The key difference between reaction rate and rate constant is that reaction rate is the speed at which reactants are converted into products whereas rate constant is a coefficient of proportionality relating the rate of a chemical reaction at a given temperature to the concentration of the reactant or to the product of the concentrations of reactants.
When one or more reactants are converted to products, they may go through different modifications and energy changes. The chemical bonds in the reactants get broken, and new bonds are formed to generate products which are totally different from the reactants. This chemical modification is known as chemical reactions. Reaction rate and rate constant are important chemical concepts that can provide more information about chemical reactions.
CONTENTS
1. Overview and Key Difference
2. What is Reaction Rate
3. What is Rate Constant
4. Side by Side Comparison – Reaction Rate vs Rate Constant in Tabular Form
5. Summary
What is Reaction Rate?
The reaction rate or rate of reaction is the speed at which reactants are converted into products. Reaction rate is simply the indication of the speed of the reaction. Therefore, we can consider it as a parameter which determines how fast or how slow the reaction is. 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 slow reaction, which takes place over the years. In contrast, reaction of a piece of potassium with water is very rapid, producing a large amount of heat; thus, it is considered as a vigorous reaction.
Consider the following reaction where reactants A and B are transformed into products C and D.
a A + b B ⟶ c C + d D
We can give the rate for the reaction in terms of either of the two reactants or products.
Rate = -(1/a) (dA/dt) = -(1/b) (dB/dt) = (1/c) (dC/dt) = (1/d) (dD/dt)
Here, a,b,c and d are stoichiometric coefficients of the reactants and products. For the reactants, we should write the rate equation with a minus sign because the products deplete as the reaction proceeds. However, as the products increase, we have to use positive signs.
Figure 01: Increase of a Particular Reaction Rate with Increasing Temperature
Chemical kinetics is the study of the reaction rates, and there are many factors affecting the speed of the reaction. These factors include concentrations of the reactants, catalysts, temperature, solvent effects, pH, product concentrations, etc. We can optimize these factors to have the maximal reaction rate, or we can adjust these factors to manipulate the required reaction rates.
What is Rate Constant?
The rate constant is a coefficient of proportionality relating the rate of a chemical reaction at a given temperature to the concentration of the reactant or to the product of the concentrations of reactants. If we write the rate equation in relation to the reactant A for the reaction given above, it is as follows.
R= -K [A]a [B]b
In this reaction, k is the rate constant. It is a proportionality constant that depends on the temperature. We can determine the rate and the rate constant of a reaction by experiments.
What is the Difference Between Reaction Rate and Rate Constant?
The key difference between reaction rate and rate constant is that reaction rate or rate of reaction is the speed at which reactants are converted into products whereas rate constant is a coefficient of proportionality relating the rate of a chemical reaction at a given temperature to the concentration of the reactant or to the product of the concentrations of reactants. Both the reaction rate and the rate constant give an indication of the reaction speed. However, the rate constant alone cannot give a valid statement of the reaction speed.
Summary – Reaction Rate vs Rate Constant
The key difference between reaction rate and rate constant is that reaction rate or rate of reaction is the speed at which reactants are converted into products whereas rate constant is a coefficient of proportionality relating the rate of a chemical reaction at a given temperature to the concentration of the reactant or to the product of the concentrations of reactants.
Reference:
1. Helmenstine, Anne Marie. “What Is the Rate Constant in Chemistry?” ThoughtCo, Sep. 27, 2018, Available here.
Image Courtesy:
1. “Reaction Rate Increase with Increasing Temperature” By Brazosport College – Own work (CC BY-SA 3.0) via Commons Wikimedia