Key chemical reactions

2. A- B + C

3. A + B^C

4. A + B^C + D

There are four main types of chemical reac­tions,which can be gener­alized by the simple equa­tions above. In 1, a single substance A undergoes a chemical change to pro­duce a different substance

B. In 2, the single substance
A decomposes to form two
new substances B and C. In
reaction type 3, two sub­
stances A and B combine to
form a third, new substance

C. And finally in 4, the reac-
tants A and B take part in a
chemical reaction to form
the two products C and D.
Reactions of type 4 are the
most common. They may in­
volve more than two reac-
tants or give more than two
products.

Electrolysisis a key chemi­cal reaction. Water, which contains a little acid to make it a better conductor of electricity, is used. When an electrical current (DC) is passed through the water, oxygen gas is evolved at the positive electrode (anode) and hydrogen gas is evolved at the negative elec­trode (cathode). The volume of hydrogen produced is ex­actly twice the volume of ox­ygen. This is because each molecule of water contains two atoms of hydrogen and one atom of oxygen.

The world around us changes continually. Some of this change is purely physical. When water boils, for example, the liquid water turns to gaseous steam. In the sub-zero tempera­tures of winter, a puddle turns to solid ice. These are both purely physical changes. Alter­ing the temperature alters the physical form of the water. But the molecules that make up the water remain unchanged. Each molecule is still composed of two hydrogen atoms linked to an oxygen atom (H₂0).

On the other hand, if we pass an electric current through water, a mixture of gases is produced. Under everyday conditions, we can­not turn these gases back into liquids, let alone solids. The molecules that make up these gases are no longer water molecules. They are a mixture of hydrogen (H₂) and oxy­gen (0₂). If a spark is applied to the mixture, it explodes as the gases recombine to produce water. Both of these are chemical, not physi­cal, changes.

This behavior demonstrates two important principles of chemistry. First, under certain conditions, substances change into wholly dif­ferent substances. A chemical change—known as a reaction—is one in which the molecules are not the same at the end of the process as they were at the beginning. Second, we can change water into different substances, then change these back into water. Thus, at least one chemical reaction must be reversible. In fact, under the right conditions, nearly all chemical reactions are reversible.

Date: 2015-12-11; view: 1786