Question

What do you understand by the rate of a reaction? Explain why the rate of a reaction cannot be measured by dividing the total amount of reactant consumed by the time taken.

Answer 1

A chemical reaction is said to occur when the reactants are converted into products. The concentration of reactants, presence of a catalyst, temperature, and pressure affect the rate of reaction.

Complete answer:
The rate of a reaction is defined as the speed of the reaction at which the reactants are converted into products. The rate is mainly affected by the time taken for the conversion of reactant substances. The reaction can occur on its own spontaneously or sometimes a catalyst has to be added to initiate the reaction. A catalyst speeds up the chemical reaction and lowers the reaction time.
The state of equilibrium is maintained when there is a balance between the concentration of reactants and products. The initial rate of the reaction is zero, but as the reaction proceeds, it increases per unit time.
Mathematically the rate of the reaction is defined as the ratio of the concentration of products to the time taken.
For example, In the reaction: $aA\text{ + bB }\to \text{ cC}$
Where A and B are the concentration of reactants, C denotes the concentration of products, and (a, b, c) are the stoichiometric coefficients.
Rate is dependent on the concentration of individual reactants.
$Rate\text{ = -}\dfrac{1}{a}\dfrac{\Delta [A]}{\Delta t}\text{ = -}\dfrac{1}{b}\dfrac{\Delta [B]}{\Delta t}\text{ = }\dfrac{1}{c}\dfrac{\Delta [C]}{\Delta t}$

Note:
The rate of a reaction is increased if the concentration of the reactants is increased. The speed of a reaction is never the same. The time interval also alters the reaction rate. While writing the rate of a reaction, place a minus sign before the rate of reactants as their concentrations are decreased.