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, primarily as heat and work.

• Internal Energy (U): The total energy contained within a system, including kinetic and potential energies of all particles.

• Enthalpy (H): A thermodynamic quantity defined as $H = U + PV$, where P is pressure and V is volume. It represents the heat content of a system 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 ($riangle H^ ext{°}$): The enthalpy change measured under standard conditions (1 atm, 25°C, 1 M concentration).

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

Calculating $riangle H$ for Reactions

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

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

• Formula: $riangle H_ ext{rxn} = riangle H_f^ ext{°}( ext{products}) - riangle H_f^ ext{°}( ext{reactants})$

Example: Calculate $riangle H$ for $2NO_2(g) + 2H_2O(l) ightarrow N_2O_4(g) + 2H_2O(g)$ using standard enthalpies of formation.

Heat Capacity and Calorimetry

• Specific Heat Capacity (c): The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• Formula: $q = mc riangle T$

• Coffee-cup Calorimeter: Used to measure heat changes at constant pressure.

Example: The specific heat capacity of a metal can be determined by measuring the temperature change of water when the metal is heated and placed in the water.

Chemical Bonding and Molecular Structure

Types of Chemical Bonds

Chemical bonds are the forces that hold atoms together in compounds.

• 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.

• Maximum Number of Bonds: The maximum number of covalent bonds between two atoms is determined by the number of available valence electrons.

Example: The triple bond in nitrogen ($N ext{≡} N$) is the maximum number of bonds between two nitrogen atoms.

Lewis Structures and Resonance

• Lewis Structure: A diagram showing the arrangement of valence electrons among atoms in a molecule.

• Resonance: Occurs when more than one valid Lewis structure can be drawn for a molecule; the actual structure is a hybrid.

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

Example: The best Lewis structure for OCl2 is the one with minimized formal charges and complete octets.

VSEPR Theory and Molecular Geometry

• VSEPR Theory: Valence Shell Electron Pair Repulsion theory predicts the shape of molecules based on electron pair repulsion.

• Common Geometries:

  • Linear: 180° bond angle

  • Trigonal planar: 120° bond angle

  • Tetrahedral: 109.5° bond angle

  • Trigonal bipyramidal: 90°, 120° bond angles

  • Octahedral: 90° bond angles

• Hybridization: Mixing of atomic orbitals to form new hybrid orbitals (e.g., sp, sp2, sp3).

Example: The geometry of SF6 is octahedral due to six bonding pairs around the central atom.

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: Calculate the volume of 1.0 mol of gas at STP (Standard Temperature and Pressure: 0°C, 1 atm).

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 gas.

• Formula: $P_ ext{total} = P_1 + P_2 + P_3 + ext{...}$

Example: The partial pressure of nitrogen in air can be calculated using Dalton's Law.

Intermolecular Forces and Phase Changes

Types of Intermolecular Forces

• London Dispersion Forces: Weak 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: Water exhibits hydrogen bonding, which accounts for its high boiling point.

Boiling Points and Phase Changes

• Boiling Point: The temperature at which a liquid changes to a gas.

• Factors Affecting Boiling Point: Strength of intermolecular forces, molecular mass, and structure.

Example: Among H2, He, and CH4, He has the lowest boiling point due to weak London dispersion forces.

Additional info:

• Some questions also cover topics such as work done by gases, entropy, and molecular orbital diagrams, which are relevant to chapters on thermochemistry, chemical bonding, and molecular structure.

• Practice questions include calculations using enthalpy, heat capacity, gas laws, and Lewis structures, providing comprehensive coverage for exam preparation.