Chemical Reactions: Types and Characteristics

1. Decomposition Reactions

The scheme of a decomposition reaction is: A → B + C + ...

Decomposition reactions are those in which a substance is transformed into simpler ones due to an energy intake that can be performed in two ways:

• Thermal decomposition: Heat energy is supplied.

2Ag₂O → 4Ag + O₂

Electrolytic Decomposition: The energy provided is electric current.

CaCl₂ → Ca + Cl₂

2. Synthesis Reactions

The scheme of a synthesis reaction is: A + B + ... → C

These are reactions where two or more simple substances, elements, or compounds react to give a single compound.

3. Substitution Reactions

The scheme of a substitution reaction is: A + BC → AC + B

These are reactions in which an element of a reagent is replaced or exchanged for an element of another reagent. They can be:

• Simple: When an element and a compound react. The element replaces another element in the compound.

C + 2CuO → 2Cu + CO₂

Double: Occurs when two compounds react. An element is exchanged between the two compounds.

They fall into two classes:

• Between two salts: AgNO₃ + NaI → NaNO₃ + AgI
• Between an acid and a hydroxide: NaOH + HCl → NaCl + H₂O

4. Combustion Reactions

Combustion reactions are those that occur between a fuel, which reacts rapidly with oxygen (the oxidizer), producing a large amount of energy, usually in the form of light or heat.

The combustion of methane gas is a common example of this type of reaction.

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) + 890.3 KJ

Incomplete combustion processes are those that occur with a limited amount of oxygen. For example, when carbon is burned in an atmosphere low in oxygen, carbon monoxide is produced.

C(s) + ½ O₂(g) → CO(g) + 110.5 KJ

Note: Oxidation reactions between metals and oxygen, and the reactions producing cells during metabolism, are not considered combustion reactions.

5. Acids and Bases

Acids and bases are substances that have been widely used since antiquity. Characteristics of acids and bases:

Acids are a group of substances having in common the following properties:

• They have a sour flavor, like lemon or vinegar.
• They react with some metals, releasing hydrogen.
• They react with carbonates to give CO₂.
• In aqueous solution, they conduct electricity.
• They react with bases, neutralizing their action.
• They change the color of plant pigments and change the color of blue litmus paper to red.

Bases are a group of substances having in common the following properties:

• Their taste is bitter.
• They are greasy and slippery to the touch, like soap.
• In an aqueous solution, they conduct electricity.
• They react with acids, neutralizing their properties.
• They change the color of plant pigments and change the color of red litmus paper to blue.

6. Chemical Behavior of Acids and Bases

The Swedish chemist S. Arrhenius justified the conductivity of acids and bases by the existence of positive and negative ions in their aqueous solutions, from the breakup of the molecules. Acids are substances that form H⁺ ions in an aqueous solution. These are surrounded by water molecules. Bases are substances that, when dissolved in water, dissociate to give OH^- ions. Acids and bases may be:

• Strong: When dissolved in water, almost all molecules are dissociated into ions, so there are many H⁺ ions and OH^- ions.
• Weak: When dissolved in water, a small part of their molecules are dissociated into their ions or react with water. They produce few H⁺ and OH^- ions.

7. pH and Indicator Substances

Indicators are substances that show a change in color, depending on the acidity or basicity of the solution to which they are added.

The pH of a solution is a numerical value between 1 and 14, which is related to the concentration of H⁺(aq).

The standard of comparison is pure water, a neutral substance, with pH = 7. The acidity or basicity is determined by pH:

• Acid solutions: pH < 7. The concentration of H⁺(aq) is greater than that of OH^-(aq).
• Basic solutions: pH > 7. The concentration of H⁺(aq) is lower than that of OH^-(aq).

Paper impregnated with several indicators is also used; it is known as universal indicator paper.

8. Neutralization Reactions

The neutralization of an acid by a base is a reaction between the H⁺ ions from the acid and the OH^- ions from the base to give H₂O. In the neutralization reaction, an acid reacts with a base, producing an aqueous solution of a salt. The products have different properties from the acid and base reactants.

Calculations on the reactions of neutralization: To produce the neutralization reaction, the number of H⁺ ions from the acid must be the same as that of OH^- ions from the base. If we have the concentration and volume of one and the concentration of the other, we can calculate the unknown quantity.