Core Chemistry Concepts: Matter, Atoms, and Reactions
States of Aggregation
Matter exists in different states based on the intensity of forces between particles:
- Solid: Forces of attraction are very intense. Solids are rigid, cannot flow, maintain their own shape and volume, and are incompressible.
- Liquid: Forces are less intense than in the solid state. Liquids can flow, do not have their own shape (taking the shape of the container), maintain their own volume, and are nearly incompressible (often treated as incompressible, though slight compressibility exists).
- Gas: Forces are very weak. Gases are fluids, occupy the entire volume and shape of the container, and are highly compressible.
Kinetic Molecular Theory (KMT)
The Kinetic Molecular Theory describes the behavior of gases:
- Gases consist of a large number of very small particles (molecules or atoms) in continuous, random motion.
- There are large gaps between gas particles. This means that although a gas occupies a large volume, the volume of the particles themselves is negligible compared to the total volume.
- Gas particles collide with one another and with the walls of their container.
- Particles do not interact (attract or repel) one another, except during collisions.
- The average kinetic energy of the particles is directly proportional to the absolute temperature of the gas. Faster movement corresponds to higher temperatures.
- Since kinetic energy depends on speed, increasing the temperature causes particles to move faster and possess more kinetic energy.
Gas Pressure Explained
Pressure in gases is the force exerted by its particles per unit area when they collide with the container walls. It is expressed in Newtons per square meter (N/m²), known as the Pascal (Pa).
Fundamental Chemistry Laws
These laws describe the quantitative relationships in chemical reactions:
- Law of Conservation of Mass: The total mass of reactants equals the total mass of products. (e.g., Mass A + Mass B = Mass C + Mass D)
- Law of Definite Proportions: A given chemical compound always contains its component elements in fixed ratios by mass, regardless of its source or method of preparation. (Represented stoichiometry: aA + bB → cC + dD)
Dalton’s Atomic Theory Principles
John Dalton proposed the following postulates:
- Elements are composed of extremely small, indivisible particles called atoms.
- All atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in these properties.
- Compounds are formed when atoms of different elements combine in simple, whole-number ratios.
- A chemical reaction involves the rearrangement, separation, or combination of atoms. Atoms are not created or destroyed in a chemical reaction.
The atom is the basic unit of a chemical element that can participate in a chemical reaction.
Bohr’s Atomic Model Explained
Addressing limitations in Rutherford’s model, Niels Bohr proposed his model in 1913, incorporating quantum ideas:
- Electrons orbit the nucleus in specific, stable, circular paths called energy levels or shells.
- While in these orbits, electrons do not radiate energy.
- An electron can jump to a different orbit by absorbing or emitting a specific amount (quantum) of energy.
The Current Atomic Model
Bohr’s model was insufficient for explaining more complex phenomena. The modern Quantum Mechanical Model, developed throughout the early 20th century, is currently accepted. It is highly mathematical and describes electrons in terms of probabilities within regions of space (orbitals), rather than fixed paths. This model has led to significant technological advancements, such as lasers and electron microscopes.
Defining Atoms: Z and A Numbers
Atoms are characterized by:
- The atomic number (Z): The number of protons in the nucleus. This defines the element.
- The mass number (A): The total number of protons (Z) and neutrons (N) in the nucleus. Formula: A = Z + N.
Physical vs. Chemical Changes
Physical changes alter the form or appearance of a substance but do not create new substances (e.g., melting ice, boiling water). Chemical changes (chemical reactions) result in the formation of new substances with different properties (e.g., burning wood, rusting iron).
Reaction Rates and Collision Theory
According to collision theory, chemical reactions occur when reactant molecules collide with sufficient energy (activation energy) and proper orientation to break existing bonds and form new ones.
The rate of a chemical reaction is quantified by observing how the amount of product increases or the amount of reactant decreases over a specific time period.
Increasing the temperature increases the kinetic energy of molecules. This leads to more frequent and more forceful collisions, increasing the likelihood that collisions will have enough energy to overcome the activation energy barrier, thus speeding up the reaction rate.