1. Hints. You will be able to understand chemistry more easily if you can recognize compound types without having to look them up. So here are some hints:

Acid: the simplest (Arrhenius) definition of an acid is that it is a compound containing hydrogen, which dissolves with water to produce H⁺ ions. Formulas of most acids have hydrogen as the first element, e.g. sulfuric acid, H₂SO₄.

Base: a compound that dissolves in water to produce OH^- ions. Metal hydroxides are bases. So are ammonia and organic amines, which, although they do not have OH^- in their formulas, react with water to produce hydroxide. For example:

NH₃ + H₂O   ==> NH₄⁺ + OH^-

Soluble bases include hydroxides of the Group 1A cations, along with ammonia and many organic amines. Sr(OH)₂ and Ba(OH)₂ are slightly soluble. Other metal hydroxides are generally insoluble.

Strong and weak acids and bases: strong acids and bases ionize completely in water; weak acids and bases ionize only a small amount. See Table 5.2, Kotz and Treichel, p. 193.

Acid oxide: compound of oxygen with a nonmetal; reacts with water to form an acid

Basic oxide: compound of oxygen with a metal; reacts with water to form a base (metal hydroxide). Soluble oxides include oxides of the Group 1A cations, along with SrO and BaO which are slightly soluble. Other metal oxides are generally insoluble.

Soluble and insoluble salts: A salt is a compound consisting of cations combined with anions other than oxide or hydroxide.

Soluble salts: almost all salts of sodium, potassium, and ammonium ions; all acetates, chlorates, perchlorates, and nitrates; most chlorides and sulfates

Insoluble salts: carbonates, phosphates, and sulfides are insoluble except for salts containing ammonium or Group 1A cations.

3. Hints. Note that when an Arrhenius acid reacts with an Arrhenius base, a salt and water are produced. The reaction type is called metathesis or double displacement.

Example. Complete and balance the following equation for the reaction between calcium hydroxide and triphosphoric acid:

Ca(OH)₂ + H₅P₃O₁₀  ==>

Solution. First predict the products of reaction. Determine the charges on the cation and anion so that you can combine them to produce the correct formulas for the products.

Now write the unbalanced equation:

Ca(OH)₂ + H₅P₃O₁₀  ==>  Ca ₅(P₃O₁₀)₂ + H₂O

The right hand side has 5 calcium ions and 2 triphosphate ions, so 5 calcium hydroxides and 2 triphosphoric acids are required on the left. The 10 hydroxide ions from calcium hydroxide and 10 hydrogen ions from triphosphoric acid combine to form 10 water molecules.

5 Ca(OH)₂ + 2 H₅P₃O₁₀  ==> Ca ₅(P₃O₁₀)₂ + 10 H₂O

5. Guidelines for balancing two types of redox equations There are no general rules that always work for any kind of equation, but the following guidelines may be helpful.

1. Combination reactions. A combination reaction is said to occur when two elements combine to make a compound. These can often be balanced "by inspection."

Example 1: Na + P₄  ==> Na₃P

Here, phosphorus occurs as P₄ molecules so balance the P's first (4 Na₃P will be formed). Now the 4 Na₃P on the right require 12 Na on the left to balance. Balanced: 12 Na + P₄  ==> 4 Na₃P

Example 2. Y + O₂  ==> Y₂O₃

In this reaction of yttrium with oxygen, the oxygen occurs as diatomic molecules but the product has 3 atoms of oxygen in each formula unit. We could balance with fractional numbers (2 Y + 1.5 O₂  ==> Y₂O₃), but whole numbers are preferred. Thus we double all the coefficients and the balanced equation is: 4 Y + 3 O₂  ==> 2 Y₂O₃

2. Combustion reactions. Many compounds of hydrogen with nonmetals such as C, B, and Si, burn in air to produce the nonmetal oxide (CO₂, B₂O₃, SiO₂, etc.) and water. The rules for balancing reactions such as these are given on pp. 152-3 in Kotz and Treichel, and examples are shown for combustion of hydrocarbons.

1. Reaction of a basic oxide with water.Example. Tl₂O is a basic oxide with Tl in the +1 oxidation state. Thus the formula for the base is TlOH.

2. Reaction of an acidic oxide with water. These are easiest if you determine the oxidation number of the nonmetal in the oxide and connect it to the corresponding acid. Example. P₄O₆ contains P(III); the two acids of phosphorus are phosphorous acid and phosphoric acid; phosphorous acid contains P(III) and has the formula H₃PO₃; thus the formula for reaction of P₄O₆ with water is H₃PO₃.
 
 

(a) Cr₂O₇^2-      (b) KCr(SO₄)₂      (c) Cr(NO₃)₃      (d) Cr₂(CH₃CO₂)₄      (e) Cr₂O₅

Answers: (a) +6; (b) +3; (c) +3; (d) +2; (e) +5

Keys to solution:

• Recognize the formulas and charges for ions other than chromium. Oxide, sulfate, nitrate, and acetate are represented above.
• Remember that the oxidation number of chromium refers to each atom of chromium. Example. In Cr₂O₅ each oxygen has an oxidation number of -2, so each Cr must have an oxidation number of +5.
• See Example 5.7 on p. 206 and the Table at the top of p. 207, Kotz and Treichel, for additional examples and guidelines.

 

 
 
 

Note: oxidizing agents are chemical species or compounds, not an individual atom in the species or compound. For example, in the reaction I₂ + 2 S₂O₃^2-  ==> 2 I^- + S₄O₆^2-:

I₂ (not I) is reduced; it is the oxidizing agent

S₂O₃^2- (not S) is oxidized; it is the reducing agent

Note that the oxidation number of sulfur in S₄O₆^2-is not an integer; it has the fractional value of 2.5!
 
 

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Last revised: August 20 2000 by M. F. Richardson
© Brock University, 1999