Notes, abstracts, papers, exams and problems of Chemistry

Ranking Elements

The first classification of chemical elements was based on their chemical and physical properties, dividing them into two large groups: metals and non-metals.

• The first classification was done by J.W. Döbereiner. He observed a link between the atomic mass and properties of elements such as barium, calcium, and strontium. He arranged 40 elements in a line, one after another, by their mass.
• John Newlands arranged the elements by atomic mass, from the lowest to the highest, in columns. This is now called the Law of Octaves.
• D.I. Mendeleev arranged the elements by atomic mass. As a result of this work, the periodic law of elements was discovered in 1869.

Forces Between Molecules

These are listed from weakest to strongest:

1. Dispersion

Reactions in Aqueous Solutions

Acid and Base Definition According to Bronsted-Lowry

Acid: A substance capable of donating protons.

Base: A substance capable of accepting protons.

For a substance to act as an acid, it must be in the presence of another substance that acts as a base and accepts the proton transferred by the acid. In aqueous solutions, water plays the role of both acid and base.

• When an acid donates protons, it becomes a species capable of accepting protons, called the conjugate base.
• When a base accepts protons, it is transformed into a species capable of donating protons, i.e., an acid, called the conjugate acid.

Equilibrium of Water Dissociation and the Concept of Kw

Water behaves as both an acid and a base according to the reaction:... Continue reading "Chemical Reactions in Aqueous Solutions and Stoichiometry" »

Chemical Bonding

Introduction

Chemical bonds are the forces that hold atoms together, leading to the formation of molecules and crystals. There are three primary types of chemical bonds:

1. Ionic Bonds

Ionic bonds occur between a metal and a nonmetal. In this type of bond, one atom loses electrons (forming a positive ion called a cation) while the other gains electrons (forming a negative ion called an anion).

2. Covalent Bonds

Covalent bonds form between two nonmetals. Atoms in a covalent bond share electrons to achieve a stable electron configuration. Each shared pair of electrons represents a covalent bond. These bonds can be visualized using Lewis diagrams.

3. Metallic Bonds

Metallic bonds occur between atoms of the same metal. In metallic bonding,... Continue reading "Understanding Chemical Bonds: A Comprehensive Look" »

Acid-Base Reactions

Acid-base reactions are essential in chemistry, allowing us to determine the concentration of an acid or base solution through titration. This process involves using a solution of known concentration (a standard solution) and an acid-base indicator.

Neutralization

Neutralization is the combination of hydrogen ions (H⁺) and hydroxide ions (OH^-) to form water (H₂O) and a salt. These reactions are exothermic, meaning they release heat.

Acid + Base → Salt + Water

Titration of a Strong Acid with a Strong Base

When titrating a strong acid (e.g., hydrochloric acid, HCl) with a strong base (e.g., sodium hydroxide, NaOH), the reaction is a straightforward neutralization:

HCl + NaOH → NaCl + H₂O

Titration of a Weak Acid with a Weak Base

Titrating... Continue reading "Acid-Base Reactions: Titration, Neutralization, and Indicators" »

1) Solution

A homogeneous mixture of two or more substances formed by a solute dispersed in a solvent.

2) Tyndall Effect

Visual of a light beam passing through a homogeneous mixture.

3) Concentration

The relationship between the amount of solute and solvent.

4) Dissolution

The process of scattering solute particles within the solvent.

5) Solubility

Expresses the maximum amount of solute that can dissolve in a given solvent.

6) Polar Molecule

A molecule with an uneven distribution of electronic charge along its structure.

7) Non-Polar Molecule

A molecule with a uniform distribution of electronic charges.

8) Types of Concentrations

• Dilute: A solution containing a small amount of solute.
• Saturated Solution: Contains the maximum amount of solute at a given temperature.

Electron Spin and Orbitals

Spin refers to the movement of an electron around the nucleus and on its own axis. There are two possible spin directions: +1/2 and -1/2. When two electrons have opposite directions of rotation, they are represented with small arrows, one pointing up and the other down.

An orbital is occupied when it contains two electrons. An orbital with a single unpaired electron is represented by a single arrow. Orbitals are represented as rectangles called quantum boxes, within which the electrons are indicated.

The electronic configuration of an atom describes how the electrons are arranged within the atom.

Hund's Rule

Hund's rule states that an electron cannot completely fill an orbital until all orbitals within that sublevel contain... Continue reading "Electron Configuration and Chemical Bonding Basics" »

Chemical Reactions

A chemical reaction is a process in which one or more substances react to form other substances with different properties.

Chemical Reactions and Chemical Equations

The chemical equation is the symbolic representation of a chemical reaction.

Activation Energy

For a chemical reaction to be possible, particles must collide with a minimum energy.

Reaction Energy

The energy exchange that occurs in the course of a chemical reaction is called the energy of reaction.

Speed of Reaction

The speed at which a chemical reaction takes place.

Factors Influencing the Reaction Rate

• Nature of Reagents: Generally, covalent substances lead to slow reactions at room temperature, while ionic substances react quickly when dissolved.
• Temperature: The reaction

Bioelements: Chemical Basis of Life

Bioelements are the chemical elements that constitute living matter. They are classified based on their abundance:

Classification of Bioelements

• Primary Bioelements

  These make up the vast majority of living matter. They include Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S) – often abbreviated as CHONPS.

• Secondary Bioelements

  These include Sodium (Na+), Potassium (K+), Calcium (Ca2+), Magnesium (Mg2+), and Chlorine (Cl-). Although present in smaller proportions than primary bioelements, they are essential for life. In aqueous environments within living organisms, they are always found in their ionized forms.

• Trace Elements (Oligoelements)

  These are required in very small amounts.

Metallic Materials

Metallic materials are inorganic substances containing one or more elements. They may contain metals and also some non-metallic elements. The metallic elements include iron, copper, aluminum, nickel, magnesium, and titanium. Non-metallic elements include carbon, nitrogen, and oxygen.

Characteristics of Metal Components

• Have a crystalline structure
• Are good thermal and electrical conductors
• Many have mechanical resistance and are ductile at room temperature
• Can also exhibit high strength at elevated temperatures

A metal alloy is a combination of two or more metals. They are normally divided into two classes:

• Ferrous metals and alloys that contain a high percentage of iron
• Nonferrous metals and alloys containing no iron or relatively

Why Life Is Based on Carbon and Not on Silicon

Although silicon (Si) also has four electrons in its valence shell and is much more abundant in the Earth's crust than carbon (C), life is based on C and not on Si. This is because C-C, C=O, and C-N bonds are weak enough to form stable chains and rings, while also being able to break to make way for other molecules. This allows living organisms to obtain the energy contained in these bonds. In contrast, Si-Si and Si-O-Si-O chains are so stable that they are unalterable.

Descriptive Biology

Descriptive biology is based on the simple observation of the structure and functioning of living things, to make descriptions of what is observed. Until the 17th century, it was based solely on simple observation... Continue reading "Carbon-Based Life and the Scientific Method" »