Stoichiometry, Solutions, Kinetics, and Chemical Equilibrium

Stoichiometry is the field of chemistry that studies the relationships between the quantities involved in a chemical reaction. The laws governing stoichiometry are:

  • Law of definite proportions: when you combine two or more elements for a given compound, they always do so with a relationship of constant mass.
  • Law of constant composition: All chemical elements in a pure state always contain the same elements.
  • Law of conservation of mass: Matter is not created or destroyed, only transformed.

Stoichiometry is determined by:

  • Limiting reagent: The reactant that determines the amount of product obtained in a chemical reaction.
  • Reaction yield: The amount of product produced in a chemical reaction.

Solutions are homogeneous mixtures of a solute in a solvent.

  • Solute: The substance dissolved.
  • Solvent: The substance that dissolves the solute.

Types of solutions:

  • Diluted: A solution with a small amount of solute relative to the solvent.
  • Saturated: A solution with the maximum amount of solute that can dissolve at a given temperature.
  • Supersaturated: A solution that contains more solute than it can normally dissolve at a given temperature.

Solubility: The maximum amount of solute that can dissolve in a given amount of solvent at a given temperature.

Factors affecting solubility:

  • Contact Surface: The solute-solvent interaction increases with more surface contact, leading to faster dissolution (e.g., spraying the solute).
  • Agitation: Shaking the solution helps separate layers and form new solute-solvent interactions, continuing dissolution.
  • Temperature: Increasing the temperature favors molecular movement, increasing the energy of solid particles and allowing them to dissolve more easily.
  • Pressure: This primarily affects the solubility of gases, with solubility being directly proportional to pressure.

Concentration: The amount of solute in a solution.

Classified as:

Quantitative:

  • Physical Units: % P/P, % P/V, % V/V
  • Chemical Units: Molality, Molarity, Normality, Molar fraction

Colligative properties of solutions are properties that depend on the number of solute particles in a given amount of solvent.

  • Vapor Pressure: Raoult’s Law states that the decrease in vapor pressure of the solvent is proportional to the mole fraction of solute.
  • Boiling Point: The temperature at which the vapor pressure equals the applied pressure (usually atmospheric pressure).
  • Freezing Point: The temperature at which the vapor pressure of liquid and solid are equal. Solutions freeze at a lower temperature than the pure solvent.
  • Osmotic Pressure: The pressure required to prevent the flow of solvent across a semipermeable membrane from a region of lower solute concentration to a region of higher solute concentration.

Chemical Kinetics is the branch of chemistry that studies the rates of chemical reactions and the factors that affect them.

Rate of chemical reaction: The change in concentration of reactants or products per unit time. Factors affecting reaction rate:

  • Nature of the reactants: The inherent properties of the reactants, such as their chemical bonds and molecular structure, influence the reaction rate.
  • Concentration of the reactants: Increasing the concentration of reactants generally increases the reaction rate.
  • Temperature: Increasing the temperature generally increases the reaction rate due to increased kinetic energy of molecules.
  • Catalysis: Catalysts speed up reactions by providing an alternative reaction pathway with a lower activation energy.

Chemical Equilibrium is a state where the rates of the forward and reverse reactions are equal, resulting in no net change in concentrations of reactants and products.

Types of reactions:

  • Direct (Irreversible) Reaction: Reactions that proceed in one direction until one or more reactants are exhausted.
  • Reverse (Reversible) Reaction: Reactions that can proceed in both forward and reverse directions, potentially reaching a state of equilibrium.

External factors affecting chemical equilibrium:

  • Adding or removing a chemical species: Adding a component favors the reaction that consumes it; removing a component favors the reaction that produces it.
  • Changing concentration: The law of mass action states that the rate of a chemical reaction is proportional to the product of the molar concentrations of the reactants.
  • Changing temperature: Increasing temperature favors endothermic reactions; decreasing temperature favors exothermic reactions.
  • Changing pressure: Increasing pressure favors the reaction that produces fewer moles of gas; decreasing pressure favors the reaction that produces more moles of gas.
  • Adding a catalyst: Catalysts do not affect the equilibrium position, but they do speed up the attainment of equilibrium.