Chemical Equilibrium, Reaction Rates, and Solutions
Mass Action Law
The mass action law states that the value of the equilibrium constant Kc is constant for a particular reaction at a constant temperature, provided that equilibrium concentrations are used. The mathematical expression representing the equilibrium constant at a certain temperature is:
aA + bB ⇌ cC + dD
Kc = ([C]c[D]d) / ([A]a[B]b)
This shows that the equilibrium constant is the ratio of the product concentrations to the reactant concentrations, each raised to the power of their stoichiometric coefficient.
Concentration of Species
In gas-phase reactions, concentration can also be expressed in terms of partial pressures. Since gas pressure (P) is directly related to molar density (n/V) by P = (n/V)RT at constant temperature, the equilibrium can be written as a function of partial pressures of reactants and products.
Le Chatelier’s Principle
Le Chatelier’s principle states that when a system at chemical equilibrium is disturbed by a change in temperature, pressure, or concentration, the system shifts its equilibrium position to counteract the effect of the disturbance. This principle is used to evaluate the effects of such changes on chemical equilibrium.
Reaction Rate
The reaction rate is the change in the amount of substance (in moles) of a reagent or product per unit time and volume.
Reaction rate is an intensive property that depends on temperature, pressure, and concentration in a homogeneous system.
Factors Affecting Reaction Rate
- Concentration: The speed of a reaction often increases with increasing reactant concentration.
- Temperature: Reactions generally proceed faster at higher temperatures.
- Catalyst: A catalyst increases the reaction rate without being consumed. Inhibitors slow down reactions.
- Surface Area: For reactions involving a solid and a gas or liquid, the surface area of the solid affects the reaction rate. A larger surface area increases the rate.
Second-Order Reactions
A second-order reaction has a rate that depends on the concentration of one reactant raised to the second power or the product of the concentrations of two different reactants, each raised to the first power. The simplest type involves only one type of molecule.
The rate law is given by: Rate = k[A][B]
Catalysts
Catalysts are substances that increase the speed of a chemical reaction without being consumed. They work by forming an intermediate that regenerates in a subsequent step. The presence of a catalyst only accelerates the attainment of equilibrium; the same equilibrium mixture is achieved with or without the catalyst.
Catalyst Classification
- Homogeneous Catalysis: Reactants and catalyst are in the same phase (usually liquid, but can be gas).
- Heterogeneous Catalysis: Reactants and catalyst are in different phases. Reactants in gas or liquid phase react at the surface of a solid catalyst.
- Enzyme Catalysis: Highly specific biological catalysts that increase the rate of biochemical reactions.
Saturated Solutions
Empirical solutions are qualitative, described by relative proportions of solute to solvent.
Saturated solutions contain the maximum amount of solute that can dissolve at a given temperature; any additional solute will not dissolve.
Supersaturated solutions contain more solute than a saturated solution at the same temperature, achieved by dissolving the solute at a higher temperature and then cooling the solution.
Factors Affecting Solubility
- Nature of solute and solvent
- Temperature
- Pressure
The rate of dissolution depends on solute particle size, agitation speed, and temperature.
Spontaneous Processes
Radioactive substances emit three main types of radiation: alpha particles (α), beta particles (β), and gamma rays (γ).
Radioactivity
Except for hydrogen, all atomic nuclei contain protons and neutrons. Unstable nuclei spontaneously emit particles and/or electromagnetic radiation, a phenomenon known as radioactivity. This occurs in all elements with atomic numbers greater than 82.