Notes, abstracts, papers, exams and problems of Chemistry

Chemical Bonding and Molecular Structure

• • Ionic Compound: A chemical substance composed of positive and negative ions.
  • Crystal: A solid with flat faces that form sharp angles, whose atoms, ions, or molecules are arranged in a regular three-dimensional arrangement.
  • Double Bond: A covalent bond in which two pairs of electrons are shared between two atoms.
  • Electronegativity: The attraction an atom in a molecule exerts on the electrons of a covalent bond.
  • Electronegative Element: Elements, particularly fluorine, oxygen, and nitrogen, have a very strong attraction for the electrons involved in chemical bonding.
  • Covalent Bond: An electron pair shared between two atoms in a molecule.
  • Nonpolar Covalent Bond: A chemical bond in which one or more pairs of electrons

Chemical Kinetics

Chemical kinetics is the study of the speed with which chemical reactions occur.

Rate of Reaction

For a homogeneous reaction between reactive gases in solution, the average speed is defined as the ratio between the variation of the concentration (expressed in mol/liter) of one of the reactants or products and the time interval in which that variation occurs.

Instantaneous speed is the speed that the reaction has at a given moment.

Collision Theory

The energy required for a collision to be effective and break the bonds necessary to produce the reaction is called activation energy.

Rate Equation

The rate equation is defined as the equation, determined experimentally, used to predict the dependence of the velocity of a chemical reaction... Continue reading "Chemical Kinetics: Reaction Rates and Equilibrium" »

Radioactivity

Radioactivity is the phenomenon whereby some substances emit radiation that can impress photographic plates, ionize gases, produce fluorescence, and cross through opaque to ordinary light. These elements are called radioactive.

Radiation can be:

• Alpha: Positively charged helium atoms, which are easily absorbed by matter.
• Beta: Electrons, which are more penetrating.
• Gamma: Electromagnetic in nature and even more penetrating.

Rutherford Model

The Rutherford model consists of:

• Nucleus: Composed of Z protons and N neutrons, responsible for the atom's mass and positive charge. The mass number A is equal to the sum of protons and neutrons (A = Z + N).
• Electron Cloud: Composed of electrons, responsible for the atom's negative electric charge.

Chemical Formulas

Definition and Types

A chemical formula expresses the composition of a substance, both qualitatively and quantitatively. It shows the elements forming the substance and the proportions of their atoms.

There are three main types of chemical formulas:

• Empirical Formula: Provides qualitative and relative quantitative information. It shows the elements and their relative proportions in the substance.
• Molecular Formula: Indicates the exact number of atoms of each element in a molecule.
• Structural Formula: Shows the distribution of atoms in the molecule and the types of bonds between them. Commonly used for organic compounds.

Determining Empirical and Molecular Formulas

The empirical formula is determined through quantitative analysis.... Continue reading "Chemical Formulas and Equations: A Comprehensive Overview" »

Copper: Chemical element, symbol Cu, atomic number 29. It is a reddish metal, soft, malleable, and ductile with a metallic gloss. Copper is the element with the highest electrical and thermal conductivity. This material is abundant in nature.

Reactions:
1. Flame Test: Copper-oxygen compounds, when introduced into the flame, give it a bright green color due to the volatile copper oxide. Moist ore with hydrochloric acid, when heated in the flame, produces a bright blue, tinged with green.
2. Blue Solution with Ammonium Hydroxide: The copper-containing acid solution is alkalized with ammonium hydroxide, resulting in a deep blue color.
3. Reduction on Charcoal: When mixing a small amount of copper ore with a reducing mixture (equal parts of sodium... Continue reading "Understanding the Importance of Copper in Health and Environment" »

Ideal Gas Equation of State

The general equation of state for ideal gases is:

p · V = n · R · T

Kinetic Theory of Gases

Gases consist of molecules that:

• Occupy no volume.
• Move randomly with a velocity whose mean depends on temperature.
• Collide elastically with each other and the vessel walls, causing pressure.

Boyle's Law

P₁ · V₁ = P₂ · V₂

At constant temperature, the volume occupied by a gas is inversely proportional to the pressure it is under.

Charles's and Gay-Lussac's Law

At constant pressure, the volume occupied by a gas is directly proportional to the absolute temperature at which it is located. At constant volume, the pressure of a gas is directly proportional to the absolute temperature at which it is located.

Dalton's Law

The total pressure... Continue reading "Ideal Gas Equation, Kinetic Theory, and Atomic Models" »

The Pluralistic School

Speculation about the physical world, initiated by the Ionians, was continued in the 5th century BC by Empedocles and Anaxagoras. They developed philosophies which replaced the description of a first Ionian substance only by the assumption of a plurality of substances. Empedocles maintained that all things are composed of four irreducible elements: air, water, earth, and fire, combined or separated by two opposing forces as a process of alternation: love and hate. Through this process, the world evolves from chaos to form and back into chaos again, in a repeated cycle. Empedocles considered the eternal cycle as the true object of religious worship and criticized the popular belief in personal gods, but failed to explain... Continue reading "Pluralism and Atomism in Ancient Greek Philosophy" »

Key Properties of Chemical Substances

Ionic Compounds

• Solid at room temperature
• Form crystal lattices, not individual molecules
• Soluble in water
• Conduct electricity when molten or dissolved
• High hardness and brittle
• High melting and boiling temperatures

Molecular Substances

• Composed of molecules
• Weak intermolecular forces
• Often gaseous (e.g., O₂, N₂, NH₃) or liquid (e.g., H₂O)
• Soft solids
• Low melting and boiling temperatures
• Poor electrical conductors
• Not very soluble in water

Covalent Crystals

• Solid
• Form crystals with atoms joined by covalent bonds in three dimensions
• Insoluble in all solvents
• Generally do not conduct electricity (except graphite)
• Very hard and brittle
• High melting and boiling temperatures (1200 to 3600 °C)

Metals

• Solid at room temperature (except

Classification of Hairstyles

• Curling: Gives hair a ring-shaped design, loops, or corkscrews, adding volume.
• Waving (Inverter): Creates S-shaped waves in the hair, an intermediate effect between straightening and curling.
• Straightening (Smooth): Gives hair a smooth appearance by eliminating waves or curls.
• Updo (Collect): Involves arranging the hair in a specific way. Updos can be total or partial.

Hairstyle Change Duration

• Temporary: A change of form that produces a modification of the hair structure that lasts for a limited time (days, weeks).
• Permanent: A change in form that produces a structural modification that persists despite washing, brushing, coloring, shampooing, etc.

Temporary Changes

Temporary changes in hair shape primarily occur due to... Continue reading "Hairstyle Types: Curling, Waving, Straightening & More" »