Chemical Equilibrium, Acids, Bases, and Redox Reactions
Chemical Equilibrium
At the beginning of a reversible reaction, the high concentration of reactants results in a fast reaction rate towards product formation. As the reaction progresses and product concentration increases, the reverse reaction rate also increases. Eventually, the forward and reverse reaction rates become equal, establishing a state of chemical equilibrium where the concentrations of reactants and products remain constant.
Equilibrium Constant
The Law of Mass Action states: “In a reversible reaction at equilibrium, the ratio of the product of the concentrations of the products to the product of the concentrations of the reactants, each raised to the power of its stoichiometric coefficient, is constant for a given temperature.”
Acids and Bases
Bronsted-Lowry Definition
An acid is a substance that donates protons (H+ ions). A base is a substance that accepts protons. For a substance to act as an acid, it must be in the presence of a base to accept the donated protons. In aqueous solutions, water acts as a base in the presence of an acid.
Relative Strength in Aqueous Media
A strong acid dissociates extensively in solution, resulting in a high hydronium ion concentration and a low concentration of the molecular form. A weak acid dissociates poorly, with a higher concentration of the molecular form compared to the ionized form. The degree of dissociation represents the mole fraction of the acid or base that dissociates.
Hess’s Law
The heat evolved or absorbed in a chemical reaction depends only on the initial and final states of the system, not on the intermediate steps. This is because the heat change at constant pressure equals the enthalpy change, which is a state function. Hess’s Law is useful for calculating the heat of reactions that cannot be measured directly.
pH Concept
To simplify working with negative powers of 10 for hydrogen ion concentration, the pH scale was introduced. A pH below 7 indicates an acidic solution, a pH above 7 indicates a basic solution, and a pH of 7 indicates a neutral solution. Similarly, a pOH of 7 is neutral, above 7 is acidic, and below 7 is basic. The sum of pH and pOH equals 14.
Oxidation-Reduction
Electronic Concept
Oxidation involves the loss of electrons, while reduction involves the gain of electrons. These processes are always coupled: if one substance loses electrons (is oxidized), another must gain them (be reduced). Therefore, these reactions are referred to as oxidation-reduction or redox reactions.
Solubility
Concept
Solubility refers to the concentration of a solute in a saturated solution at a given temperature and with a specific solvent.
Factors Affecting Solubility
- Nature of the solute and solvent: Some substances are highly soluble in a given solvent, while others are sparingly soluble or insoluble.
- Temperature: Generally, the solubility of solids in liquids increases with temperature.
- Presence of a common ion: The solubility of a salt is lower in a solution containing one of its ions compared to a solution without the common ion.
Elements and Compounds
Matter is anything that occupies space and has mass. A material system is a defined portion of matter. Material systems are classified as heterogeneous or homogeneous. A heterogeneous system has distinct regions with different properties. A homogeneous system appears uniform throughout. Homogeneous systems that cannot be separated into simpler components by physical means are called pure substances. Homogeneous systems that can be separated by physical processes are called solutions. Pure substances that cannot be separated into simpler substances by chemical means are called elements. Pure substances that can be separated into two or more elements by chemical processes are called compounds. An atom is the smallest unit of an element that retains its chemical properties. A molecule is the smallest unit of a pure substance that retains its chemical properties.