SAT Subject Test Chemistry
REVIEW OF MAJOR TOPICS
Some Representative Groups and Families
The most common member of this family is nitrogen itself. It is a colorless, odorless, tasteless, and rather inactive gas that makes up about four-fifths of the air in our atmosphere. The inactivity of N2 gas can be explained by the fact that the two atoms of nitrogen are bonded by three covalent bonds that require a great deal of energy to break. Since nitrogen must be “pushed” into combining with other elements, many of its compounds tend to decompose violently with a release of the energy that went into forming them.
Nature “fixes” nitrogen, or makes nitrogen combine, by means of a nitrogen-fixing bacteria found in the roots of beans, peas, clover, and other leguminous plants. Discharges of lightning also cause some nitrogen fixation with oxygen to form nitrogen oxides.
An important compound of nitrogen is nitric acid. This acid is useful in making dyes, celluloid film, and many of the lacquers on cars.
Its physical properties are: it is a colorless liquid (when pure), it is one and one-half times as dense as water, it has a boiling point of 86°C, the commercial form is about 68% pure, and it is miscible with water in all proportions.
Its outstanding chemical properties are: the dilute acid shows the usual properties of an acid except that it rarely produces hydrogen when it reacts with metals, and it is quite unstable and decomposes as follows:
4HNO3(aq) → 2H2O(l)+4NO2(g)+O2(g)
Because of this ease of decomposition, nitric acid is a good oxidizing agent. When it reacts with metals, the nitrogen product formed will depend on the conditions of the reaction, especially the concentration of the acid, the activity of the metal, and the temperature. If the nitric acid is concentrated and the metal is copper, the principal reduction product will be nitrogen dioxide (NO2), a heavy, red-brown gas with a pungent odor.
Cu(s)+4HNO3(aq) → Cu(NO3)2(aq)+2NO2(g)+2H2O(l)
With dilute nitric acid, this reaction is:
3Cu+8HNO3 → 3Cu(NO3)2+4H2O + 2NO(g)
The product NO, called nitric oxide, is colorless and is immediately oxidized in air to NO2 gas.
With still more dilute nitric acid, considerable quantities of nitrous oxide (N2O) are formed; with an active metal like zinc, the product may be the ammonium ion (NH4+).
When nitric acid is mixed with hydrochloric acid, the mixture is called aqua regia because of its ability to dissolve gold.