Notes, abstracts, papers, exams and problems of Chemistry

Scientific models used to explain why and how atoms form molecules:

• Lewis dot structure
• Valence bond theory

Valence bond theory describes a covalent bond as the overlap of half-filled atomic orbitals (each containing a single electron) that yield a pair of electrons shared between the two bonded atoms. According to valence bond theory, a covalent bond results when two conditions are met: (1) an orbital on one atom overlaps an orbital on a second atom and (2) the single electrons in each orbital combine to form an electron pair.

• Molecular orbital theory

model that describes the behavior of electrons delocalized throughout a molecule in terms of the combination of atomic wave functions. It describes the distribution of electrons in molecules in much... Continue reading "Atomic Models, Periodic Trends, and Chemical Bonding" »

IONIC: thet are solids at room temperatures and have medium to high meltin and boiling points. Ionic bond is strong because it is electrostatic. They are very hard. It is not eassy to scratch an ionic compound. They are fragile. Hitting them forces ions with the same signs together, and they repel and break the crystal. Most disolve well in water, water molecules may surround the ions,weakening their bonds and separating them from the crystal lattice. This is called solvation. They do not conduct electricity well in solid state but hey do in solution or liquid state. SODIUM CHLORIDE

Mixing and Measuring

Agitator

An agitator is a machine used for mixing liquids, typically with an electromagnet.

Beaker

A beaker is a cylindrical container made of glass that is very commonly used in the laboratory, mainly to prepare or heat substances.

Burette

A burette is used for decantation and for transferring and dispensing liquids or fluids.

Graduated Cylinder

A graduated cylinder is used to measure volumes of liquids.

Micropipettes

Micropipettes are used to measure and extract very small amounts of liquids from a solution.

Pipette

A pipette is used to accurately measure volumes of liquids.

Volumetric Flask

A volumetric flask is used to prepare solutions and to know the volume.

Heating and Support

Bunsen Burner

A Bunsen burner is used to heat chemical... Continue reading "Essential Lab Equipment: Uses and Functions" »

Paradigmatic (vertical) axis

The words organizations, groups, and Amnesty International are related to each other because they all belong to the same semantic group, a relationship known as hyponymy. Organizations serve as a hypernym (a more general or superordinate word), while human rights groups and Amnesty International serve as hyponyms (more specific words, in this case types of organizations). The other two kinds of paradigmatic relationships are those of synonymy (prompted = caused) and antonymy (weak/strong).

Syntagmatic (horizontal) axis
 Unlike the paradigmatic relationships, the syntagmatic relationships of a word are not about meaning. They are about the lexical company the word keeps (collocation) and the grammatical patterns

Its function is to complete the measurement circuit, providing a passage from the conductivity-sensing electrode, through the solution, to the reading device. The four parts form an electrical circuit.

Types of Electrodes

1. Hydrogen Electrode
2. Calomel Electrode: Consists of a mixture of mercury and mercurous chloride in contact with a potassium chloride solution.
3. Silver Chloride Electrode
4. Glass Electrode:
   • Parts: Lead-plated, resin-filled rod, internal electrode, sensitive glass.
5. Glass Electrode and Calomel: External calomel reference, two different cells connected in series.

Analysis Methodology

To analyze a substance using potentiometric techniques (anions, cations, organic and inorganic), the substance, or some of its possible complexes, must be electroactive... Continue reading "Potentiometric Analysis: Electrodes, Methods, and Factors" »

1. Origin of Fire

1.1. The Fire Tetrahedron

For a fire to occur, a chemical reaction between a fuel and an oxidizer is necessary, along with an activation energy. These three factors make up the fire triangle. The fire tetrahedron is formed by adding the chain reaction, which is a reaction that does not need external energy to be maintained.

• Fuel: A substance that reacts easily with an oxidizer to produce heat.
• Oxidizer: A substance that activates the fuel (e.g., oxygen).
• Activation Energy: The minimum energy required to initiate the combustion process.
• Chain Reaction: A self-sustaining reaction that does not require external energy.

Types of Fire

• Class A: Solid fuels like wood.
• Class B: Liquid or gas fuels like oil or acetylene.
• Class C: Electrical

What is Matter?

Matter is anything that occupies space and has mass. This includes land, water, air, and more. Essentially, if something exists physically, it is considered matter.

Theories of Matter

Historically, there were two main theories regarding the divisibility of matter:

• Matter is discontinuous: This theory proposed that there is a fundamental, indivisible unit of matter. This smallest unit was called an "atom."
• Matter is continuous: This theory suggested that matter could be infinitely divided into smaller and smaller parts.

It was eventually proven that matter is discontinuous, meaning atoms exist.

Thompson's Atomic Model

J.J. Thompson's model, often called the "plum pudding" model, depicted the atom as a sphere of positive charge with negatively... Continue reading "Understanding Matter: Atoms, Structure, and Properties" »

Volumetric Analysis

Requirements for Volumetric Reactions

For a reaction to be used in volumetric analysis, it must meet the following criteria:

• The reaction between the titrant and the titrated substance must be quantitative, without adding excess reagent. The reaction must be complete at the equivalence point.
• The reaction must be fast. If it is slow, it is possible to accelerate it with temperature or catalysts.
• The reaction must be stoichiometric and definite.
• To determine the endpoint, a sensitive method must be available.

Characteristics of a Standard Solution

To prepare a standard solution, there are two methods:

1. Directly dissolve a standard substance and dilute to an exact volume (using a volumetric flask).
2. Prepare a solution of approximate concentration

Quantum Numbers

By solving the equations of quantum mechanics for an atom, quantum numbers appear as a mathematical consequence. These describe the behavior of electrons in the atom.

• The principal quantum number, n: Represents an energy level. It can assume any positive integer (1, 2, ...). The first level is the lowest energy, and subsequent levels, increasingly distant from the nucleus, have greater energies.
• The orbital angular momentum quantum number, l: Determines the orbital shape and energy within each level. It takes values between 0 and n-1 inclusive.
• The magnetic quantum number, m_l: Describes the orientation of the orbital in space. Among other things, it explains the splitting of spectral lines when an external magnetic field is applied.

What is a Mixture?

A mixture is a substance formed by combining two or more substances without a chemical reaction occurring that changes its components.

• A homogeneous mixture is one in which no substance loses its original properties and can be separated by physical means. It has a uniform composition throughout.
• A heterogeneous mixture is one that has a non-uniform composition in which components can be distinguished by the naked eye and consists of two or more physically distinct substances, unevenly distributed.

Physical Separation Techniques

Distillation

Distillation is used to separate two liquids with different boiling points by heating and subsequent condensation of the substances. The distillation process consists of two phases: first, the... Continue reading "Understanding Mixtures and Separation Techniques" »