Difference Between Reaction Rate and Reaction Time

Reaction Rate vs Reaction Time
 

Reaction rate and reaction time are inter-dependant variables. Reaction rate of a reaction determines the time it will take to complete the reaction to a certain extent.

Reaction Rate

Reaction rate is simply the indication of the speed of the reaction. Therefore, it can be regarded as a parameter, which determines how fast or how slow is a reaction. Naturally, some reactions are very slow, so we cannot even see the reaction taking place unless we observe it for a very long time. For example, rock weathering by chemical processes is a very slow reaction, which takes place over the years. In contrast, reaction of a piece of potassium with water is very rapid; thus, producing a large amount of heat and it is considered as a vigorous reaction.

Consider the following reaction where reactants A and B are going to the products C and D.

a A + b B  → c C + d D

The rate for the reaction can be given in terms of either of two reactants or products.

Rate =  -(1/a) d[A]/dt = -(1/b) d[B]/dt = (1/c) d[C]/dt = (1/d) d[D]/dt

Here, a, b,c and d are stoichiometric coefficients of the reactants and products.  For the reactants, the rate equation is written with a minus sign, because the products are depleting as the reaction proceeds. However, as the products are increasing, they are given positive signs. 

Chemical kinetics is the study of the reaction rates, and there are many factors affecting the speed of the reaction. These factors are concentrations of the reactants, catalysts, temperature, solvent effects, pH, sometimes the product concentrations, etc. These factors can be optimized to have the maximal reaction rate or can be adjusted to manipulate the required reaction rates. If we write the rate equation in relation to the reactant A for the above-given reaction, it will be as follows.

R= -K [A]^a [B]^b

In this reaction, k is the rate constant.  It is a proportionality constant, which depends on the temperature. Rate and the rate constant of a reaction can be found by experiments.

Reaction Time

When one or more reactants are converting to products, they may go through different modifications and energy changes. The chemical bonds in the reactants break and new bonds are formed, to generate products, which are totally different from the reactants. This kind of chemical modification is known as chemical reaction. The time taken to complete a reaction to a certain extent is known as reaction time. The time depends on various factors that affect the reaction. For example, particle size of reactants, concentrations, their physical states, temperature, and pressure are some of the factors, which affect the time to complete a reaction. Other than for the completion time of reaction, we can measure time throughout the reaction. For example, we can measure the half reaction time. Therefore, there is no specific definition for reaction time. Rather, we measure time according to our experiment needs.