Energy and chemical change
In Introducing Physical and Chemical Change, we learnt about physical and chemical changes. In this tutorial, we will learn about the energy changes that occur in chemical reactions. The concepts of exothermic and endothermic reactions are introduced. We will also learn about activation energy. The following list summarises the concepts to be covered next.
Bond energy and how it leads to the energy changes in reactions.
In Bonding and Atomic Combinations, the concept of bond energy and graphs of potential energy versus atomic distance were covered. These two concepts form the cornerstone to understanding the energy changes in chemical reactions. In this topic the fact that bond forming requires energy and bond breaking releases energy is introduced. These two concepts are linked to the potential energy graph for bonding.
Exothermic and endothermic reactions.
Two classes of chemical reaction exist: exothermic and endothermic. It is important to note that these are not different types of reactions. For example an acid-base reaction can be exothermic or endothermic. All chemical reactions will either be exothermic or endothermic and this is determined by the energy of bond formation and bond breaking.
Energy diagrams for reactions with and without activation energy.
We can draw energy diagrams to show how a reaction proceeds. These diagrams give the reactants energy and the products energy. These diagrams show the energy of the system as a whole and are not concerned with just one reactant or one product.
A practical demonstration that can be done is to burn magnesium ribbon in air and in oxygen to investigate the concept of activation energy.
All reactions (even the exothermic ones) need something to get them going. This may be very small or may be very large. At the maximum energy of the reaction the transition state or activated complex occurs. This is the point at which the reaction is somewhere between forming the products and breaking apart the reactants.
Introducing Energy Changes In Chemical Reactions
You have probably seen a fire burning or burnt fuel for warmth or cooking or light. A fire burning is one of the most noticeable examples of a chemical reaction that produces a lot of energy.
All chemical reactions involve energy changes. In some reactions, we are able to observe these energy changes as either an increase or a decrease in the overall energy of the system. In some reactions we see this as a change in the temperature. In other reactions we can observe this change when a reaction starts to give off light or when a reaction will only work after light is shone on it.
The study of energy changes (particularly heat) in chemical reactions is known as chemical thermodynamics. This is also sometimes called thermochemistry.