Displacement Reactions - In displacement reactions, there is an exchange of atoms (or groups of atoms) between molecules. There are two types of displacement reactions, single and double displacement. In a single displacement reaction, such as A + BC « AC + B, one element shifts position. In living systems, for example, hemoglobin (an iron-containing molecule) can combine with CO2 from the cells to form carbamino hemoglobin. When this carbamino hemoglobin is transported by the blood to the lungs, the carbon dioxide is displaced by O2. This single displacement reaction can be represented as follows:
In a double displacement, as in AB + CD « AC + BD, two elements shift position. A double displacement reaction occurs when silver nitrate reacts with hydrochloric acid to form silver chloride and nitric acid.
All four types of reactions take place during the various life processes in living things, such as the utilization of my breakfast. Often the reactions occur in pairs something is taken apart (degraded) in order to be put together (synthesized) in another arrangement. Such an arrangement is called a paired reaction. It is rather like cutting apart several types of board in order to build a bookcase. You have the same wood when you have finished that you had when you started, but the structure and arrangement are different. Paired reactions are an essential characteristic of chemical reactions in living things. In other words, if you put things together, you can usually break them apart. In fact, most chemical reactions are reversible. Chemists use a pair of arrows to indicate a reversible reaction:
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