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

Chemical Reactions and Stoichiometry

A chemical reaction is a process by which substances are transformed into others with the release or absorption of energy. Initial substances are called reactants, and the final substances are called products.

Balancing Chemical Equations

Stoichiometry indicates the proportion of molecules (microscopic scale) or moles (macroscopic scale) involved in a reaction to ensure the conservation of atoms. Common balancing methods include:

• Mathematical Method: A matching method consisting of solving a system of equations arising from the equality of atoms on either side of the equation.
• Trial and Error: A method where coefficients are adjusted starting with elements involved in the fewest substances and ending with those

Fundamentals of Polymers

Polymer: Giant organic molecules with molecular weights ranging from 10,000 to 1,000,000 g/mol.

Monomer Unit: The structural entity that is repeated along the polymer molecule.

Monomer + Catalyst → Reactive Monomer Unit

Classification of Polymers

• Mechanism of Polymerization: Addition and condensation.
• Molecular Structure: Linear, branched, and cross-linked.
• Thermal Behavior:
  • Thermoplastics: Plastic polymers that soften when heated and harden upon cooling. This process is reversible; they are typically soft and ductile (e.g., Polyamide via spinning).
  • Thermosets: Plastic polymers that harden irreversibly when heated; they are hard, tough, and fragile (e.g., Melamine via compression).
• Nature of Monomer Units: Homopolymers and

Organic Reactions

Halogenation Substitution of Alkanes

CH₄ + Cl₂ → CH₃Cl + HCl

Obtaining Alcohol from Halogenated Alkanes

CH₃-CH₂-CH₂-CH₂Cl + KOH → CH₃-CH₂-CH₂OH + KCl

Addition Reactions to Double and Triple Bonds

Hydrogenation

Hydrogen is added to alkenes and alkynes to give alkanes in the presence of a metal catalyst.

CH₃-CH=CH-CH₃ + H₂ → CH₃-CH₂-CH₂-CH₃

Halogenation

Alkenes and alkynes react with halogens to give halogenated compounds.

CH₃-CH=CH₂ + Br₂ → CH₃-CHBr-CH₂Br

Adding Hydracids

Acid hydrogen is added to the carbon with more hydrogens (Markovnikov's rule).

CH₂=CH₂ + HI → CH₃CH₂I

Water Addition

Water is added to alkenes in the presence of acid and also follows Markovnikov's rule.

Voltaic Battery

A voltaic battery is a device that allows an... Continue reading "Understanding Organic Reactions, Voltaic Cells, and Salt Hydrolysis" »

Understanding Electrification Phenomena

When rubbing certain materials, two electrical phenomena can be observed: attraction or repulsion. Charles inferred the existence of these two classes of electrical phenomena.

Electrification by Contact

This occurs when an isolated, uncharged body comes into contact with another body already charged by friction. To study this, we utilize an electric pendulum. These experiments show that bodies which have come into contact with other previously electrified bodies also become electrified.

Electrification by Induction

To electrify a body, it is not essential for the charged body to make direct contact with the body you wish to electrify (they can be closer but not touching).

Electric Charge: Definition and Properties

Electric

The material can be a mixture or a pure substance. Materials can be pure substances, which include chemical elements or chemical compounds. For example, Oxygen is a chemical element that cannot be decomposed by chemical procedures. Water is a compound ($\text{H}_2\text{O}$) that can be decomposed into oxygen and hydrogen by chemical methods (like passing an electric current).

The Atom: Building Block of Matter

An atom is the smallest unit that exists in a chemical element. A material is discontinuous and formed by tiny particles called atoms. The size of atoms is on the order of $10^{-12}$ meters (which equals 1 picometer, $\text{pm}$, or $0.000000000001\text{m}$).

Subatomic Particles

• Protons: Carry a positive electric charge ($+$). The charge

Water Hardness

Hard water is water that contains appreciable amounts of dissolved calcium and magnesium compounds. These compounds form insoluble precipitates with soap. Ion exchangers are used to remove the hardness.

