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

Industrial Applications of Electrolysis

Electrolytic processes have significant applications in the industrial field. Some of these include:

• The acquisition of active metals.
• Metallic coatings.
• The purification of metals.

Metallic Coatings

Through electrolysis, it is possible to deposit a thin layer of metal onto another metal. This process is called electroplating and has many applications, including:

• Gold and silver coatings: Used for the aesthetic beautification of various objects.
• Zinc, nickel, chromium, and copper coatings: Designed to provide protection for metal objects against corrosion.

In zinc coating (galvanization), the electrolyte is a salt of the metal, the anode is formed by a pure zinc bar, and the cathode is the workpiece to be coated.... Continue reading "Industrial Electrolysis Applications: Metal Refining and Plating" »

Covalent Molecular vs. Atomic Covalent Solids

Covalent molecular substances are linked together by intermolecular forces, which are generally weak. Consequently, molecular substances are usually gases or liquids at Standard Temperature and Pressure (STP). In contrast, in atomic covalent solids such as diamond or silica, the molecules are linked together by covalent bonds. These bonds are very strong, resulting in greater cohesion between the molecules; therefore, they are solid at STP.

Molecular substances have low boiling temperatures because small thermal agitation can break the intermolecular bonds. However, covalent solids are much stronger; they only melt or boil at very high temperatures because these links offer strong resistance to being... Continue reading "Chemical Bonds and Properties of Matter" »

Physical and Chemical Changes in Matter

A physical change is a transformation that does not vary the nature of matter. Before and after the change, the matter is represented by the same chemical formula. A chemical change is a transformation in which the nature of matter varies. Before the change, the substance is represented by one chemical formula, and afterwards, by a different formula.

The Collision Theory of Reactions

Collision theory: Chemical reactions occur when molecules of reactants collide and break. Atoms that have been released are rearranged to form new molecules.

Measurement of Mass: The Mole

The mole: A mole of atoms is the amount of a chemical element that is equivalent to its atomic mass in grams. In the SI system, the mole is... Continue reading "Chemical Reactions and the Mole Concept" »

1. Introduction to Atomic Structure

1.1 Fundamental Particles

• Proton: An elementary particle located in the nucleus of an atom. The number of protons, known as the atomic number, determines the chemical properties of an element. Protons carry a positive electric charge and have a mass 1836 times greater than that of an electron.
• Electron: A subatomic fermionic particle that surrounds the nucleus. Electrons have a significantly smaller mass than protons. Their movement generates electrical current in most metals and they play a crucial role in chemical bonding and atomic attraction.

1.2 Atomic and Mass Numbers

• Atomic Number (Z): All atoms of a specific chemical element contain the same number of protons in their nucleus. This value characterizes

Ferrous Metals Classification

Based on Carbon Content

• Irons (0.01% to 0.03% C): White, difficult to obtain, with few industrial applications.
• Steels (0.03% to 1.76% C): Iron-carbon alloys that may contain other elements.
• Cast Irons (1.76% to 6.67% C): Iron-carbon alloys that may have other elements.
• Graphites (more than 6.67% C): Very fragile with no industrial applications.

Types of Cast Iron

Regular Cast Iron

• White Cast Iron: Very hard and brittle, used to make malleable iron.
• Gray Cast Iron: Used in malleable castings.
• Mottled Cast Iron: An intermediate between white and gray cast iron.

Alloyed Cast Iron

Contains iron, carbon, and other elements to improve its chemical properties.

Special Cast Iron

• Whiteheart Malleable Iron: A piece is molded in white

Chlorides

Chlorine, as an element, has a huge tendency to form the chloride ion (Cl^-, a chlorine atom with a negative charge). This ion, in turn, will tend to form salts with other positively charged elements or cations (Na⁺ sodium, K⁺ potassium, Mg²⁺ magnesium, etc.). The chloride ion is one of the main inorganic anions in natural water and wastewater. The chloride content of water is variable and is mainly due to the nature of the terrain traversed. Typically, the chloride ion content of natural waters is less than 50 mg/L. In drinking water, the salt produced by the Cl^- is variable and depends on the water chemistry.

Chlorite and Chlorate

These are salts derived from chloric acid and chlorous acid. These compounds tend to form chlorine gas,... Continue reading "Chloride: Properties, Uses, and Health Effects" »

Chemical Kinetics and Reaction Rates

Factors Influencing Reaction Velocity

• Rate Equation: V = k · [A]^a · [B]^b
• Temperature: The Arrhenius Law: k = k₀ · e^{-E_a / (R · T)}
• Concentration and Pressure: P · V = n · R · T
  P = (n · R · T) / V → n / V = concentration
  P = [X] · R · T
• Standard State and Physical State:
  • Solid: Few collisions result in slow reactions. The level of division is key; more particles mean a greater impact surface.
  • Liquid: Average collision frequency results in average reaction speeds.
  • Gas: Many collisions result in faster reactions.
• Catalysts: Positive catalysts increase speed. Negative catalysts (inhibitors) slow it down. Catalysts do not change thermochemical quantities (ΔH, ΔG, ΔS...), the quantity of product, or

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

Minerals and Alloy Fundamentals

Minerals: A usable part is formed by the metal and rich-looking material, named the ore, and another not usable part because it is very poor in metal, called the gangue (bargain).

Alloys: A product obtained from the union of two or more chemical elements which presents the characteristics of a metal. An alloy modifies its crystalline structure, increasing its hardness and mechanical resistance but lowering its conductivity.

Solidification of Alloys

• Area liquid phase: Liquidus line for liquid (it will always be liquid).
• Area solid + liquid phase: Contains a mass of solid and another of liquid.
• Solid-phase area: Below the solidus line (always a solid phase).

Classification of Metal Products

Shop-metal products:

• Ferrous

Atomic Structure: Models and Definitions

The Plum Pudding Model

The Plum Pudding Model considers the atom as a ball of positive charge with electrons (negative charge) embedded.

The Rutherford Model

Rutherford's Bombardment Experiment

Rutherford tested the Rutherford Model by bombarding metal foil atoms with alpha particles (projectiles harder than atoms).

Experimental Results

• Result 1: The majority of alpha particles passed through the foil undeflected.
• Result 2: Some of these particles suffered deviation.
• Result 3: Very few rebounded and turned back.

Interpretation and Consequences

The interpretation was that particles that crossed the foil showed that atoms are not solid, but contain much empty space.

The Consequences of the Rutherford Atomic Model... Continue reading "Atomic Structure Fundamentals: Models, Particles, and Radioactivity" »