Thermochemistry and Heat Capacity: Study Notes for General Chemistry

Thermochemistry

Introduction to Thermochemistry

Thermochemistry is the study of energy and heat associated with chemical reactions and physical changes. It is a foundational topic in general chemistry, focusing on how energy is transferred and transformed in chemical systems.

• Energy: The capacity to do work or produce heat.

• Thermochemical processes involve changes in energy, often measured as heat or work.

Classification of Energy

Energy in chemistry is classified based on its source and effect.

• Mechanical Energy: Includes kinetic energy (energy of motion) and potential energy (energy due to position).

• Chemical Energy: Stored in chemical bonds of atoms and molecules.

• Thermal Energy: Related to the temperature and movement of particles.

Table: Classification of Energy

Type	Description
Kinetic Energy	Energy due to motion of atoms or molecules
Potential Energy	Energy due to position or arrangement
Chemical Energy	Energy stored in chemical bonds
Thermal Energy	Energy due to temperature

Energy Conversion Factors

Energy can be measured in different units, and conversion factors are essential for calculations.

• SI Unit for Energy: Joule (J)

• Common conversions:

  • 1 calorie (cal) = 4.184 J

  • 1 Calorie (Cal) = 1,000 cal = 4,184 J

  • 1 kilowatt-hour (kWh) = 3.60 x 106 J

Mechanical Energy

Kinetic and Potential Energy

Mechanical energy is the sum of kinetic and potential energy in a system.

• Kinetic Energy (K.E.): Energy due to motion. $K.E. = \frac{1}{2}mv^2$ where m is mass and v is velocity.

• Potential Energy (P.E.): Energy due to position in a gravitational field. $P.E. = mgh$ where m is mass, g is acceleration due to gravity, and h is height.

Energy Interconversions

Kinetic and potential energy can be converted into each other, especially in mechanical systems.

• Example: A falling object converts potential energy to kinetic energy as it descends.

First Law of Thermodynamics

Energy Conservation

The first law states that energy cannot be created or destroyed, only transferred between a system and its surroundings.

• System: The part of the universe being studied.

• Surroundings: Everything outside the system.

Heat and Work

Definitions

The transfer of energy between system and surroundings occurs as heat or work.

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

• Work (w): Energy transferred when a force moves an object.

Heat & Work Diagrams

Heat Application	Work Application
System absorbs/releases heat from/to surroundings	System does work on surroundings or vice versa

Internal Energy

Definition and Calculation

Internal energy ($E$ or $U$) is the total energy from all forms of kinetic and potential energy in a system.

• Internal Energy Formula: $\Delta E = q + w$ where $q$ is heat and $w$ is work.

• Work (Pressure-Volume Work): $w = -P\Delta V$ where $P$ is pressure and $\Delta V$ is change in volume.

Enthalpy

• Enthalpy ($H$): The amount of heat released or absorbed at constant pressure during a chemical reaction. $\Delta H = \Delta E + P\Delta V$

Endothermic and Exothermic Reactions

Endothermic Reactions

Endothermic reactions absorb heat from the surroundings, resulting in a temperature decrease in the surroundings.

• Example: Melting ice, water boiling.

• Energy diagram: Reactants have lower energy than products.

Exothermic Reactions

Exothermic reactions release heat to the surroundings, causing a temperature increase in the surroundings.

• Example: Combustion, water freezing.

• Energy diagram: Reactants have higher energy than products.

Heat–Temperature Relationship

Proportionality

The amount of heat absorbed or released by an object is directly proportional to its temperature change.

• Relationship: $q \propto \Delta T$

Molar & Specific Heat Capacity

Definitions

• Heat Capacity (C): Amount of heat required to change the temperature of a substance by 1 K.

• Specific Heat Capacity (c): Amount of heat required to change the temperature of 1 g of substance by 1 K.

• Molar Heat Capacity (Cm): Amount of heat required to change the temperature of 1 mole of substance by 1 K.

Heat Capacity Formulas

Type	Formula
Molar Heat Capacity	$C_m = \frac{q}{n\Delta T}$
Specific Heat Capacity	$c = \frac{q}{m\Delta T}$

Specific Heat Capacity Formula

Calculation

The amount of heat ($q$) absorbed or released by a substance is calculated as: $q = mc\Delta T$ where m is mass, c is specific heat capacity, and $\Delta T$ is temperature change.

Heat Capacity Comparison Table

Substance	Specific Heat Capacity (J/g·°C)
Aluminum, Al	0.900
Copper, Cu	0.385
Compounds (Ethanol, C2H5OH)	2.450
Wood	1.760

Calorimetry

Definition

Calorimetry is the measurement of heat changes in chemical reactions or physical processes.

• Calorimeter: An insulated container used to measure heat transfer.

• Standard Heat Capacity: $C = \frac{q}{\Delta T}$ where $C$ is standard heat capacity, $q$ is heat, and $\Delta T$ is temperature change.

Coffee Cup Calorimeter

Constant Pressure Calorimetry

A coffee cup calorimeter is used to measure heat changes at constant pressure, typically in liquid solutions.

• Heat lost by one substance is gained by another: $m_1c_1\Delta T_1 = m_2c_2\Delta T_2$

Practice Problems and Examples

Applications

Throughout the notes, practice problems and examples illustrate the application of formulas and concepts, such as calculating heat changes, internal energy, and enthalpy in various chemical and physical scenarios.

• Example: Calculating the heat released when water cools, or the final temperature in a calorimeter experiment.

• Practice: Determining which substance will have the greatest temperature change given specific heat capacities.

Summary Table: Key Formulas

Formula	Description
$K.E. = \frac{1}{2}mv^2$	Kinetic energy
$P.E. = mgh$	Potential energy
$\Delta E = q + w$	Change in internal energy
$w = -P\Delta V$	Pressure-volume work
$q = mc\Delta T$	Heat change for a substance
$C = \frac{q}{\Delta T}$	Standard heat capacity

Additional info:

• These notes cover essential thermochemistry topics for a general chemistry course, including energy classification, heat and work, internal energy, calorimetry, and heat capacity.

• Practice problems and examples are included to reinforce understanding and application of concepts.