General Chemistry Exam 3 Study Guide: Thermochemistry, Chemical Bonding, and Molecular Geometry

Thermochemistry

Internal Energy, Enthalpy, and the First Law of Thermodynamics

Thermochemistry studies the energy changes that occur during chemical reactions and changes of state. The internal energy of a system is the sum of all kinetic and potential energies of its particles. The First Law of Thermodynamics states that energy cannot be created or destroyed, only transferred.

• Internal Energy Change ($\Delta E$): $\Delta E = q + w$, where $q$ is heat and $w$ is work.

• Exothermic Process: Energy is released to the surroundings; $\Delta H$ is negative.

• Endothermic Process: Energy is absorbed from the surroundings; $\Delta H$ is positive.

• Work: Work done by the system on the surroundings is negative; work done on the system is positive.

Example: If a system gives off 48.0 kJ of heat and does 86.0 kJ of work on the surroundings, $\Delta E = -48.0\,\text{kJ} + (-86.0\,\text{kJ}) = -134.0\,\text{kJ}$.

Calculating Heat and Calorimetry

Calorimetry measures the heat exchanged in chemical reactions. The heat absorbed or released is calculated using:

• $q = mC_s\Delta T$

• Where: $m$ = mass, $C_s$ = specific heat capacity, $\Delta T$ = temperature change

Example: Heating 1400 g of water from 20.0°C to 100.0°C with $C_s = 4.18\,\text{J/g} \cdot \text{°C}$:

• $q = 1400 \times 4.18 \times (100.0 - 20.0) = 468,000\,\text{J} = 468\,\text{kJ}$

Hess's Law

Hess's Law allows calculation of enthalpy changes for reactions by combining known enthalpy changes of related reactions.

• Formula: $\Delta H_{rxn} = \sum \Delta H_{products} - \sum \Delta H_{reactants}$

Example: For the reaction $\text{CaO(s) + CO_2(g) \rightarrow CaCO_3(s)}$, combine the enthalpy changes of related reactions to find $\Delta H_{rxn}$.

Chemical Bonding and Lewis Structures

Lewis Dot Structures

Lewis structures represent the arrangement of valence electrons in molecules. They help predict molecular shape, bond order, and formal charges.

• Steps:

  1. Count total valence electrons.

  2. Arrange atoms and connect with single bonds.

  3. Distribute remaining electrons to complete octets.

  4. Assign formal charges as needed.

• Formal Charge: $\text{FC} = \text{valence electrons} - \text{nonbonding electrons} - \frac{1}{2}(\text{bonding electrons})$

Example: For $\text{SCN}^-$, assign formal charges to S, C, and N based on their electron counts.

Bond Order and Bond Strength

Bond order is the number of chemical bonds between a pair of atoms. Higher bond order means stronger and shorter bonds.

• Bond Order Formula: $\text{Bond Order} = \frac{\text{number of bonding pairs}}{\text{number of atom pairs}}$

• Example: The bond order of $\text{H}_2^+$ is 0.5.

Polarity and Dipole Moments

A polar covalent bond occurs when electrons are shared unequally between atoms, resulting in a dipole moment.

• Dipole Moment: $\mu = Q \times r$

• Example: HCl has a polar covalent bond due to the difference in electronegativity between H and Cl.

Molecular Geometry and VSEPR Theory

Electron and Molecular Geometry

The Valence Shell Electron Pair Repulsion (VSEPR) Theory predicts the shapes of molecules based on electron pair repulsion.

• Electron Geometry: Arrangement of all electron groups (bonding and lone pairs).

• Molecular Geometry: Arrangement of only the atoms (ignoring lone pairs).

Molecule	Electron Geometry	Molecular Geometry
BeF2	Linear	Linear
BF3	Trigonal Planar	Trigonal Planar
CH4	Tetrahedral	Tetrahedral
NH3	Tetrahedral	Trigonal Pyramidal
SF6	Octahedral	Octahedral

Hybridization

Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding.

• sp3 Hybridization: Tetrahedral geometry (e.g., CH4).

• sp2 Hybridization: Trigonal planar geometry (e.g., BF3).

• sp Hybridization: Linear geometry (e.g., BeF2).

Selected Physical Constants and Reference Tables

Physical Constants

Physical constants are essential for calculations in chemistry. Common constants include Avogadro's number, the gas constant, and the speed of light.

Constant	Symbol	Value
Avogadro's constant	NA	6.02214076 × 1023 mol−1
Ideal Gas Constant	R	8.3144621 J·mol−1·K−1
Speed of light in vacuum	c	2.99792458 × 108 m·s−1
Boltzmann constant	kB	1.380649 × 10−23 J·K−1
Electron rest mass	me	9.10938356 × 10−31 kg

Periodic Table and Activity Series

The Periodic Table organizes elements by increasing atomic number and groups elements with similar chemical properties. The activity series ranks metals by their reactivity, useful for predicting single displacement reactions.

• Example: Alkali metals (Group 1) are highly reactive; noble gases (Group 18) are inert.

Additional info:

• Some questions and tables were inferred to provide complete academic context.

• Reference tables and periodic table are standard resources for general chemistry exams.