General Chemistry for the Health Care Professions

General Chemistry
for the
Health Care Professions

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2-5. EXTERNAL CONDITIONS AFFECTING CHEMICAL REACTIONS

External conditions that affect reactions are usually types of energy that are put into a reaction, such as heat or light. Chemical reactions are always accompanied by an energy change. Either energy is released or it is acquired. When the amount of energy is changed, so is the amount of matter. This is called the Law of Conservation of Matter and Energy. However, ordinary chemical reactions involve such small matter changes that they go undetected and may be ignored.

Heat. Generally, heat is the form of energy we are most concerns us most. It may affect a reaction in one of two ways.

Exothermic reactions. If a reaction gives off heat, it is called an exothermic reaction. External heat, if supplied to this type of reaction, will slow down the rate of reaction.
Endothermic reactions. If a reaction takes in heat, it is an endothermic reaction. lf heat is added to an endothermic reaction, the rate of reaction will increase. This may be of value in the preparation of medicinal products.

Light. Light is a form of energy that may cause many chemicals to decompose. For this reason, it is necessary to protect some drugs from contact with light by placing them in dark-colored or opaque containers. These containers prevent most or all of the outside light from coming into contact with the drug.

Original content of this course is supplied by Academy of Health Sciences
Fort Sam Houston, Tx.