Chapter 5: Thermochemistry

Introduction to Thermochemistry

Thermochemistry is the study of energy changes, particularly heat, that accompany chemical reactions and physical changes. This chapter focuses on understanding how energy interacts with matter, how it is transferred, and how it can be measured and calculated in chemical systems.

• Key Concepts: Energy transfer, heat, work, enthalpy, calorimetry, and thermodynamic processes.

Energy in Chemistry

Forms and Transfer of Energy

Energy is a property of matter that enables it to do work. In chemistry, energy is often transferred as heat or work during chemical and physical processes.

• Potential Energy (PE): Energy due to position. In chemistry, this often refers to electrostatic energy between charged particles, such as ions in a lattice or electrons in bonds.

• Kinetic Energy (KE): Energy of motion. For molecules and atoms, kinetic energy is related to their movement.

Key Equations:

• Electrostatic potential energy: where J·m/C2, and are charges, is distance.

• Kinetic energy: where is mass (kg), is velocity (m/s).

Energy Transfer: Heat and Work

Energy is usually transferred between a system and its surroundings as either heat () or work ().

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

• Work (): Energy transferred when an object is moved by a force.

Joule (J): The SI unit of energy.

Calorie (cal): Another unit of energy.

Systems, Surroundings, and Energy Flow

Definitions

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

• Surroundings: Everything outside the system.

Energy can be transferred between the system and surroundings as heat () or work ().

• Endothermic process: System gains heat from surroundings ().

• Exothermic process: System loses heat to surroundings ().

Transferring Heat Energy

• Heat always flows from high to low temperature.

• Loss of heat (): System temperature decreases, surroundings gain heat ().

• Gain of heat (): System temperature increases, surroundings lose heat ().

Specific Heat and Calorimetry

Specific Heat Capacity ()

Specific heat capacity is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C (or 1 K).

• Formula: where is heat (J), is mass (g), is temperature change ().

• Each substance has a unique specific heat due to its composition and structure.

Example 1: Calculating Specific Heat

Problem: An unknown metal (23.4 g) is heated to 100.0°C and transferred to 2.0 L of water. The metal releases 669.4 J of heat, and the final temperature of the metal is 25.7°C. Find the specific heat () of the metal.

• Identify system and surroundings.

• Use , plug in values, and solve.

Calorimetry

Calorimetry is the measurement of heat flow. A calorimeter is an insulated device used to measure heat absorbed or released during a process.

• Heat lost by system = Heat gained by surroundings:

• Standard calorimetry is usually done at constant pressure.

Example 2: Calculations Using Specific Heat

Problem: A sample of NaCl (0.563 g) absorbs 22.18 kJ of heat. Initial temperature is 20.5°C, . Find the final temperature.

• Rearrange to solve for .

• Plug in values and solve.

Example 3: Calorimetry Calculations

Problem: 0.563 g of NaCl is dissolved in 100.0 mL of water at 23.0°C in a calorimeter; final temperature is 18.7°C. , .

• Add masses to find total mass of solution.

• Use for surroundings.

• Flip sign of to reflect .

Bomb Calorimetry

Bomb calorimetry is used to measure heat of combustion at constant volume. The heat capacity () of the calorimeter is known.

• Formula:

• Units for : J/°C or J/K.

Internal Energy, Work, and Enthalpy

Internal Energy ()

The internal energy of a system is the sum of all kinetic and potential energies. It changes when heat or work is transferred:

• Formula:

Work ()

Work is energy transferred when an object is moved by a force. In chemistry, work is often done by gases expanding or contracting at constant pressure:

• General formula:

• For gases:

• Units: 1 L·atm = 101.3 J

Summary Table: Work, Heat, and Internal Energy

Enthalpy ()

Enthalpy is the heat content of a system at constant pressure. It is a state function and is used to quantify heat changes in chemical reactions.

• Formula: (at constant pressure)

• Relationship to internal energy:

Calculating Enthalpy

• Use and moles of substance:

Enthalpy of Reaction ()

The enthalpy of reaction is the difference between the enthalpy of products and reactants:

• Formula:

• If , is positive (endothermic).

• If , is negative (exothermic).

Summary of Key Concepts

• Energy can be transferred as heat or work.

• Specific heat capacity allows calculation of heat absorbed or released.

• Calorimetry measures heat changes in chemical processes.

• Internal energy and enthalpy are state functions used to track energy changes.

• Sign conventions are important for interpreting , , and .