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

Fundamental Concepts of Chemical Change

Phenomenon

Any change that takes place within material systems.

Physical Change

Events in which there is no change in the composition or identity of the substances involved.

Chemical Change

Events in which the identity of the substances involved is altered.

Chemistry (Chemical Transformation)

The transformation of one substance into another. This process involves breaking existing chemical bonds and forming new ones.

Final Substance (Reaction Products)

The substances resulting from a chemical reaction.

Chemical Reaction Dynamics

Reversible Reactions

Chemical equations where the conversion of reactants to products occurs simultaneously with the conversion of products back to reactants (equilibrium).

Irreversible Reactions

Reactions... Continue reading "Chemical Reactions, Conservation Laws, and Consumer Price Index (CPI)" »

Chemical Kinetics

Reaction Rate: The change in the concentration of reactants or products per unit time.

For a reaction aA + bB → cC + dD, the rate (v) can be expressed as:

v = -1/a * Δ[A]/Δt = -1/b * Δ[B]/Δt = 1/c * Δ[C]/Δt = 1/d * Δ[D]/Δt

Rate Law: The relationship between the reaction rate and the concentrations of reactants.

v = k [A]^m[B]ⁿ

Where k is the rate constant, and m and n are the reaction orders with respect to A and B, respectively.

Factors Influencing Reaction Rate

• Nature of Reactants: The physical state and chemical properties of reactants affect the rate. Homogeneous reactions (same phase) are often faster than heterogeneous reactions (different phases). In heterogeneous reactions, increasing the surface area increases the

Understanding Hard Water

Older generations coined the phrase "hard water" because its mineral content makes cleaning more difficult. This difficulty is primarily due to dissolved compounds containing calcium and magnesium. Freshwater sources naturally contain varying quantities of these minerals.

As water travels, it dissolves, suspends, or exchanges compounds and trace elements from the materials it contacts. For example, flowing through limestone increases water hardness, while contact with peat can soften it.

Total Water Hardness

Total water hardness measures the combined concentration of calcium and magnesium, the two most common divalent metal ions. However, in some geographic locations, iron, aluminum, and manganese may also be present at... Continue reading "Hard Water Explained: Causes, Types, and Classification" »

Types of Solid Solutions

Metal alloys are solid solutions between two or more elements. Depending on the arrangement of solvent and solute atoms, there are two types:

• Substitution Solid Solutions: Solvent and solute atoms have similar crystalline structures. Solute atoms replace solvent atoms within the crystal lattice.
• Interstitial Solid Solutions: Occur when solute atoms are very small compared to solvent atoms and are placed within the crystal lattice.

Constitution of Fe-C Alloys

Fe-C alloys are composed of constituents whose nature varies with chemical composition and temperature. The most representative constituents are:

• Ferrite: A solid solution of carbon in alpha iron.
• Cementite: The hardest and most brittle constituent of steel.
• Pearlite: A

Fundamental Concepts in Chemistry

Molar Volume Definition and Calculation

The molar volume is the volume occupied by a gas measured at 273 K (0 ºC) and 1 atm (Standard Temperature and Pressure, STP).

It has been estimated that the molar volume of any gas at 273 K and 1 atm is 224 L.

In 224 L of any gas, measured at 273 K and 1 atm, there are 6.022 × 10²³ atoms or molecules (Avogadro's number).

Atomic Structure: Nucleus and Electron Shell

The atom has a central core, the nucleus, in which almost all its mass is concentrated. This mass is provided by the neutrons and protons.

Outside the nucleus is the electron shell (or cortex), containing negatively charged electrons. These negative charges are offset by the positive charge of protons in the nucleus,... Continue reading "Chemistry Fundamentals: Atoms, Volume, and Naming" »

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" »

Fundamental Properties of Elements

Classification of Elements

Metallic Properties

• Conductivity: Excellent conductors of heat and electricity.
• Physical State: Solid at room temperature, except for mercury (Hg), which is liquid.
• Ions: Tend to lose electrons, forming positive ions (cations).
• Other Characteristics: Approximately 75% of all elements are metals. They possess metallic luster and are ductile.

Nonmetallic Properties

• Conductivity: Poor conductors of heat and electricity.
• Physical State: Can be solid (e.g., Carbon, Sulfur), liquid (e.g., Bromine), or gas (e.g., Oxygen, Nitrogen) at room temperature.
• Ions: Tend to gain electrons, forming negative ions (anions).
• Other Characteristics: Most nonmetal solids are soft.

Noble Gases

• Physical State: Gases

Matter and Material Systems

Matter and Its Appearance.

Heterogeneous Material Systems

Heterogeneous material systems are those that have different properties in different parts of the system. Distinct parts can be distinguished either by the naked eye or microscopically.

Separating Heterogeneous Mixtures

A heterogeneous material system formed by various substances is called a heterogeneous mixture. To separate heterogeneous mixtures, various techniques are used:

• Components with Different States of Aggregation

  To separate heterogeneous mixtures where, for example, one component is a solid and the other a liquid, filtration can be used.

• Components with Different Density

  In this case, techniques such as sedimentation, decantation, or centrifugation are

Bohr's Atomic Model

Rutherford's atomic model was incomplete and unstable. The Danish physicist Bohr proposed a new atomic model based on four postulates, among which was the continuity of orbits:

1. The atom consists of a nucleus with a positive charge and contains most of the mass of the atom, and a cortex in which the electrons are moving.
2. Electrons move in circular orbits around the nucleus.
3. Only orbits in which the electron's angular momentum is a multiple of Planck's constant are possible. In these orbits, the electron does not emit or absorb energy.
4. The step from one orbit to another involves the absorption or emission of radiation. The atom will absorb or emit only the radiation just enough to move from one orbit to another.

The Bohr model can... Continue reading "Bohr and Rutherford Atomic Models: Key Differences" »