General Chemistry Practice Exam Study Guide: Thermochemistry, Bonding, Gases, and Molecular Structure

Thermochemistry and Chemical Energy

Internal Energy and Enthalpy

The study of thermochemistry involves understanding how energy is transferred in chemical reactions and physical changes. Internal energy (E) is the total energy contained within a system, including both kinetic and potential energy of its particles.

• Internal energy (E): The sum of all kinetic and potential energies of the system's components.

• Enthalpy (H): Defined as $H = E + PV$, where P is pressure and V is volume. It is useful for processes occurring at constant pressure.

• Exothermic process: Releases heat to the surroundings; $riangle H$ is negative.

• Endothermic process: Absorbs heat from the surroundings; $riangle H$ is positive.

• Standard enthalpy change ($riangle H^ ext{0}$): The enthalpy change measured under standard conditions (1 atm, 25°C).

Example: The combustion of titanium with oxygen to produce titanium dioxide is an exothermic reaction.

Calculating Enthalpy Changes

• Hess's Law: The total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps.

• Standard enthalpy of formation ($riangle H_f^ ext{0}$): The enthalpy change when one mole of a compound is formed from its elements in their standard states.

• Calorimetry: Used to measure heat changes in chemical reactions. The heat capacity (C) and specific heat (c) are important for calculations.

Formula:

• $q = mc riangle T$ (heat = mass × specific heat × temperature change)

• $riangle H = riangle E + P riangle V$

Example: Calculating the heat required to raise the temperature of a metal sample using its specific heat.

Chemical Bonding and Molecular Structure

Types of Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds. The main types are ionic, covalent, and metallic bonds.

• Ionic bonds: Formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions.

• Covalent bonds: Formed by the sharing of electrons between atoms.

• Metallic bonds: Involve a 'sea' of delocalized electrons around metal ions.

Maximum number of bonds: Determined by the number of available valence electrons.

Lewis Structures and Resonance

Lewis structures represent the arrangement of electrons in a molecule. Resonance structures are alternative ways to draw a molecule that differ only in the placement of electrons.

• Octet rule: Atoms tend to form bonds until they are surrounded by eight valence electrons.

• Formal charge: Used to determine the most stable Lewis structure.

• Resonance: Delocalization of electrons across multiple atoms; actual structure is a hybrid.

Example: Choosing the best Lewis structure for OCl2 and identifying resonance forms for NCO-.

Hybridization and Molecular Geometry

Hybridization explains the shapes of molecules by combining atomic orbitals into new hybrid orbitals.

• sp, sp2, sp3 hybridization: Correspond to linear, trigonal planar, and tetrahedral geometries, respectively.

• VSEPR theory: Predicts molecular shapes based on electron pair repulsion.

• Bond angles: Determined by the number of electron domains around the central atom.

Example: The geometry of BeF2 is linear due to sp hybridization.

Gases: Properties and Behavior

Gas Laws

Gas laws describe the relationships between pressure, volume, temperature, and amount of gas.

• Boyle's Law: $P_1V_1 = P_2V_2$ (at constant temperature)

• Charles's Law: $V_1/T_1 = V_2/T_2$ (at constant pressure)

• Ideal Gas Law: $PV = nRT$

• Avogadro's Law: Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

Example: Calculating the volume of a gas at STP or determining the molar mass from density.

Gas Mixtures and Partial Pressures

• Dalton's Law of Partial Pressures: The total pressure of a mixture of gases is the sum of the partial pressures of each component.

• Collecting gases over water: Must account for vapor pressure of water.

Intermolecular Forces and Phase Changes

Types of Intermolecular Forces

Intermolecular forces determine the physical properties of substances, such as boiling and melting points.

• London dispersion forces: Present in all molecules, especially nonpolar ones.

• Dipole-dipole interactions: Occur between polar molecules.

• Hydrogen bonding: Strong dipole-dipole interaction involving H bonded to N, O, or F.

Example: Identifying which molecule has the lowest boiling point based on intermolecular forces.

Additional Topics

Work, Heat, and Calorimetry

• Work (w): In chemistry, work is often associated with the expansion or compression of gases. $w = -P riangle V$

• Calorimetry: Measurement of heat flow using devices like coffee-cup or bomb calorimeters.

Bond Energies and Reaction Enthalpies

• Bond dissociation energy: The energy required to break a specific bond in a molecule.

• Calculating reaction enthalpy: $riangle H = ext{Sum of bond energies of reactants} - ext{Sum of bond energies of products}$

Electron Configuration and Molecular Orbitals

• Electron configuration: Arrangement of electrons in atomic or molecular orbitals.

• Molecular orbital diagrams: Show the relative energies and occupancy of molecular orbitals in diatomic molecules.

Example: Determining the paramagnetic or diamagnetic nature of a molecule based on its MO diagram.

Summary Table: Key Thermochemistry Equations

Equation	Description
$q = mc riangle T$	Heat absorbed or released by a substance
$riangle H = riangle E + P riangle V$	Relationship between enthalpy and internal energy
$PV = nRT$	Ideal Gas Law
$w = -P riangle V$	Work done by a gas at constant pressure
$riangle H = ext{Sum of bond energies of reactants} - ext{Sum of bond energies of products}$	Calculating reaction enthalpy from bond energies

Practice Applications

• Apply Hess's Law to determine enthalpy changes for multi-step reactions.

• Draw and interpret Lewis structures and resonance forms for polyatomic ions.

• Use VSEPR theory to predict molecular shapes and bond angles.

• Calculate gas volumes, densities, and molar masses using the ideal gas law.

• Identify intermolecular forces and predict physical properties such as boiling points.

Additional info: These study notes synthesize the main topics and concepts covered in the provided practice exam, including thermochemistry, chemical bonding, molecular structure, gas laws, and intermolecular forces, as relevant to a General Chemistry college course.