Thermochemistry

Energy and Thermodynamics

Thermochemistry is the study of energy changes, particularly heat, that accompany chemical reactions. Energy is the ability to do work or transfer heat, and in chemistry, it is often discussed in terms of potential energy, especially electrostatic potential energy between charged particles.

• Electrostatic Potential Energy (Eel): The energy due to interactions between charged particles.

• Joule (J): The SI unit of energy.

• Energy Conversion: Examples include chemical energy converted to heat (heating homes) and sunlight converted to chemical energy (photosynthesis).

First Law of Thermodynamics

The first law states that energy can be converted from one form to another, but it cannot be created or destroyed.

• System: The part of the universe being studied (e.g., reactants in a reaction).

• Surroundings: Everything outside the system.

• Types of Systems:

  • Open System: Exchanges both heat and mass with surroundings.

  • Closed System: Exchanges only heat, not mass.

  • Isolated System: Exchanges neither heat nor mass.

Internal Energy and Its Change

The internal energy (E) of a system is the sum of all kinetic and potential energies of its components. The change in internal energy, ΔE, is given by:

• If ΔE > 0: System absorbs energy.

• If ΔE < 0: System releases energy.

Thermodynamic Quantities and Sign Conventions

Thermodynamic quantities have three parts: a number, a unit, and a sign. The sign indicates the direction of energy flow.

• (where q = heat, w = work)

Endothermic and Exothermic Processes

• Endothermic: Heat is absorbed by the system (ΔH > 0).

• Exothermic: Heat is released by the system (ΔH < 0).

State Functions

State functions depend only on the current state of the system, not the path taken. Internal energy (E) and enthalpy (H) are state functions, but heat (q) and work (w) are not.

Work and Enthalpy

• Work (w): For a gas, (work done by the system is negative).

• Enthalpy (H):

• At constant pressure,

• Since and , at constant pressure.

Enthalpy of Reaction

• This is called the enthalpy of reaction or heat of reaction.

Calorimetry

Calorimetry is the measurement of heat flow. A calorimeter is used to measure the heat absorbed or released during a reaction.

Hess’s Law and Enthalpy of Formation

• Hess’s Law allows calculation of ΔH for a reaction using known ΔH values.

• Enthalpy of formation (): The enthalpy change when a compound is formed from its elements.

Energy in Foods and Fuels

Most energy in foods comes from carbohydrates, fats, and proteins.

• Carbohydrates: 17 kJ/g

• Fats: 38 kJ/g

• Proteins: 17 kJ/g

Electronic Structure of Atoms

Nature of Electromagnetic Radiation

Understanding the electronic structure of atoms requires knowledge of electromagnetic radiation, which travels as waves at the speed of light.

• Wavelength (λ): Distance between corresponding points on adjacent waves.

• Frequency (ν): Number of waves passing a point per unit time.

• Speed of Light (c): ; m/s

Quantum Theory and Atomic Spectra

• Energy is quantized; it comes in packets called quanta.

• Planck's constant: J·s

• Energy of a photon:

• Emission spectra: Atoms emit light at specific wavelengths.

Bohr Model and Quantum Numbers

• Electrons exist in discrete energy levels described by quantum numbers.

• Energy is involved in transitions between levels.

• Principal quantum number (n): Energy level (n ≥ 1).

• Angular momentum quantum number (l): Orbital shape (0 ≤ l ≤ n-1).

• Magnetic quantum number (ml): Orbital orientation (−l ≤ ml ≤ l).

• Spin quantum number (ms): Electron spin (+½ or −½).

Relationship Among Quantum Numbers

Orbital Types and Shapes

• s Orbitals: Spherical shape (l = 0).

• p Orbitals: Two lobes with a node (l = 1).

• d Orbitals: Four lobes or a doughnut shape (l = 2).

• f Orbitals: Complex shapes (l = 3).

Electron Spin and Pauli Exclusion Principle

• Each orbital can hold two electrons with opposite spins.

• No two electrons in the same atom can have the same set of four quantum numbers.

Electron Configurations and Orbital Diagrams

Electron configuration describes the arrangement of electrons in an atom. The most stable configuration is the ground state.

• Notation: Number (energy level), letter (orbital type), superscript (number of electrons).

• Example: Lithium (Li) has configuration 1s22s1.

Electron Configurations of Several Lighter Elements

Condensed Electron Configurations

• Valence electrons: Electrons in the outermost shell.

• Core electrons: Filled inner shell electrons.

• Condensed notation uses noble gas symbols in brackets for core electrons.

Periodic Table and Electron Filling

• Orbitals are filled in order of increasing energy.

• s and p blocks: Main-group elements.

• d block: Transition elements.

• f block: Lanthanides and actinides.

Example:

• Argon (Ar): 1s22s22p63s23p6

• Potassium (K): [Ar]4s1

Additional info: The notes have been expanded to include definitions, formulas, and examples for clarity and completeness, as well as logical grouping of fragmented points.