Laboratory manual of chemistry, for secondary schools by unknow

Author:unknow
Language: eng
Format: epub
Tags: Chemistry
Publisher: New York, D. Appleton and company
Published: 1905-03-25T05:00:00+00:00

It will now be our purpose to study the metallic or base-forming elements; and it appears best to take up first those having the most strongly marked characteristics of metals. Those elements are sodium, potassium of Group I, magnesium and calcium of Group II, etc. There are, however, certain incidental properties of solubility and precipitation which enable us to detect the presence of metals by analysis.

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LABORATORY MANUAL OF CHEMISTRY 106

For purposes of analysis the classification of metallic elements does not follow strictly the order by families shown in the periodic system. In our study of metals we shall, therefore, follow the best order for analysis, and not adhere strictly to the periodic system.

We shall take up sodium, potassium, lead, silver, mercury, in the order named. Follow with a study of copper, iron and aluminium, zinc, calcium, barium, and magnesium. (For study of sodium and potassium, see text.)

Note. —In the following experiments use only chemically pure (C.P.) reagents and distilled water.

LEAD

Occurrence. The most important source of lead in nature is lead sulphide, PbS, known as galena. Lead occurs also in combination as the sulphate, carbonate, and chromate.

Metallurgy, Definition: Metallurgy is the process of extracting a metal from its ores. Two methods are in use for the extraction of lead from its ores:

1. By heating lead sulphide with metallic iron, free lead and iron sulphide are obtained. PbS + Fe = FeS

+ pb.

2. By roasting the sulphide it is partly changed to lead oxide and lead sulphate. Upon heating a mixture of the two compounds in a closed vessel with more lead sulphide, the whole of the lead is obtained as the free element.

Solubility. Nitric acid is the best solvent for lead. Lead nitrate, nitric oxide, and water are the products. The chemical equations for all reactions of this kind are based upon accurate quantitative experiments. An inspection, however, of the class of reactions represented by the solution of lead reveals the fact that each reaction takes place in accordance with a definite law of oxidation and

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106 LABORATORY MANUAL OF CHEMISTRY

reduction. A clear understanding of this law is a great aid in chemical calculations. In its several clauses, therefore, we shall state it at this point of our work.

Laws of Oxidation and Beduction

1. Measure. Oxidation (or reduction) is measured in units of valence; the valence of hydrogen = 1. In compounds, H is the standard unit.

2. Free elements. The valence of a free element is zero.

Z. Hydrogen and oxygen. In compounds, the valence of hydrogen is always -|- 1, of oxygen always — 2. The valence for other elements varies; for hydrogen and oxygen in compounds it is constant.

4. Oxidation and reduction. An increase in valence from a negative valence up to zero, or from zero to a positive unit of valence, is oxidation. Arty decrease in valence is reduction.

5. Relation between oxidation and reduction. In any reaction, the total amount of oxidation in units of valence always equals the total reduction in units of valence. We hase our equations upon this principle.

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