DEFINITION OF RATE OF REACTION

Definition of rate of reaction

The reaction rate is the rate at which a species loses its chemical identity per unit volume.

The rate of a reaction can be expressed as the rate of disappearance of a reactant or as the rate of appearance of a product.

Following is single phase reaction

Rate of reaction of component A

-rA = rate of disappearance of A, minus sign means disappearance
The rates of reaction of all components in above reaction are related by

The rate of change in a number of moles of any component i due to reaction is dNi/dt, then the rate of reaction in its various forms is defined as follows.
Based on unit volume of reacting fluid,


Based on unit mass of solid in fluid-solid systems


Based on unit interfacial surface in two-fluid systems or based on unit surface of solid in gas-solid systems


Based on unit volume of solid in gas-solid systems


Based on unit volume of reactor, if different from the rate based on unit volume of fluid




In homogeneous systems the volume of fluid in the reactor is often identical to the volume of reactor. In such a case V and Vr are identical.
so that

RATE OF REACTIONS

Rate of reactions

The rate of reaction tells us how fast a number of moles of one chemical species are being consumed to form another chemical species.

• The term chemical species refers to any chemical component or element with a given identity.
• The identity of a chemical species is determined by the kind, number, and configuration of that species atoms.

Chemical reaction has taken place when a detectable number of molecules of one or more species have lost their identity and assumed a new form by a change in the kind or number of atoms in the compound and/or by a change in structure or configuration of these atoms.

• In this classical approach to chemical change, it is assumed that the total mass is neither created nor destroyed when a chemical reaction occurs.
• The mass referred to is the total collective mass of all the different species in the system.
• However when considering the individual species involved in a particular reaction, we do speak of the rate of disappearance of mass of a particular species.
• The rate of disappearance of a species say species A is the number of A molecules that lose their chemical identity per unit time per unit volume through the breaking and subsequent re-forming of chemical bonds during the course of the reaction.
• In order for a particular species to "appear" in the system some prescribed fraction of another species must lose its chemical identity.
• There are three basic ways a species may lose its chemical identity:

1. Decomposition:- ln decomposition the molecule loses its identity by being broken down into smaller molecules. atoms. or atom fragments. 
2. Combination:- A molecule may lose its species identity is through combination with another molecule or atom.
3. Isomerization:- Here, although the molecule neither adds other molecules to itself nor breaks into smaller molecules. it still loses its identity through a change in configuration.