Rate Law and Specific Rate Constant

Learning Objectives

  • Define rate law.
  • Define specific rate constant.

ExamplesWhat are the migration patterns of the current population?

Map of North Carolina showing the population changes in counties. The rate of change of a quantity is often very important.

State of North Carolina Population Change. Courtesy of the US Census/Wikimedia.

Where are people moving from and where are they moving to? How fast is the population changing in different areas? These are important questions for people involved in deciding about where to build schools or hospitals or where to open new businesses. If an area is growing rapidly, action needs to be taken soon to accommodate the growth. How fast the growth is will determine how many schools to build. Rates of change affect a lot of decisions.

Rate Law and Specific Rate Constant

Consider a simple chemical reaction in which reactant A is converted into product B according to the equation below.

A rightarrow B

The rate of reaction is given by the change in concentration of A as a function of time. The rate of disappearance of A is also proportional to the concentration of A .

-frac{Delta [A]}{Delta t} alpha [A]

Since the rate of a reaction generally depends upon collision frequency, it stands to reason that as the concentration of A increases, the rate of reaction increases. Likewise, as the concentration of A decreases, the rate of reaction decreases. The expression for the rate of the reaction can be shown as follows:

text{rate}=-frac{Delta [A]}{Delta t} quad text{or} quad text{rate}=k[A]

The proportionality between the rate and [A] becomes an equal sign by the insertion of a constant (k). A rate law is an expression showing the relationship of the reaction rate to the concentrations of each reactant. The specific rate constant (k) is the proportionality constant relating the rate of the reaction to the concentrations of reactants. The rate law and the specific rate constant for any chemical reaction must be determined experimentally. The value of the rate constant is temperature dependent. A large value of the rate constant means that the reaction is relatively fast, while a small value of the rate constant means that the reaction is relatively slow.

Summary

  • The rate law and specific rate constant are defined.

Practice

Read the material at the link below and answer the following questions:

http://www.differencebetween.com/difference-between-reaction-rate-and-vs-specific-rate-constant/

  1. Why is the sign for reactant concentration negative?
  2. Why is the sign for product concentration positive?
  3. What do the lower-case letters in the rate equation represent?

Review

  1. What is a rate law?
  2. What is a specific rate constant?
  3. How are these parameters determined?

 Glossary

  • rate law: An expression showing the relationship of the reaction rate to the concentrations of each reactant.
  • specific rate constant (k) : The proportionality constant relating the rate of the reaction to the concentrations of reactants.