Average Rate Vs Instantaneous Reaction Rate

As per the general definition, the rate of chemical reaction is defined as a speed with which a reactant is converted into product or a speed at which a reaction takes place. 
Thus, we can say that when a reaction is takes place, then the concentration of reactant decreases with time while the concentration of the product increases with time and after the completion of the reaction, the whole reactant is converted into product.
......."The Rate Of A Chemical Reaction Is The Change In The Concentrations Of Reactants Or Products In A Given Time Interval"........

Average Rate Law Vs Instantaneous Rate Law???? 
When we define the rate of change in the concentration of a reactant over a time interval, the reaction rate is known as the average rate of that reaction. Whereas, when we define the rate of change in the concentration of a reactant at a particular time, the reaction rate is known as the instantaneous rate of that reaction.
A ➞ B
➩ ravg = - ∆[A]/∆t =∆[B]/∆t 
If, ∆t ➝ 0 then;
➩ rInst = - d[A]/dt = d[B]/dt 
Here,                                                                                
rIns – Represent instantaneous reaction rate 
ravg – Represent average reaction rate

......."Instantaneous Reaction Rate Is defined for Tangent Line (At A Particular Instant Of Time) While Average Reaction Rate Is Defined For Secant Line ( For Time Interval)".....
.....Average Reaction Rate Between Time t₁&t₂ Whereas Instantaneous Reaction Rate At Any Instant Of Time.......

Average Vs Instantaneous
Average reaction rate or instantaneous reaction rate both the reaction rates are useful. Let's see a small comparison between them;
  • Average reaction rate is defined for the entire time interval, whereas instantaneous reaction rate is defined at a particular instant of time.
  • Average rate of reaction remains constant for a given time interval and do not give any idea about a reaction at a particular instant of time. This is where the instantaneous rate of reaction comes into picture because it gives us idea about a reaction at any particular instant of time.
  • For all practical purposes, the instantaneous rate of reaction is used because average rate of reaction doesn't give the exact information in most cases about the completion of the reaction.
  • When the time interval "∆t" tends to zero, the average rate of reaction is converted into instantaneous rate of reaction.

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