Chemistry

I

Weight Relations

The atomic weight of an element, which is given on the periodic table, is the average mass of atoms of an element (since most elements consist of different isotopes, the differing weights of these isotopes are taken into account). For example, in a naturally occurring sample of nitrogen (N), the average weight of the nitrogen atoms is 14.00674 atomic mass units. The proportions by weight of elements in a compound can be determined from the chemical formula of the compound and the atomic weights of the component elements. For example, the proportions by weight of elements in Fe₂O₃, ferric oxide, which forms when iron rusts, can be determined by finding the atomic weights of iron and oxygen on the periodic table and adding them:

The fraction of iron in pure ferric oxide is 111.6940/159.6922 = 0.6994, or 69.94 percent. The quantity 159.6922 is called the formula weight or molecular weight of the compound.

Extending these weight concepts to chemical equations makes possible the calculation of how much of each reactant will be needed to react (without leaving one in excess) and how much of the various products are formed. Thus, in the oxidation of iron: 4Fe_(s) + 3O_2(g) → 2Fe₂O_3(s), 223.3880 grams (4 x 55.8470) of iron reacts with 95.9964 grams (6 × 15.9994) of oxygen to produce 319.3844 grams (2 × 159.6922; or 223.3880 + 95.9964) of ferric oxide. In this way, the amount of ferric oxide formed from a given amount of iron can be calculated.

J

Gas Volumes in Chemical Reactions

At the same temperature and pressure, equal volumes of different gases contain identical numbers of molecules (see Avogadro’s Law). The volumes and number of molecules in a chemical reaction all relate to one another in identical ratios (see The Gas Laws section below).

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When methane (CH₄), the main constituent of natural gas, is burned, the balanced equation for the reaction is CH_4(gas) + 2O_2(gas) → CO_2(gas) + 2H₂O_(gas). This equation shows that one volume of methane needs two volumes of oxygen to burn it, and that one volume of carbon dioxide and two volumes of water vapor are produced. The volumes are compared at the same temperature and pressure.

IV

Physical Properties

Physical properties of a material are those properties that do not depend on the chemical behavior of the material. Physical properties include the state of a material (gas, liquid, or solid), melting point, boiling point, crystal structure, and electrical conductivity.

A

State

The state of a material is determined by the attraction between its atoms or molecules and by the temperature of the material. In the solid state, the attraction between the atoms or molecules is so strong that it holds them rigidly in place. The energy of vibration of the molecules of a material increases with a rise in temperature. As the temperature rises, the molecules eventually acquire enough energy to break away from their fixed positions, and the solid either melts or transforms directly into gas (a process called sublimation). The material melts if the molecular attraction remains great enough to hold the molecules together, and the material sublimes to a gas (in which the molecules are free to move randomly) if the attraction is too small.

B

Melting Point

The melting point (or freezing point) of a substance is the temperature at which the solid form of the substance changes to a liquid (or from liquid to solid). The melting point of water is 0° on the Celsius (centigrade) temperature scale and 32° on the Fahrenheit scale (see Freezing Point).

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