Ch. 5 Thermochemistry and Energy Changes in Chemical Reactions

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.

Type of Process	Direction of Heat Flow	System Effect
Exothermic	System → Surroundings	System loses energy
Endothermic	Surroundings → System	System gains energy

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:

$\Delta H = H_{\text{products}} - H_{\text{reactants}}$

Example: For the reaction $\mathrm{Mg} + \frac{1}{2} \mathrm{O}_2 \rightarrow \mathrm{MgO}$, $\Delta H = -601.6\ \mathrm{kJ}$ (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:

$\mathrm{Mg} + \frac{1}{2} \mathrm{O}_2 \rightarrow \mathrm{MgO}$, $\Delta H = -601.6\ \mathrm{kJ}$ per mole of Mg.

If you have 2 moles of Mg: $\Delta H = 2 \times (-601.6\ \mathrm{kJ}) = -1203.2\ \mathrm{kJ}$

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:

$\mathrm{Mg}(s) + \frac{1}{2} \mathrm{O}_2(g) \rightarrow \mathrm{MgO}(s)\quad \Delta H = -601.6\ \mathrm{kJ}$

Standard Enthalpy Change

Standard enthalpy change ($\Delta H^\circ$) 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:

$\Delta E = q + w$

Where $\Delta E$ is the change in internal energy, $q$ is heat, and $w$ is work.

Summary Table: Types of Energy Changes

Type	Description	Example
Physical	Change in state or temperature	Melting ice
Chemical	Change in chemical composition	Combustion of methane

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.