Chapter 5: Thermochemistry – Study Notes

Chapter 5: Thermochemistry

Energy

Thermochemistry is the study of energy changes, particularly heat, that accompany chemical and physical processes. Understanding energy, its forms, and how it is measured is fundamental to this topic.

• Definition of Energy: Energy is the capacity to do work or transfer heat. It exists in various forms, such as kinetic and potential energy.

• Units of Energy: The SI unit of energy is the joule (J). Another common unit is the calorie (cal), where $1\ \text{cal} = 4.184\ \text{J}$.

• Kinetic Energy (KE): The energy of motion, given by $KE = \frac{1}{2}mv^2$.

• Potential Energy (PE): The energy stored due to position or composition.

• Internal Energy (U): The total energy contained within a system, including both kinetic and potential energy.

• Work (w): Energy used to move an object against a force. $w = F \times d$ (force times distance).

• Heat (q): Energy transferred due to temperature difference.

Example: When gasoline burns in a car engine, chemical potential energy is converted to kinetic energy and heat.

System and Surroundings

In thermochemistry, it is important to define the system (the part of the universe being studied) and the surroundings (everything else).

• System: The portion of the universe under study (e.g., the contents of a beaker).

• Surroundings: Everything outside the system.

• Types of Systems:

  • Open System: Can exchange both matter and energy with surroundings.

  • Closed System: Can exchange energy but not matter.

  • Isolated System: Cannot exchange matter or energy.

Example: A sealed thermos bottle is an example of an isolated system.

Sign Conventions and Enthalpy

Thermochemical calculations require careful attention to the sign conventions for heat and work.

• Exothermic Process: Releases heat to surroundings ($q < 0$).

• Endothermic Process: Absorbs heat from surroundings ($q > 0$).

• Enthalpy (H): The heat content of a system at constant pressure. The change in enthalpy ($\Delta H$) is given by:

$\Delta H = H_{\text{final}} - H_{\text{initial}}$

• State Function: A property that depends only on the current state of the system, not on the path taken to reach that state (e.g., internal energy, enthalpy).

Example: The combustion of methane is exothermic, releasing heat to the surroundings.

Calorimetry

Calorimetry is the measurement of heat flow. It is used to determine the heat exchanged in chemical reactions or physical changes.

• Calorimeter: A device used to measure heat flow.

• Specific Heat Capacity (c): The amount of heat required to raise the temperature of 1 gram of a substance by 1°C.

• Heat Transfer Equation:

$q = mc\Delta T$

• Units: Heat (q) in joules (J), mass (m) in grams (g), specific heat (c) in J/g·°C, temperature change ($\Delta T$) in °C.

• Constant Pressure Calorimetry: Used for reactions in solution (e.g., coffee-cup calorimeter).

• Constant Volume Calorimetry: Used for combustion reactions (e.g., bomb calorimeter).

Example: Measuring the heat released when dissolving salt in water using a coffee-cup calorimeter.

Enthalpy of Reaction and Hess's Law

The enthalpy change for a reaction can be determined directly or indirectly using Hess's Law.

• Standard Enthalpy of Formation ($\Delta H_f^\circ$): The enthalpy change when one mole of a compound is formed from its elements in their standard states.

• Hess's Law: The total enthalpy change for a reaction is the sum of the enthalpy changes for individual steps.

$\Delta H_{\text{reaction}} = \sum \Delta H_{\text{products}} - \sum \Delta H_{\text{reactants}}$

• Application: Allows calculation of enthalpy changes for reactions that are difficult to measure directly.

Example: Calculating the enthalpy change for the formation of CO2 from C and O2 using known enthalpies of formation.

Summary Table: Types of Systems

System Type	Matter Exchange?	Energy Exchange?	Example
Open	Yes	Yes	Open beaker
Closed	No	Yes	Sealed flask
Isolated	No	No	Thermos bottle

Additional info:

• These notes expand on the learning goals and key concepts outlined in the provided syllabus for Chapter 5: Thermochemistry.

• Examples and equations have been added for clarity and completeness.