14.3: Reaction Conditions and Rate

Introduction

In this section we will introduce some of the conditions that influence the rate of reaction. Later in this chapter we will take a deeper look at each of these conditions, and we are covering them now to give you a big picture of where we are going. The four major conditions we will cover are:

1. Concentration
2. Temperature
3. Catalyst
4. Surface Areas (heterogeneous systems)

Note: Many texts also discuss the nature of the reactants, which could be added to the above list.

Concentration

The effects of concentration can vary depending on the reaction mechanism. Many reactions require two reactant particles to collide as reactant bonds are broken and product bonds are formed. From bimolecular collision theory, the frequency of collisions depends not simply on the total number of molecules present, but on their concentration. The more concentrated the reactants are, the greater the collision frequency and, therefore, potentially the faster the reaction rate. Other processes do not require collisions. In radioactive decay, for example, an unstable nucleus spontaneously decomposes into another nuclide while emitting radiation. The total rate of radioactive decay depends on the number of radioactive nuclei present, but not on their collision frequency. The following YouTube video demonstrates a reaction whose rate is influenced by concentration..

Video \(\PageIndex{1}|) 2:18 YouTube uploaded by Learn Science demonstrating the effect of reactant concentration on reaction rate (https://youtu.be/LEkXQxHtbMM)

Temperature

Increased temperature means increased molecular kinetic energy, which can increase reaction rate in several ways. Most importantly, at higher temperatures a larger fraction of molecules have enough energy for collisions to overcome the activation-energy barrier and lead to reaction. We will examine this in more detail when we discuss the Arrhenius equation. Higher temperature also increases collision frequency because molecules are moving faster, but this effect is generally less important than the increase in the fraction of collisions having sufficient energy to react. Conversely, cooling slows reaction rates. Refrigerating food, for example, slows oxidation and other chemical reactions, allowing food to last longer. The following YouTube video shows the effect of temperature on an iodide clock reaction.

Video \(\PageIndex{2}\): 2:34 YouTube uploaded by Katherine Mirica demonstrating the temperature effect on a chemical reaction.

In the lab we will be running a similar experiment to this.

Catalyst

A catalyst influences the mechanism of a reaction. It is a reactant that is consumed in an early step and produced in a later step, and thus can be used over and over. We will be discussing these in great detail when we get to section section 14.5.

Surface Area

Test Yourself

Homework: Section 14.3

Robert E. Belford (University of Arkansas Little Rock; Department of Chemistry). The breadth, depth and veracity of this work is the responsibility of Robert E. Belford, rebelford@ualr.edu. You should contact him if you have any concerns. This material has both original contributions, and content built upon prior contributions of the LibreTexts Community and other resources, including but not limited to:

• Liliane Poirot