2.5 Chemical Reactions

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Ch. 2 Contents

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------------------Online------------------ ----wilcox@sunline.net Ch. 2 : The Chemical Context of Life

2.5 Chemical reactions make and break chemical bonds

Reactant molecules are said to enter into a chemical reaction to produce product molecules. The example below illustrates two gasses, hydrogen and oxygen, reacting to produce water. The ball and stick models illustrate what is happening, and below is the chemical equation written using the chemical symbols. The numbers in the equation allow the equation to be "balanced." This reaction goes to completion, with all the reactants being converted into product.

Many chemical reactions do not proceed to completion. As product builds up, the product molecules can break apart into the reactants. The hydrogen gas below combines with nitrogen gas to form ammonia; and ammonia then breaks apart to form hydrogen and nitrogen gases. Such reactions reach an equilibrium point where the concentration of reactants and products stay the same.

Such reactions are often said to be "dynamic equilibrium" reactions, because the reactions are still proceeding in both directions, but the amount of reactants and product stays the same.

In what way does your "checking account" resemble a dynamic chemical equilibrium?

Photosynthesis

The chemical equation for photosynthesis, showing the reactants and products, is written below.

Most of the oxygen we breath is a byproduct of photosynthesis.

All of the food we eat comes directly or indirectly from photosynthesis.

Summary: You should understand

"reactant" molecules are converted into "product" during a "chemical reaction"
some reactions can run in only one direction and they proceed to completion
other reactions can run reverse, and these are called "equilibrium reactions"
if the reactions can run in reverse, and forward, at the same time, it is called a dynamic equilibrium
chemical reactions can be written in "equation" form
photosynthesis produces both sugar and oxygen as "product" molecules

Test your knowledge: Practice Quiz # 51.

Go to Ch. 2 Contents and click on
"Terms and Figures."