RRE - Reaction Rates (Lesson)
Reaction Rates
Introduction
Chemical reactions differ significantly in the speed at which they occur. Some are basically instantaneous, while others may take years to reach equilibrium. The Reaction Rate for a chemical reaction is the measure of the change in concentration of the reactants or the change in concentration of the products per unit time. Let’s take a closer look at reaction rates to gain an understanding of variables that influence how fast or slow a reaction occurs.
Reaction Rates Presentation
This lesson introduces kinetics and looks at how molecular motion affects gases. Students learn about collision theory and reaction rates. The teacher demonstrates how substances react with each other, using an antacid tablet immersed in water. The students design their own antacid tablet experiment to determine which conditions create the fastest rate of reaction.
Download the note taking guide for Chemistry Matters Unit 9 Segment A. Links to an external site.
Collision Theory
Before we describe reaction rates, we will use a theory to help us understand how reactions take place and how they are affected by different factors. We call this the collision theory. It states that the rate of a reaction depends upon the number of effective collisions per second among the reactant molecules. An effective collision is defined as one that actually produces a product. Only a very small percentage of collisions can really lead to a change. Why? Two conditions must be met in order for a collision to be effective.
Molecular Orientation
In most reactions, molecules have to be oriented in a certain way so that the atoms that need to collide can do so. Below is a diagram of the reaction, Cl2 + H2 → 2HCl. In this image, the Cl2 molecule and H2 molecule are oriented end to end as they approach each other. This is not the proper orientation to form 2 HCl. So, the molecules collide and bounce off each other unchanged.
In the image below, the Cl2 molecule and H2 molecule are oriented side to side so that when they meet, they are oriented properly to form 2 HCl .
Molecular Kinetic Energy
Even if the molecules are oriented correctly, they must have enough kinetic energy to overcome the repulsion of their outer electrons and actually produce a reaction. If they do not have enough energy, the molecules will simply bounce apart. The minimum kinetic energy required for the reaction to take place is called activation energy, Ea.
Potential Energy Diagrams
As mentioned in the previous module, we visualize the changes in energy that take place as molecules approach each other and then react by using a potential energy diagram. The y-axis represents the changes in potential energy as the particles collide. The x-axis is called the reaction coordinate or reaction pathway. You can think of this as the passing of time. The arrows in the activity below show how a reaction progresses and to review what each portion on the diagram represents.
Potential Energy and the Reaction Process Revealed Practice
Factors Affecting Reaction Rates
Anything that will increase the number of effective collisions in a period of time will increase the reaction rate. There are five factors that are commonly known to affect reaction rates. We will investigate each one in terms of collisions.
Reaction Rates Extension
IMAGES CREATED BY GAVS OR FREE TO USE