Notes, summaries, assignments, exams, and problems for Chemistry

Atomic Theory: From Democritus to Quantum Models

Democritus' Atomic Theory

Democritus, in ancient Greece, founded the atomist school. This school, taking a pluralistic approach, proposed that atoms were the fundamental particles. The core principles of atomism are:

1. Atoms move in a vacuum.
2. The shape and size of atoms determine the substance.

Aristotle, conversely, believed that matter was a single, continuous entity.

Dalton's Atomic Theory

John Dalton proposed his atomic theory in the early 19th century. Its key postulates are:

1. Elements are composed of indivisible and indestructible particles called atoms.
2. Atoms of the same element have the same mass and properties.
3. Atoms of different elements have different masses and properties.
4. Chemical compounds are

Understanding Material Systems and the Atmosphere

Background: A material system is a set of interrelated elements considered as a unit for study.

Homogeneous and Heterogeneous Material Systems

Material systems can be homogeneous or heterogeneous.

In a homogeneous system, different substances are not readily apparent.

A mixture is a material system (either homogeneous or heterogeneous) from which different substances can be separated by physical processes.

Solutions and Their Components

A solution is a homogeneous mixture formed by two or more substances in variable amounts, which can be separated by physical means.

In a solution, the substance present in the greater proportion is called the solvent; the others are called solutes.

Separation Techniques

Chemical Bonding and Structure

Forming Molecules

In composites, clustering involves forming molecules. Atoms of different non-metal elements are joined to form molecules.

Ionic Crystals

When metals bind to nonmetals, they form ions of two types. Metals typically form positive ions, and nonmetals form negative ions, which are held together by electrostatic attraction.

For example, a sodium atom gives one electron to a chlorine atom, becoming Na⁺ and Cl^- ions. These ions attract each other. When this happens, many atoms can form a crystal lattice where the ions alternate in a three-dimensional network.

Atomic and Molecular Mass

The atomic mass of an atom is the average mass of its isotopes. It is typically found in the periodic table, often below the... Continue reading "Chemical Bonding, Mass, and Element Types" »

Classification of Matter

Matter is classified as homogeneous or heterogeneous according to its appearance.

• Homogeneous: It looks the same everywhere.
• Heterogeneous: It does not have the same aspect throughout.

Classification of Substances

Substances can be classified according to their material composition into pure substances or mixtures.

• Pure substances: Are formed by a single substance.

Classification of Pure Substances

Pure substances are classified into chemical elements and compounds

• Chemical Elements: Are pure substances that cannot be decomposed into simpler ones.
• Compounds: Are pure substances that can be decomposed into simpler ones.

• Mixtures: A mixture is formed by several pure substances. Mixtures have no fixed composition.

Separation of Mixing

Drinking Water Purification

Raw Water Treatment

1. Screening: Raw water passes through gratings, filters, and screens to remove coarse solids.
2. Aeration: Oxidation of undesirable organic species improves taste and odor.
3. Coagulation: Colloidal particles are coagulated and flocculated.
4. Sedimentation: Solids are allowed to settle and are removed.
5. Filtration: Removal of any remaining coagulated solids.
6. Disinfection: Control of bacteria.
7. Softening: Using lime and soda methods.
8. Demineralization: By distillation or ion exchange resins.

Wastewater Treatment

Primary Treatment

1. Screening/Roughing: Removal of large objects and solids through sieves.
2. Grit Removal
3. Skimming
4. Sedimentation: Takes place in tanks or chambers where suspended solids settle. This removes approximately

Arrhenius Theory of Acids and Bases

According to the Arrhenius theory, acids and bases exist only in aqueous solution:

• Acid: Any substance that in aqueous solution dissociates with the formation of hydrogen ions, H⁺.
• Base: Any substance that in aqueous solution dissociates with the formation of hydroxyl ions, OH^-.

Neutralization Reaction

Neutralization is the complete reaction of an acid and a base with the formation of a salt and water. HA + BOH → AB + H₂O.

Brønsted-Lowry Theory

According to the Brønsted-Lowry theory, the concepts of acid and base were expanded:

• Acid: Any chemical species, molecular or ionic, capable of yielding an H⁺ ion to another substance.
• Base: Any chemical species, molecule or ion, capable of receiving an H⁺ ion from another

Mixtures

A mixture is defined as a material system formed by combining two or more substances whose proportions can vary.

• Homogeneous mixtures: These consist of two or more components that cannot be distinguished visually.
• Heterogeneous mixtures: These are formed by two or more components that can be visually distinguished. They have different properties depending on the proportion of the sample taken.

Pure Substances

A pure substance is one in which, regardless of its state of aggregation, its properties are characteristic and allow differentiation from other substances.

• Chemical elements: These are pure substances that cannot be decomposed into simpler substances through normal physical and chemical processes.
• Compounds: These are pure substances

Porcelain Dish Experiment

Procedure for Porcelain Dish

• Heat the charcoal in a porcelain dish.
• Add the wine from the first beaker, which initially held the charcoal.
• Add the hot charcoal to the wine and then remove it. Place the flask under a clean funnel and filter the contents into the flask.
• Place a quantity of pure wine (IVF) into the first test tube.
• Into the second test tube, add a quantity of discolored, filtered wine.
• Add the wine over charcoal.
• Mix and let stand for a couple of minutes.
• Filter.

Determination of Gas Density

Materials for Gas Density Experiment

• Erlenmeyer flask
• Glass tube
• Silicone or rubber tubing
• Crystallizing dish
• Support stand and ring
• Sample container
• Cap with a hole

Reagents for Gas Density Experiment

• Half an effervescent tablet
• Distilled

Chemical Bonds and Atomic Structure

Most atoms are present together with other peers, forming elements or different ones to form compounds. The physical states evolve toward states of minimum potential energy, which correspond with maximum stability.

Chemical Bonding

Two or more atoms are held together if the energy of the system is less when they are together than when separated.

The Octet Rule

The noble gases have very stable atoms that do not combine with other atoms and do not alter their electronic structure. All of them have 8 electrons except helium. It follows that the electronic provision of a noble gas property is a particularly stable arrangement called the octet.

Octet Rule

In forming compounds, atoms exchange electrons to acquire 8 electrons... Continue reading "Chemical Bonds and Atomic Structure: Understanding Stability" »