Chemical Reactions and Matter: Understanding the Basics

Posted on Jan 16, 2025 in Chemistry

Chemistry: Reactions and Matter

Chemical Reactions

Chemical reactions are the processes by which one or more substances are transformed into one or more different substances.

Pure Substances

A pure substance is a type of material that cannot be separated into other types of matter by the simplest physical methods and has a constant composition.

  • Element: A pure substance that cannot be decomposed by chemical processes.
  • Compound: A pure substance formed by two or more chemically bonded elements in fixed proportions. It can be separated by chemical methods.

Mixtures

A mixture is a type of matter formed by two or more substances that do not react with each other, while keeping their properties, and can be separated by physical methods. They have a variable composition.

  • Homogeneous: Components form a single phase.
  • Heterogeneous: Two or more recognizable phases can be seen.

Physical Methods of Separation

  • Crystallization: Based on the difference in solubility of the components of a mixture, separating a solid from its solution.
  • Filtration: Based on the diversity of particle size, it separates a solid from a liquid.
  • Distillation: Based on the different boiling points of the components.
  • Settling: Based on the different densities of two immiscible liquids.

The Mole Concept

A mole is the amount of a substance that contains as many elementary entities as there are atoms in 12g of carbon-12. One mole of atoms has a mass in grams whose numerical value matches the atomic mass. One mole of molecules has a mass in grams whose numerical value matches that of the mass of a molecule. The mass of one mole of a substance is called its molar mass.

Molar Volume

Molar volume is the volume that one mole of a substance occupies. At 0ºC and 1 atm, 1 mol of any gas occupies a volume of 22.41 L. This is possible because one mole of any substance contains the same number of molecules, and the same number of molecules of a gas occupies the same volume.

Laws of Chemical Combination

  • Law of Mass Conservation: In a chemical reaction, matter is neither created nor destroyed but is transformed. That is, the total mass of the reacting substances coincides with the total mass of the substances formed. The total mass does not change.
  • Law of Definite Proportions: When two or more elements combine chemically to give a particular compound, they always do so in a constant mass ratio.
  • Law of Multiple Proportions: When two elements combine to give different compounds, there is a simple numerical relationship of whole numbers between the various quantities of one element that are combined with a fixed amount of the other.

Dalton’s Atomic Theory

  • Elements are made of atoms.
  • All atoms of the same element have identical mass and properties.
  • Compounds are formed by the union of atoms.
  • Atoms are grouped differently than they were initially.

Modifications to Dalton’s Theory

  1. The atom is made up of other smaller particles: protons, neutrons, and electrons.
  2. Not all atoms of the same element have the same mass (isotopes).
  3. In nuclear reactions, atoms can be created and destroyed.

Law of Combining Volumes

Under the same conditions of pressure and temperature, the volumes of gaseous substances involved in a chemical reaction have a simple numerical relationship between them. This law is only valid for gases.

Avogadro’s Hypothesis

At the same pressure and temperature, equal volumes of any gas contain equal numbers of molecules.

Molecular Mass

Molecular mass is the sum of the masses of the atoms that form the molecule.

Atomic Mass

The atomic mass of an atom is the weighted average of the mass of the isotopes of the element that exists.

Empirical Formula

The empirical formula indicates the simplest whole-number ratio of atoms in a compound.

Molecular Formula

The molecular formula indicates the actual number of atoms of each element in a molecule.

Percent Composition

Percent composition is the percentage by mass of each element in a compound.

Atomic Mass Unit (amu)

An amu is one-twelfth of the mass of a carbon-12 atom.