Thermochemistry and Energy Changes

Introduction to Thermochemistry

Thermochemistry is the study of energy changes, particularly heat, that occur during physical and chemical processes. It is a branch of thermodynamics focused on the transfer of energy as heat between a system and its surroundings.

• Thermodynamics: The broader study of energy changes in chemical and physical processes.

• Thermochemistry: Specifically examines energy changes during chemical reactions.

• System: The part of the universe under study (e.g., the chemicals in a reaction vessel).

• Surroundings: Everything outside the system.

Example: In a reaction flask, the chemicals are the system; the flask, air, and lab are the surroundings.

System & Surroundings

Understanding the distinction between system and surroundings is crucial for analyzing energy flow.

• System: The matter or region being studied.

• Surroundings: Everything else that can exchange energy with the system.

• Energy transfer is measured as heat (q) or work (w).

Diagram: The notes include a diagram showing the system as a circle within a larger circle representing the surroundings.

Types of Heat Transfer

Heat can be transferred in two main ways during chemical or physical changes:

• Exothermic Process: Heat is released from the system to the surroundings.

• Endothermic Process: Heat is absorbed by the system from the surroundings.

Example: Combustion of gasoline is exothermic; melting ice is endothermic.

How Does Heat Travel?

Heat always flows from a hotter object to a cooler one until thermal equilibrium is reached.

• Heat transfer continues until both objects reach the same temperature.

• Measured in joules (J) or calories (cal).

Enthalpy Changes (ΔH)

Enthalpy change (ΔH) is the heat transferred at constant pressure during a chemical reaction.

• ΔH > 0: Endothermic reaction (system absorbs heat).

• ΔH < 0: Exothermic reaction (system releases heat).

• Enthalpy is a state function, meaning it depends only on the initial and final states, not the path taken.

Formula:

Example: For the reaction , (exothermic).

Stoichiometry of Enthalpy Changes

The change in enthalpy is directly proportional to the quantity of reactants undergoing a change.

• Enthalpy changes are often given per mole of reactant or product.

• To calculate enthalpy for a given amount, use stoichiometry.

Example Calculation:

, per mole of Mg.

If you have 2 moles of Mg:

Thermochemical Equations

Thermochemical equations show both the chemical change and the associated enthalpy change.

• Must specify physical states of reactants and products.

• Enthalpy change refers to the reaction as written.

Example:

Standard Enthalpy Change

Standard enthalpy change () is measured under standard conditions: 1 atm pressure and 25°C (298 K).

• Allows comparison of enthalpy changes for different reactions.

Key Laws and Principles

Several laws govern energy changes in chemical reactions:

• Law of Conservation of Energy: Energy cannot be created or destroyed, only transformed.

• First Law of Thermodynamics: The total energy of an isolated system is constant.

Formula:

Where is the change in internal energy, is heat, and is work.

Summary Table: Types of Energy Changes

End-of-Chapter Questions

The notes reference end-of-chapter questions for further practice: 5.28, 42-44, 46, 47.

Additional info: Some diagrams and tables were inferred from context and standard chemistry curriculum. The notes cover foundational concepts in thermochemistry, suitable for General Chemistry students.