Velocity of a Reactions

Introduction to Reactions:

The reactions are defined as " the total number reacting special whose concentration actually alters during the course of chemical reactions. In other words, it is the number of concentration terms which determines the dependence of rate of reactions. For a general reactions numerical value of order of reactions are the sum of all the exponents to which the concentrations in the rate equation are raised.

Velocity of a Reactions:

• The velocity or rate of chemical reactions can be defined and measured as "the rate of formation of  one or more of its products or as the rate of consumption of one or more of its reactants".
• If the reactions are homogeneous and occurs entirely in the gaseous phase, the partial pressure of its components could be measured, and if the reactions takes place in solution.
• Thus, the units of velocity of a reactions in solution will be units of concentration per unit time.
• The velocity of a reactions, depends on the concentration or pressure of the reactants.
• As the concentration of the reactants keep on decreasing from moment to moment so, velocity of a reactions decreases with time.
• In order to decide  the velocity of  reactions at any time.

Expressing velocity of reaction:

• The velocity of reactions are measured in terms of either the rate of decrease in concentration of reaction(s) or the rate of increase in concentration of product(s). Thus, for the reactions A`->` B.

• The velocity of reaction is given by:

`(-d[A])/(dt)` =`(+d[B])/(dt)`

• It must be clearly understood that the velocity of reaction is invariably a positive quantity.
• The minus sign given to -d[A]/dt simply indicates that the concentration of the reactant, A is falling.
• While positive sign given to +d[B]/dt implies that the concentration of the product, B is increasing with the time.

Simultaneous Reactions:

• Some of the simultaneous reactions are:

Parallel reactions:

• The reactions giving the main product is called main reactions, while the other is called parallel or side reactions.
• By altering the conditions of the experiment, the relative rates of the side reactions can be varied and sometimes to such an extent that a side reaction is changed into main or principal reactions and vice-versa.

Consecutive reactions:

• This reactions in which the first reaction product is subsequently converted into a second product.

Consider the process,

Reversible reactions:

• In which the rate of the back reaction in significant.

• Reaction products react among themselves to form the original reactants at significant speed.