Lecture 1st Counting Atoms, Acids, and Bases: Fundamental Concepts in GOB Chemistry

Counting Atoms: Avogadro’s Number

Introduction to Avogadro’s Number

In chemistry, counting atoms and molecules is essential for understanding chemical reactions and stoichiometry. Avogadro’s Number is the fundamental constant that defines the number of particles in one mole of a substance.

• Definition: Avogadro’s Number is the number of atoms, molecules, or particles in one mole of a substance.

• Value: $6.022 \times 10^{23}$ particles per mole.

• Analogy: Just as a dozen means 12 items, a mole means $6.022 \times 10^{23}$ items.

• Application: Used to convert between atomic/molecular scale and macroscopic quantities.

Example: 1 mole of carbon atoms contains $6.022 \times 10^{23}$ carbon atoms.

Acids

Definition and Properties of Acids

The term acid comes from the Latin word acidus, meaning sour. Acids are substances that exhibit characteristic properties and play important roles in chemical reactions and biological systems.

• Taste: Sour (e.g., vinegar, lemons, citrus fruits).

• Common Examples: Hydrochloric acid (HCl), acetic acid (CH3COOH), citric acid.

Arrhenius Acids

Arrhenius acids are defined by their ability to produce hydrogen ions (H+) when dissolved in water.

• Definition: An Arrhenius acid increases the concentration of H+ ions in aqueous solution.

• General Reaction:

$\mathrm{HCl}(g) \rightarrow \mathrm{H}^+(aq) + \mathrm{Cl}^-(aq)$

• Electrolyte: Acids are electrolytes because they produce H+ ions in water.

• Other Properties:

  • Turn blue litmus paper red

  • Corrode some metals

  • May sting to the touch

Example: Hydrochloric acid (HCl) in water dissociates to form H+ and Cl- ions.

Indicators and Acid-Base Color Changes

Certain chemical indicators, such as 7-hydroxyphenoxazone, change color depending on the acidity or basicity of the solution.

• In acidic conditions: red

• In basic conditions: blue

• In neutral conditions: purple

Bases

Definition and Properties of Bases

Bases are substances that exhibit characteristic properties and are important in neutralization reactions and many industrial processes.

• Taste: Bitter or chalky

• Feel: Soapy and slippery

• Common Examples: Sodium hydroxide (NaOH), potassium hydroxide (KOH)

Arrhenius Bases

Arrhenius bases are defined by their ability to produce hydroxide ions (OH-) when dissolved in water.

• Definition: An Arrhenius base increases the concentration of OH- ions in aqueous solution.

• General Reaction:

$\mathrm{NaOH}(s) \xrightarrow{H_2O} \mathrm{Na}^+(aq) + \mathrm{OH}^-(aq)$

• Electrolyte: Bases are electrolytes because they produce OH- ions in water.

• Other Properties:

  • Turn red litmus paper blue

  • Turn phenolphthalein indicator pink

  • Feel soapy and slippery

Example: Sodium hydroxide (NaOH) in water dissociates to form Na+ and OH- ions.

Indicators and Base Color Changes

Indicators such as 7-hydroxyphenoxazone and phenolphthalein are used to detect the presence of bases by color change.

• Phenolphthalein: Colorless in acid, pink in base

• 7-hydroxyphenoxazone: Blue in base, red in acid, purple in neutral

Characteristics of Acids and Bases

Comparison Table: Properties of Acids and Bases

The following table summarizes the main characteristics of acids and bases according to the Arrhenius definition and their observable properties.

Characteristic	Acids	Bases
Arrhenius	Produce H+	Produce OH-
Electrolyte	Yes	Yes
Taste	Sour	Bitter, chalky
Feel	May sting	Soapy, slippery
Litmus	Red	Blue
Phenolphthalein	Colorless	Pink
Neutralization	Neutralize bases	Neutralize acids

Summary

Understanding the definitions and properties of acids and bases, as well as the concept of Avogadro’s Number, is fundamental in GOB Chemistry. These concepts are essential for describing chemical reactions, solution chemistry, and laboratory analysis.