Question

Explain reaction rate with definition.

Answer 1

Hint: Reaction rate is the speed at which a chemical reaction is taking place. It can be defined in the terms of the amount of reactants consumed or the amount of products formed during a chemical reaction. So, basically it can be expressed as the change in concentration of reactants and products in unit time.

Complete answer:
In order to understand what a reaction rate is, we need to understand what happens in a chemical reaction. In a reaction the reactants get consumed and convert into products.
So, we can say that the reaction rate can be defined as the change in concentration of reactants (as reactants are being consumed) or change in concentration of products (as products are being formed) in unit time in a particular reaction.
Let us understand reaction rate with the help of an example:
Suppose, there is one mole of reactant A which in a reaction produces one mole of product B.
 $A\to B$
Initially, at time $t_1$ , the concentration of reactant A is $[A{]}_1$ and concentration of product B is $[B{]}_1$ , While at time $t_2$ , the concentration of A is $$ {[A]_2} $$ and B is $[B{]}_2$ .
Here, the square brackets represent the molar concentration.
Now, we can determine the change in time,
 $\mathrm{\Delta }t=t_2-t_1$
Change (decrease) in concentration of reactant A ,
 $\mathrm{\Delta }[A]=[A{]}_2-[A{]}_1$
Therefore, change in concentration of reactant A will be a negative value as concentration of reactants decrease with time because they get consumed in a chemical reaction.
Change (increase) in concentration of product B,
 $\mathrm{\Delta }[B]=[B{]}_2-[B{]}_1$
Therefore, change in concentration of product B will be a positive value as concentration of products increase with time because they get produced in a chemical reaction.
Now, the rate of disappearance of reactant A is given by,
 $Rate=-{\displaystyle \frac{\mathrm{\Delta }A}{\mathrm{\Delta }t}}\mathrm{\_}\mathrm{\_}\mathrm{\_}\mathrm{\_}\mathrm{\_}\mathrm{\_}(1)$
Rate of appearance of product B is given by,
 $Rate=+{\displaystyle \frac{\mathrm{\Delta }B}{\mathrm{\Delta }t}}\mathrm{\_}\mathrm{\_}\mathrm{\_}\mathrm{\_}\mathrm{\_}\mathrm{\_}(2)$
Therefore, we can say that the average rate of reaction is given by equations (1) and (2). Where, the average rate of reaction is defined as the change in concentration of reactants or products in unit time in specified time periods.
Unit of rate of a reaction is, $$ mol{L^{ - 1}}{s^{ - 1}} $$ ( concentration is in mol per litre and time is in seconds).
Now, if we express Instantaneous rate of reaction, it is defined as the rate of reaction at a particular instant of time (infinitesimally small interval of time).
So, instantaneous rate of reaction is given as:
 $r_{inst}=-{\displaystyle \frac{d[A]}{dt}}=+{\displaystyle \frac{d[B]}{dt}}$
A rate law is also given which expresses reaction rate in terms of molar concentrations of reactants with each raised to some power which may or may not be equal to the stoichiometric coefficients of that reactant in a balanced chemical equation.
Let us consider a chemical reaction:
 $aA+bB\to cC+dD$
Where, A, B are reactants , C,D are products and a,b,c,d are stoichiometric coefficients of these reactants and products.
Therefore, the rate law expression can be written as:-
 $rate=k[A{]}^x[B{]}^y$
Where, k is a rate constant.

Note:
There are some factors which influence the rate of reaction such as, concentration of reactants and products, temperature, pressure, etc. The rate depends on the power of molar concentrations of reactants in rate law, which is known as order of reaction. Order can be calculated by adding the powers of all molar concentrations of reactants in the experimental rate law expression. Therefore, order is an experimental value.