of products. Only the particles having the increased energy - active particles, are capable to carry out the act of chemical reaction. With temperature increase kinetic energy of particles increases and the number of active particles increases, therefore, chemical reactions at high temperatures proceed quicker, than at low temperatures. Dependence of reaction rate on temperature is defined by the rule of Guys - Goffa: The rule of Guys - Goffa is confidant and is applicable only for approximate impact assessment of temperature on reaction rate.
catalyst	Catalysts are substances which increase the speed of chemical reaction. They enter interaction with reagents with formation of intermediate chemical compound and is released at the end of reaction. The impact exerted by catalysts on chemical reactions is called catalysis. On aggregate state in which there are catalyst and reactants it is necessary to distinguish: homogeneous catalysis (the catalyst forms with reactants homogeneous system, for example, gas mixture; heterogeneous catalysis (the catalyst and reactants are in different phases; the catalysis goes on interfaces of phases).
contact area of reactants	For increase in contact area of reactants, they are crushed. The greatest reduction ratio is reached by dissolution of substances. Quicker than all substance react in solutions.
nature of reactants	For example, metals magnesium and iron react with hydrochloric acid of identical concentration with different speed. It is connected with different reactivity of metals.
Movie "Speed of Chemical Reactions"

   

 

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