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Kinetics Reaction Rates Study Guide

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Updated on Sep 25, 2011

Introduction

The rate of a reaction describes the speed at which the reactants disappear and products appear. Depending on the chemical reaction and components, reactions rates may be linear (constant) or change over time (accelerate or decelerate).

Reaction Rates

The rates of separate chemical reactions can be drastically different. Some reactions occur over a long period of time, such as the convergent of carbon into diamond. Other reactions occur very quickly, such as when sodium carbonate is placed in water. The rate of a reaction is defined as the change in concentration per unit time:

Rate Laws and Reaction Orders

Rates are affected by three properties:

1. Temperature: The rates of reactions increase with the temperature as more collisions among particles occur at higher temperatures.
2. Particle size: Smaller particles react faster as they collide, often at any given temperature and concentration.
3. Concentration: A high concentration of reacting particles increases the rate of chemical reactions among them.

These factors are incorporated in the rate equation for a chemical reaction. For the reaction of A + B yields C, the rate equation is

The k is the rate constant that is dependent on the temperature. The a and b exponents represent the order of the concentrations of A and B. These orders can be proportional to the stoichiometry of the reaction or other experimentally determined value. The concentrations of the reactants can be plotted over time to determine the exact order of each reactant. The overall reaction order is the sum of all the exponents. Several reactions and their rate laws are provided in Table 17.1.

Table 17.1 Rate Laws

Example:

The following equation represents the oxidation of the bromide ion:

5Br + BrO3 + 6H+→ 3Br2 + 3H2O

The reaction is first-order bromide ion, first-order bromate ion, and second-order hydrogen ion. What is the rate law and overall order? If the proton concentration quadruples, what is the effect on the rate?

Solution:

Rate = k[Br][BrO3 ][H+]2

Order = 1 + 1 + 2 = 4th order (summation of the exponents of the rate law equation) If the proton concentration quadruples, then the relative effect will be 42, or 16 times faster.

Activation Energy

Activation energy is the minimum amount of energy required for reactants to be transformed into products (i.e., to overcome the energy barrier between reactants and products).When the activation energy is high, the reaction is slower.

Catalysis

Catalysts are species that speed the reaction rate by lowering the activation energy of the reaction. They are not consumed in the reaction.

 

Practice problems for these concepts can be found at - Kinetics Reaction Rates Practice Questions

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