Thermochemistry and the Nature of Energy

Definitions and Units of Energy

Thermochemistry is the study of energy changes that occur during chemical reactions and physical transformations. Energy is defined as the ability to transfer heat or do work. The SI unit of energy is the joule (J), where . Other units include the calorie (cal) () and the kilojoule (kJ) (). Nutritional calories (Cal) are equivalent to .

Types of Energy

Objects and molecules possess two main types of energy:

• Kinetic Energy (KE): Energy due to motion. The formula is , where m is mass and v is velocity.

• Potential Energy (PE): Energy due to position or arrangement. Important types in chemistry include electrostatic potential energy (from interactions between charged particles) and nuclear potential energy (from attractions within the nucleus).

Kinetic Energy in Molecules

Molecules exhibit complex motions contributing to their kinetic energy:

• Translation: Straight-line motion, most significant in gases.

• Rotation: Rotational movement, important in gases and liquids.

• Vibration: Oscillatory motion, present in solids, liquids, and gases.

Temperature and Energy

Temperature is a measure of the average kinetic energy of a system. It is an intensive property (does not depend on the amount of substance), while heat is an extensive property (depends on the amount).

Conservation of Energy and Thermodynamic Systems

The First Law of Thermodynamics

The law of conservation of energy states that energy cannot be created or destroyed, only transformed. The total energy of the universe is constant.

System and Surroundings

In thermochemistry, the universe is divided into a system (the part under study) and its surroundings. Types of systems:

• Open system: Exchanges both energy and matter with surroundings.

• Closed system: Exchanges energy but not matter.

• Isolated system: Exchanges neither energy nor matter.

Examples of Systems

• Open system: Reaction in an open beaker (CO2 escapes).

• Closed system: Reaction in a cylinder with a movable piston.

• Isolated system: Bomb calorimeter or thermos flask.

Internal Energy and Its Changes

Internal Energy (U)

Internal energy (U) is the sum of all kinetic and potential energies of a system's components. Changes in internal energy () are monitored by temperature (for KE) and volume (for PE).

Factors Affecting Internal Energy

• Type of substance

• Amount of substance

• Temperature

• Phase (solid, liquid, gas)

Change in Internal Energy ()

The change in internal energy is calculated as:

Sign conventions:

• : System absorbs energy (gain).

• : System releases energy (loss).

Relating to Heat and Work

Energy exchange between system and surroundings occurs as heat (q) and/or work (w):

Work is often mechanical (e.g., pressure-volume work):

State Functions

A state function depends only on the current state, not the path taken. is a state function, but q and w are not.

Enthalpy and Heat Changes in Reactions

Definition of Enthalpy (H)

Enthalpy (H) is the energy absorbed or released during a reaction at constant pressure. It is defined as:

Change in enthalpy:

At constant pressure:

Endothermic and Exothermic Processes

• Endothermic: System absorbs heat (). Example: Vaporization of water.

• Exothermic: System releases heat (). Example: Condensation of water.

Enthalpy of Reaction ()

The enthalpy change for a reaction is:

Extensive Property and State Dependence

depends on the amount of reactants and the physical state of reactants and products.

Hess's Law

Hess's Law states that the enthalpy change for a reaction carried out in multiple steps is the sum of the enthalpy changes for each step:

Standard Enthalpy of Formation ()

The standard enthalpy of formation is the enthalpy change for forming 1 mole of a substance from its elements in their most stable form under standard conditions (usually 25°C, 1 atm, 1 M concentration).

Calculation formula:

Heat Capacity and Calorimetry

Heat Capacity (C) and Specific Heat (Cs)

Heat capacity (C) is the amount of heat required to raise the temperature of a substance by 1°C or 1 K. Specific heat (Cs) is the heat capacity per gram, and molar heat capacity (Cm) is per mole.

Formulas:

Calorimetry

Calorimetry is the measurement of heat based on temperature changes. Two main types:

• Coffee cup calorimeter: Measures heat at constant pressure ().

• Bomb calorimeter: Measures heat at constant volume ().

Summary Table: Types of Thermodynamic Systems

Key Equations