Chemical Reactions and Laws: Essential Concepts
Chemical Reactions and Fundamental Laws
Chemical Reaction: A process involving the combination of two or more initial substances, called reagents, resulting in new products. These products possess physical and chemical properties entirely distinct from the starting substances.
Key Laws Governing Chemical Reactions
Lavoisier’s Law (Law of Conservation of Mass): In any chemical reaction, the total mass of the reactants is equal to the total mass of the products. In essence, “In a chemical reaction, matter is neither created nor destroyed, only transformed.”
Proust’s Law (Law of Definite Proportions): This experimental law states that “Elements always combine to form a compound in a defined and constant mass ratio.”
Volumetric Laws (Gay-Lussac’s Laws):
- (I) At the same pressure (P) and temperature (T), the volumes (V) of reacting gases maintain a constant relationship, expressible by small, whole numbers (n).
- (II) At the same P and T, the volume of the gaseous products formed in a reaction maintains a constant relationship with the volumes of the reacting gases, also expressible by simple whole numbers.
Avogadro’s Hypothesis: Avogadro clarified the distinction between atoms and molecules. Initially, his ideas were not accepted by Dalton and his followers, who believed all elements were monatomic. Avogadro explained Gay-Lussac’s laws based on two hypotheses:
- (I) Equal volumes of different gases under the same conditions of P and T contain the same number of molecules.
- (II) Common gaseous elements are composed of diatomic molecules (two identical atoms), which can separate during a chemical reaction.
Atomic and Molecular Properties
Atomic Mass: The mass of an atom expressed in atomic mass units (u). It is defined as the number of times the mass of the atom contains one-twelfth of the mass of the carbon-12 isotope.
Molecular Mass: The mass of a molecule, expressed in atomic mass units (u). It is calculated by summing the masses of all the atoms that constitute the molecule.
Mole: The number of grams of an element that numerically equals its atomic mass.
Molecule (of a compound): The number of grams of a compound that numerically equals its molecular mass.
Avogadro’s Number: The number of atoms in one mole of atoms of any element, or the number of molecules in one mole of molecules of any compound. Its value is 6.023 x 1023.
Gas Laws
General Gas Equation: A combination of Boyle’s Law and Charles’s/Gay-Lussac’s Laws. It states that for a given mass of gas, the product of the gas pressure and its volume, divided by its temperature in Kelvin, is a constant.
Ideal Gas Equation: A combination of Boyle’s Law, Charles’s Law, and Avogadro’s Law.
Normal Molar Volume of a Gas: One mole of any gas at standard conditions (1 atm pressure and 0°C) occupies a fixed volume of 22.4 liters.
Chemical Formulas
Proximate Calculation: Determining the percentage of each element present in a compound.
Empirical Formula: Indicates the simplest whole-number ratio of atoms in a molecule of a compound.
Molecular Formula: Indicates the actual number of atoms of each element in a molecule of a compound.
Ions and Atomic Structure
Cation: A positively charged ion, meaning it has lost electrons.
Ion: A charged particle formed when a neutral atom or group of atoms gains or loses electrons.
Atomic Number (Z): A positive integer equal to the total number of protons in the nucleus of an atom.
Mass Number (A): The sum of the number of protons and neutrons in the nucleus of an atom.