Thermochemistry

Endothermic and Exothermic Reactions

Thermochemistry studies the energy changes that occur during chemical reactions. Reactions are classified as endothermic or exothermic based on heat flow.

• Endothermic reactions: Absorb heat from the surroundings; the system gains energy.

• Exothermic reactions: Release heat to the surroundings; the system loses energy.

• Example: Combustion of methane is exothermic; melting of ice is endothermic.

System and Surroundings

The system is the part of the universe under study (e.g., the chemicals in a reaction vessel), while the surroundings are everything else.

• Energy exchange occurs between the system and surroundings as heat (q) or work (w).

Units of Heat and Specific Heat Capacity

• Heat (q) units: Joule (J), calorie (cal), kilocalorie (kcal)

• 1 cal = 4.184 J; 1 kcal = 1000 cal

• Specific heat capacity (c): Amount of heat required to raise the temperature of 1 g of a substance by 1°C (or 1 K). Units: J/g·°C or J/g·K

Calculating Kinetic Energy

Kinetic energy (KE) of an object is given by:

• m = mass (kg), v = velocity (m/s)

• Example: Calculate the kinetic energy of a car (mass = 1575 kg) moving at 89.5 km/h.

Calculating Heat Transfer

The heat (q) absorbed or released by a substance is calculated as:

• m = mass (g), c = specific heat (J/g·°C), = change in temperature (°C)

• Example: Calculate the heat required to raise 25 g of Fe from 37°C to 75°C, given J/g·°C.

Electromagnetic Radiation

• Types of radiation: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray

• Frequency (), wavelength (), and speed of light (c) are related by:

• c = speed of light ( m/s)

Atomic Structure and Electron Configuration

Subshells and Orbitals

Each principal energy level (n) contains n subshells. Subshells are labeled s, p, d, f.

• s: 1 orbital, 2 electrons

• p: 3 orbitals, 6 electrons

• d: 5 orbitals, 10 electrons

• f: 7 orbitals, 14 electrons

Electron Configuration

• Filling order: 1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p

• Hund's rule, Pauli exclusion principle, and Aufbau principle govern electron filling.

• Transition metals may have exceptions due to stability of half-filled and fully-filled d subshells.

Quantum Numbers

• Principal quantum number (n): energy level

• Angular momentum quantum number (l): subshell (s=0, p=1, d=2, f=3)

• Magnetic quantum number (): orientation

• Spin quantum number (): +1/2 or -1/2

Periodic Properties of the Elements

Trends in the Periodic Table

• Atomic radius: Increases down a group, decreases across a period.

• Ionization energy: Energy required to remove an electron; increases across a period, decreases down a group.

• Electron affinity: Energy change when an atom gains an electron; generally more negative across a period.

• Metallic character: Increases down a group, decreases across a period.

Group and Period Similarities

• Elements in the same group have similar chemical properties due to similar valence electron configurations.

Metallic and Nonmetallic Oxides

• Metallic oxides (e.g., Na2O, K2O) form basic solutions in water.

• Nonmetallic oxides (e.g., CO2, SO2, NO2) form acidic solutions in water.

Thermodynamics and Hess's Law

First Law of Thermodynamics

Energy cannot be created or destroyed; it can only be transferred or transformed.

• Internal energy change () is the sum of heat (q) and work (w):

Enthalpy ()

• Enthalpy is the heat content of a system at constant pressure.

• Exothermic: is negative (heat released)

• Endothermic: is positive (heat absorbed)

Hess's Law

• The enthalpy change for a reaction is the same, regardless of the number of steps.

• If a reaction is reversed, the sign of is reversed.

• If a reaction is multiplied by a factor, is multiplied by the same factor.

Practice Problems and Applications

• Identify endothermic/exothermic processes from reaction equations.

• Calculate enthalpy changes using Hess's Law.

• Determine which elements have similar chemical properties based on group/period.

• Predict trends in atomic radius, ionization energy, and electron affinity.

• Classify oxides as acidic or basic based on their metallic or nonmetallic nature.

• Apply quantum numbers to determine electron configurations and subshell capacities.

Key Equations and Constants

• 1 cal = 4.184 J

• Speed of light, m/s

Additional info:

• Students should be familiar with the periodic table and be able to identify trends and exceptions, especially for transition metals.

• Understanding the relationship between electronic structure and chemical properties is essential for predicting reactivity and bonding.