This chapter covers material from Topics 6 and 16 of the IB syllabus.

CORE

SPREAD 1: Collision theory and rates of reaction

Species react as a result of collisions of sufficient energy and proper orientation.

The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.

Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and color.

Introduction

Chemical reactions occur: we know that, but one of the truths of all chemical reaction is that we can have some influence in how quickly the reaction goes.

Kinetics is a topic that has almost certainly been met in studying chemistry on prior courses and this book will not dwell on the basics, although there are essentially five ways in which this can be sped up:

Students would not be expected to recall these but might be expected to recall why they affect reaction rate.

For a chemical reaction to occur reactant particles (which could be atoms, molecules, ions or radicals) must collide in the right orientation and with sufficient energy. All chemical reactions have an activation energy that is uniquely associated with it (see previous chapter); this must be overcome if the collision is to be successful. The orientation is related to precisely where the collision occurs on each molecule. This is less relevant for ions, which are often spherical but for complicated organic molecules only one small part of the molecule might actually be “reactive” to the other reagent.

What is rate of reaction?

Students often get confused as to what reaction rate actually is, confusing it with time. Reaction rate is the speed of the chemical reaction, and it is usually expressed as the change in concentration of a particular reactant/product per unit time:

For example, moldm-3s-1

However if a reaction is particularly slow this could be changed to be per minute, or even per hour.