Temporary Hardness

Temporary hardness is caused by the presence of calcium and magnesium bicarbonates. It can be eliminated by heating the water to precipitate the calcium carbonate:

Ca(HCO₃)₂ → CaCO₃↓ + CO₂ + H₂O

Permanent Hardness

Permanent hardness refers to the remaining calcium and magnesium salts (not bicarbonates). It requires the addition of chemicals for removal, for example:

Na₂CO₃ + CaCl₂ → CaCO₃↓ + 2NaCl

Water Pollution

Thermal Pollution

Heat, in general, is not considered a contaminant, but its effects on water can... Continue reading "Water Hardness, Pollution, Cleaners, and Sampling" »

Selectivity in Chemical Analysis

A reaction used in chemical analysis is selective when the reaction is characteristic of few substances, showing a preference for some of these. When the reaction is carried out only on a particular substance, it is specific.

Limit of Detection (LOD)

Refers to the minimum amount of a substance that can be detected in a trial.

Sensitivity of Analytical Methods

The ability of a method to discriminate a change in concentration caused by a variation in the measurement of the analytical signal.

Qualitative vs. Quantitative Analysis

The science that studies the principles and techniques of chemical analysis. It is divided into:

• Qualitative analysis: Its purpose is the identification of the constituents of a sample (organic

Key Discoveries in Atomic Theory

Thomson's Discovery of Electrons

J.J. Thomson demonstrated that atoms contain tiny, negatively charged particles called electrons. He showed that atoms could lose electrons, suggesting a static atomic model.

Rutherford's Atomic Model

Ernest Rutherford established that atoms are mostly empty space. He proposed that a small, dense region, which he named the nucleus, resides at the atom's center. This nucleus contains protons and, as Rutherford suspected, neutrons. In his model, the number of protons equals the number of electrons, with the latter orbiting the nucleus in circular paths.

The Atom as a Planetary System

The Rutherford model is analogous to a planetary system because the nucleus represents the sun, and the... Continue reading "Atomic Structure and the Periodic Table" »

Biological Molecules and Biomolecules

Inorganic: H₂O, O₂, CO

Organic (Macromolecules): Proteins, Nucleic Acids, Carbohydrates, Lipids

Structure and Function of Macromolecules

• Polysaccharides: Polymers with structural functions (cellulose) or energy storage (glycogen).
• Nucleic Acids: Polymers of 4 nucleotides. Function associated with the storage, transmission, and expression of genetic information.
• Proteins: Combinations of 20 amino acids. Different functions (catalytic, structural, transport, hormones, antibodies, receptors).
• Lipids: Do not polymerize, they associate. Structural or functional roles.

Characteristics of Living Matter

Living things are made of chemical molecules interacting in an aqueous fluid, acquiring new physicochemical properties... Continue reading "Biomolecules and the Role of Water in Living Matter" »

The Krebs Cycle

The Krebs cycle, also known as the Citric Acid Cycle, is an amphibolic pathway because:

• It facilitates the degradation of Acetyl-CoA.
• Its intermediate compounds serve as precursors for other synthetic processes.

Stages of the Krebs Cycle

The cycle is divided into the following stages:

1. Formation of Citric Acid

   Citric acid is formed from oxaloacetic acid and Acetyl-CoA.

   • Modifications: Oxaloacetic acid + Acetyl-CoA → Citric acid
   • Inputs/Outputs: Enters H₂O and Acetyl-CoA

2. Isomerization of Citric Acid to Isocitric Acid

   • Modifications: Citric acid → Isocitric acid
   • Inputs/Outputs: None

3. Oxidation of Isocitric Acid

   Isocitric acid is oxidized, reducing NAD⁺ to NADH, forming alpha-ketoglutarate, and releasing CO₂.

   • Modifications: Isocitric acid →